#2051, #2084 Add new @Condition annotation for custom presence check methods

core/src/main/java/org/mapstruct/Condition.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * This annotation marks a method as a <em>presence check method</em> to check check for presence in beans.
+ * <p>
+ * By default bean properties are checked against {@code null} or using a presence check method in the source bean.
+ * If a presence check method is available then it will be used instead.
+ * <p>
+ * Presence check methods have to return {@code boolean}.
+ * The following parameters are accepted for the presence check methods:
+ *     <ul>
+ *         <li>The parameter with the value of the source property.
+ *         e.g. the value given by calling {@code getName()} for the name property of the source bean</li>
+ *         <li>The mapping source parameter</li>
+ *         <li>{@code @}{@link Context} parameter</li>
+ *     </ul>
+ *
+ * <strong>Note:</strong> The usage of this annotation is <em>mandatory</em>
+ * for a method to be considered as a presence check method.
+ *
+ * <pre><code>
+ * public class PresenceCheckUtils {
+ *
+ *   &#64;Condition
+ *   public static boolean isNotEmpty(String value) {
+ *      return value != null &#38;&#38; !value.isEmpty();
+ *   }
+ * }
+ *
+ * &#64;Mapper(uses = PresenceCheckUtils.class)
+ * public interface MovieMapper {
+ *
+ *     MovieDto map(Movie movie);
+ * }
+ * </code></pre>
+ *
+ * The following implementation of {@code MovieMapper} will be generated:
+ *
+ * <pre>
+ * <code>
+ * public class MovieMapperImpl implements MovieMapper {
+ *
+ *     &#64;Override
+ *     public MovieDto map(Movie movie) {
+ *         if ( movie == null ) {
+ *             return null;
+ *         }
+ *
+ *         MovieDto movieDto = new MovieDto();
+ *
+ *         if ( PresenceCheckUtils.isNotEmpty( movie.getTitle() ) ) {
+ *             movieDto.setTitle( movie.getTitle() );
+ *         }
+ *
+ *         return movieDto;
+ *     }
+ * }
+ * </code>
+ * </pre>
+ *
+ * @author Filip Hrisafov
+ * @since 1.5
+ */
+@Target({ ElementType.METHOD })
+@Retention(RetentionPolicy.CLASS)
+public @interface Condition {
+
+}

core/src/main/java/org/mapstruct/Mapping.java

@@ -316,6 +316,74 @@ public @interface Mapping {
      */
     String[] qualifiedByName() default { };
 
+    /**
+     * A qualifier can be specified to aid the selection process of a suitable presence check method.
+     * This is useful in case multiple presence check methods qualify and thus would result in an
+     * 'Ambiguous presence check methods found' error.
+     * A qualifier is a custom annotation and can be placed on a hand written mapper class or a method.
+     * This is similar to the {@link #qualifiedBy()}, but it is only applied for {@link Condition} methods.
+     *
+     * @return the qualifiers
+     * @see Qualifier
+     * @see #qualifiedBy()
+     * @since 1.5
+     */
+    Class<? extends Annotation>[] conditionQualifiedBy() default { };
+
+    /**
+     * String-based form of qualifiers for condition / presence check methods;
+     * When looking for a suitable presence check method for a given property, MapStruct will
+     * only consider those methods carrying directly or indirectly (i.e. on the class-level) a {@link Named} annotation
+     * for each of the specified qualifier names.
+     *
+     * This is similar like {@link #qualifiedByName()} but it is only applied for {@link Condition} methods.
+     * <p>
+     *   Note that annotation-based qualifiers are generally preferable as they allow more easily to find references and
+     *   are safe for refactorings, but name-based qualifiers can be a less verbose alternative when requiring a large
+     *   number of qualifiers as no custom annotation types are needed.
+     * </p>
+     *
+     *
+     * @return One or more qualifier name(s)
+     * @see #conditionQualifiedBy()
+     * @see #qualifiedByName()
+     * @see Named
+     * @since 1.5
+     */
+    String[] conditionQualifiedByName() default { };
+
+    /**
+     * A conditionExpression {@link String} based on which the specified property is to be checked
+     * whether it is present or not.
+     * <p>
+     * Currently, Java is the only supported "expression language" and expressions must be given in form of Java
+     * expressions using the following format: {@code java(<EXPRESSION>)}. For instance the mapping:
+     * <pre><code>
+     * &#64;Mapping(
+     *     target = "someProp",
+     *     conditionExpression = "java(s.getAge() &#60; 18)"
+     * )
+     * </code></pre>
+     * <p>
+     * will cause the following target property assignment to be generated:
+     * <pre><code>
+     *     if (s.getAge() &#60; 18) {
+     *         targetBean.setSomeProp( s.getSomeProp() );
+     *     }
+     * </code></pre>
+     * <p>
+     * <p>
+     * Any types referenced in expressions must be given via their fully-qualified name. Alternatively, types can be
+     * imported via {@link Mapper#imports()}.
+     * <p>
+     * This attribute can not be used together with {@link #expression()} or {@link #constant()}.
+     *
+     * @return An expression specifying a condition check for the designated property
+     *
+     * @since 1.5
+     */
+    String conditionExpression() default "";
+
     /**
      * Specifies the result type of the mapping method to be used in case multiple mapping methods qualify.
      *

documentation/src/main/asciidoc/chapter-10-advanced-mapping-options.asciidoc

@@ -229,6 +229,78 @@ The source presence checker name can be changed in the MapStruct service provide
 Some types of mappings (collections, maps), in which MapStruct is instructed to use a getter or adder as target accessor see `CollectionMappingStrategy`, MapStruct will always generate a source property
 null check, regardless the value of the `NullValueCheckStrategy` to avoid addition of `null` to the target collection or map.
 ====
+
+[[conditional-mapping]]
+=== Conditional Mapping
+
+Conditional Mapping is a type of <<source-presence-check>>.
+The difference is that it allows users to write custom condition methods that will be invoked to check if a property needs to be mapped or not.
+
+A custom condition method is a method that is annotated with `org.mapstruct.Condition` and returns `boolean`.
+
+e.g. if you only want to map a String property when it is not `null, and it is not empty then you can do something like:
+
+.Mapper using custom condition check method
+====
+[source, java, linenums]
+[subs="verbatim,attributes"]
+----
+@Mapper
+public interface CarMapper {
+
+    CarDto carToCarDto(Car car);
+
+    @Condition
+    default boolean isNotEmpty(String value) {
+        return value != null && !value.isEmpty();
+    }
+}
+----
+====
+
+The generated mapper will look like:
+
+.try-catch block in generated implementation
+====
+[source, java, linenums]
+[subs="verbatim,attributes"]
+----
+// GENERATED CODE
+public class CarMapperImpl implements CarMapper {
+
+    @Override
+    public CarDto carToCarDto(Car car) {
+        if ( car == null ) {
+            return null;
+        }
+
+        CarDto carDto = new CarDto();
+
+        if ( isNotEmpty( car.getOwner() ) ) {
+            carDto.setOwner( car.getOwner() );
+        }
+
+        // Mapping of other properties
+
+        return carDto;
+    }
+}
+----
+====
+
+[IMPORTANT]
+====
+If there is a custom `@Condition` method applicable for the property it will have a precedence over a presence check method in the bean itself.
+====
+
+[NOTE]
+====
+Methods annotated with `@Condition` in addition to the value of the source property can also have the source parameter as an input.
+====
+
+<<selection-based-on-qualifiers>> is also valid for `@Condition` methods.
+In order to use a more specific condition method you will need to use one of `Mapping#conditionQualifiedByName` or `Mapping#conditionQualifiedBy`.
+
 [[exceptions]]
 === Exceptions
 

processor/src/main/java/org/mapstruct/ap/internal/gem/GemGenerator.java

@@ -12,6 +12,7 @@ import org.mapstruct.AfterMapping;
 import org.mapstruct.BeanMapping;
 import org.mapstruct.BeforeMapping;
 import org.mapstruct.Builder;
+import org.mapstruct.Condition;
 import org.mapstruct.Context;
 import org.mapstruct.DecoratedWith;
 import org.mapstruct.EnumMapping;
@@ -61,6 +62,7 @@ import org.mapstruct.tools.gem.GemDefinition;
 @GemDefinition(ValueMappings.class)
 @GemDefinition(Context.class)
 @GemDefinition(Builder.class)
+@GemDefinition(Condition.class)
 
 @GemDefinition(MappingControl.class)
 @GemDefinition(MappingControls.class)

processor/src/main/java/org/mapstruct/ap/internal/model/BeanMappingMethod.java

@@ -1155,6 +1155,7 @@ public class BeanMappingMethod extends NormalTypeMappingMethod {
                             .dependsOn( mapping.getDependsOn() )
                             .defaultValue( mapping.getDefaultValue() )
                             .defaultJavaExpression( mapping.getDefaultJavaExpression() )
+                            .conditionJavaExpression( mapping.getConditionJavaExpression() )
                             .mirror( mapping.getMirror() )
                             .options( mapping )
                             .build();

processor/src/main/java/org/mapstruct/ap/internal/model/MethodReferencePresenceCheck.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.internal.model;
+
+import java.util.Objects;
+import java.util.Set;
+
+import org.mapstruct.ap.internal.model.common.ModelElement;
+import org.mapstruct.ap.internal.model.common.PresenceCheck;
+import org.mapstruct.ap.internal.model.common.Type;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class MethodReferencePresenceCheck extends ModelElement implements PresenceCheck {
+
+    protected final MethodReference methodReference;
+
+    public MethodReferencePresenceCheck(MethodReference methodReference) {
+        this.methodReference = methodReference;
+    }
+
+    @Override
+    public Set<Type> getImportTypes() {
+        return methodReference.getImportTypes();
+    }
+
+    public MethodReference getMethodReference() {
+        return methodReference;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if ( this == o ) {
+            return true;
+        }
+        if ( o == null || getClass() != o.getClass() ) {
+            return false;
+        }
+        MethodReferencePresenceCheck that = (MethodReferencePresenceCheck) o;
+        return Objects.equals( methodReference, that.methodReference );
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash( methodReference );
+    }
+}

processor/src/main/java/org/mapstruct/ap/internal/model/PresenceCheckMethodResolver.java

@@ -0,0 +1,149 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.internal.model;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.mapstruct.ap.internal.model.common.Parameter;
+import org.mapstruct.ap.internal.model.common.PresenceCheck;
+import org.mapstruct.ap.internal.model.common.Type;
+import org.mapstruct.ap.internal.model.source.Method;
+import org.mapstruct.ap.internal.model.source.ParameterProvidedMethods;
+import org.mapstruct.ap.internal.model.source.SelectionParameters;
+import org.mapstruct.ap.internal.model.source.SourceMethod;
+import org.mapstruct.ap.internal.model.source.selector.MethodSelectors;
+import org.mapstruct.ap.internal.model.source.selector.SelectedMethod;
+import org.mapstruct.ap.internal.model.source.selector.SelectionCriteria;
+import org.mapstruct.ap.internal.util.Message;
+
+/**
+ * @author Filip Hrisafov
+ */
+public final class PresenceCheckMethodResolver {
+
+    private PresenceCheckMethodResolver() {
+
+    }
+
+    public static PresenceCheck getPresenceCheck(
+        Method method,
+        SelectionParameters selectionParameters,
+        MappingBuilderContext ctx
+    ) {
+        SelectedMethod<SourceMethod> matchingMethod = findMatchingPresenceCheckMethod(
+            method,
+            selectionParameters,
+            ctx
+        );
+
+        if ( matchingMethod == null ) {
+            return null;
+        }
+
+        MethodReference methodReference = getPresenceCheckMethodReference( method, matchingMethod, ctx );
+
+        return new MethodReferencePresenceCheck( methodReference );
+
+    }
+
+    private static SelectedMethod<SourceMethod> findMatchingPresenceCheckMethod(
+        Method method,
+        SelectionParameters selectionParameters,
+        MappingBuilderContext ctx
+    ) {
+        MethodSelectors selectors = new MethodSelectors(
+            ctx.getTypeUtils(),
+            ctx.getElementUtils(),
+            ctx.getTypeFactory(),
+            ctx.getMessager()
+        );
+
+        Type booleanType = ctx.getTypeFactory().getType( Boolean.class );
+        List<SelectedMethod<SourceMethod>> matchingMethods = selectors.getMatchingMethods(
+            method,
+            getAllAvailableMethods( method, ctx.getSourceModel() ),
+            Collections.emptyList(),
+            booleanType,
+            booleanType,
+            SelectionCriteria.forPresenceCheckMethods( selectionParameters )
+        );
+
+        if ( matchingMethods.isEmpty() ) {
+            return null;
+        }
+
+        if ( matchingMethods.size() > 1 ) {
+            ctx.getMessager().printMessage(
+                method.getExecutable(),
+                Message.GENERAL_AMBIGUOUS_PRESENCE_CHECK_METHOD,
+                selectionParameters.getSourceRHS().getSourceType().describe(),
+                matchingMethods.stream()
+                    .map( SelectedMethod::getMethod )
+                    .map( Method::describe )
+                    .collect( Collectors.joining( ", " ) )
+            );
+
+            return null;
+        }
+
+        return matchingMethods.get( 0 );
+    }
+
+    private static MethodReference getPresenceCheckMethodReference(
+        Method method,
+        SelectedMethod<SourceMethod> matchingMethod,
+        MappingBuilderContext ctx
+    ) {
+
+        Parameter providingParameter =
+            method.getContextProvidedMethods().getParameterForProvidedMethod( matchingMethod.getMethod() );
+        if ( providingParameter != null ) {
+            return MethodReference.forParameterProvidedMethod(
+                matchingMethod.getMethod(),
+                providingParameter,
+                matchingMethod.getParameterBindings()
+            );
+        }
+        else {
+            MapperReference ref = MapperReference.findMapperReference(
+                ctx.getMapperReferences(),
+                matchingMethod.getMethod()
+            );
+
+            return MethodReference.forMapperReference(
+                matchingMethod.getMethod(),
+                ref,
+                matchingMethod.getParameterBindings()
+            );
+        }
+    }
+
+    private static List<SourceMethod> getAllAvailableMethods(Method method, List<SourceMethod> sourceModelMethods) {
+        ParameterProvidedMethods contextProvidedMethods = method.getContextProvidedMethods();
+        if ( contextProvidedMethods.isEmpty() ) {
+            return sourceModelMethods;
+        }
+
+        List<SourceMethod> methodsProvidedByParams = contextProvidedMethods
+            .getAllProvidedMethodsInParameterOrder( method.getContextParameters() );
+
+        List<SourceMethod> availableMethods =
+            new ArrayList<>( methodsProvidedByParams.size() + sourceModelMethods.size() );
+
+        for ( SourceMethod methodProvidedByParams : methodsProvidedByParams ) {
+            // add only methods from context that do have the @Condition annotation
+            if ( methodProvidedByParams.isPresenceCheck() ) {
+                availableMethods.add( methodProvidedByParams );
+            }
+        }
+        availableMethods.addAll( sourceModelMethods );
+
+        return availableMethods;
+    }
+}

processor/src/main/java/org/mapstruct/ap/internal/model/PropertyMapping.java

@@ -13,6 +13,9 @@ import java.util.Objects;
 import java.util.Set;
 import javax.lang.model.element.AnnotationMirror;
 
+import org.mapstruct.ap.internal.gem.BuilderGem;
+import org.mapstruct.ap.internal.gem.NullValueCheckStrategyGem;
+import org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem;
 import org.mapstruct.ap.internal.model.assignment.AdderWrapper;
 import org.mapstruct.ap.internal.model.assignment.ArrayCopyWrapper;
 import org.mapstruct.ap.internal.model.assignment.EnumConstantWrapper;
@@ -32,6 +35,7 @@ import org.mapstruct.ap.internal.model.common.PresenceCheck;
 import org.mapstruct.ap.internal.model.common.SourceRHS;
 import org.mapstruct.ap.internal.model.common.Type;
 import org.mapstruct.ap.internal.model.presence.AllPresenceChecksPresenceCheck;
+import org.mapstruct.ap.internal.model.presence.JavaExpressionPresenceCheck;
 import org.mapstruct.ap.internal.model.presence.NullPresenceCheck;
 import org.mapstruct.ap.internal.model.presence.SourceReferenceMethodPresenceCheck;
 import org.mapstruct.ap.internal.model.source.DelegatingOptions;
@@ -40,9 +44,6 @@ import org.mapstruct.ap.internal.model.source.MappingOptions;
 import org.mapstruct.ap.internal.model.source.Method;
 import org.mapstruct.ap.internal.model.source.SelectionParameters;
 import org.mapstruct.ap.internal.model.source.selector.SelectionCriteria;
-import org.mapstruct.ap.internal.gem.BuilderGem;
-import org.mapstruct.ap.internal.gem.NullValueCheckStrategyGem;
-import org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem;
 import org.mapstruct.ap.internal.util.Message;
 import org.mapstruct.ap.internal.util.NativeTypes;
 import org.mapstruct.ap.internal.util.Strings;
@@ -52,12 +53,12 @@ import org.mapstruct.ap.internal.util.accessor.AccessorType;
 
 import static org.mapstruct.ap.internal.gem.NullValueCheckStrategyGem.ALWAYS;
 import static org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem.IGNORE;
+import static org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem.SET_TO_DEFAULT;
+import static org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem.SET_TO_NULL;
 import static org.mapstruct.ap.internal.model.ForgedMethod.forElementMapping;
 import static org.mapstruct.ap.internal.model.ForgedMethod.forParameterMapping;
 import static org.mapstruct.ap.internal.model.ForgedMethod.forPropertyMapping;
 import static org.mapstruct.ap.internal.model.common.Assignment.AssignmentType.DIRECT;
-import static org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem.SET_TO_DEFAULT;
-import static org.mapstruct.ap.internal.gem.NullValuePropertyMappingStrategyGem.SET_TO_NULL;
 
 /**
  * Represents the mapping between a source and target property, e.g. from {@code String Source#foo} to
@@ -142,6 +143,7 @@ public class PropertyMapping extends ModelElement {
         // initial properties
         private String defaultValue;
         private String defaultJavaExpression;
+        private String conditionJavaExpression;
         private SourceReference sourceReference;
         private SourceRHS rightHandSide;
         private FormattingParameters formattingParameters;
@@ -182,6 +184,11 @@ public class PropertyMapping extends ModelElement {
             return this;
         }
 
+        public PropertyMappingBuilder conditionJavaExpression(String conditionJavaExpression) {
+            this.conditionJavaExpression = conditionJavaExpression;
+            return this;
+        }
+
         public PropertyMappingBuilder forgeMethodWithMappingReferences(MappingReferences mappingReferences) {
             this.forgeMethodWithMappingReferences = mappingReferences;
             return this;
@@ -557,13 +564,19 @@ public class PropertyMapping extends ModelElement {
             // simple property
             else if ( !sourceReference.isNested() ) {
                 String sourceRef = sourceParam.getName() + "." + ValueProvider.of( propertyEntry.getReadAccessor() );
-                return new SourceRHS( sourceParam.getName(),
+                SourceRHS sourceRHS = new SourceRHS(
+                    sourceParam.getName(),
                     sourceRef,
-                                      getSourcePresenceCheckerRef( sourceReference ),
+                    null,
                     propertyEntry.getType(),
                     existingVariableNames,
                     sourceReference.toString()
                 );
+                sourceRHS.setSourcePresenceCheckerReference( getSourcePresenceCheckerRef(
+                    sourceReference,
+                    sourceRHS
+                ) );
+                return sourceRHS;
             }
             // nested property given as dot path
             else {
@@ -598,11 +611,15 @@ public class PropertyMapping extends ModelElement {
                 String sourceRef = forgedName + "( " + sourceParam.getName() + " )";
                 SourceRHS sourceRhs = new SourceRHS( sourceParam.getName(),
                                                      sourceRef,
-                                                     getSourcePresenceCheckerRef( sourceReference ),
+                                                     null,
                                                      sourceType,
                                                      existingVariableNames,
                                                      sourceReference.toString()
                 );
+                sourceRhs.setSourcePresenceCheckerReference( getSourcePresenceCheckerRef(
+                    sourceReference,
+                    sourceRhs
+                ) );
 
                 // create a local variable to which forged method can be assigned.
                 String desiredName = propertyEntry.getName();
@@ -613,7 +630,25 @@ public class PropertyMapping extends ModelElement {
             }
         }
 
-        private PresenceCheck getSourcePresenceCheckerRef(SourceReference sourceReference ) {
+        private PresenceCheck getSourcePresenceCheckerRef(SourceReference sourceReference,
+                                                          SourceRHS sourceRHS) {
+
+            if ( conditionJavaExpression != null ) {
+                return new JavaExpressionPresenceCheck( conditionJavaExpression );
+            }
+
+            SelectionParameters selectionParameters = this.selectionParameters != null ?
+                this.selectionParameters.withSourceRHS( sourceRHS ) :
+                SelectionParameters.forSourceRHS( sourceRHS );
+            PresenceCheck presenceCheck = PresenceCheckMethodResolver.getPresenceCheck(
+                method,
+                selectionParameters,
+                ctx
+            );
+            if ( presenceCheck != null ) {
+                return presenceCheck;
+            }
+
             PresenceCheck sourcePresenceChecker = null;
             if ( !sourceReference.getPropertyEntries().isEmpty() ) {
                 Parameter sourceParam = sourceReference.getParameter();

processor/src/main/java/org/mapstruct/ap/internal/model/common/SourceRHS.java

@@ -7,7 +7,6 @@ package org.mapstruct.ap.internal.model.common;
 
 import java.util.Collection;
 import java.util.Collections;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 import java.util.stream.Stream;
@@ -31,7 +30,7 @@ public class SourceRHS extends ModelElement implements Assignment {
     private String sourceLoopVarName;
     private final Set<String> existingVariableNames;
     private final String sourceErrorMessagePart;
-    private final PresenceCheck sourcePresenceCheckerReference;
+    private PresenceCheck sourcePresenceCheckerReference;
     private boolean useElementAsSourceTypeForMatching = false;
     private final String sourceParameterName;
 
@@ -65,6 +64,10 @@ public class SourceRHS extends ModelElement implements Assignment {
         return sourcePresenceCheckerReference;
     }
 
+    public void setSourcePresenceCheckerReference(PresenceCheck sourcePresenceCheckerReference) {
+        this.sourcePresenceCheckerReference = sourcePresenceCheckerReference;
+    }
+
     @Override
     public Type getSourceType() {
         return sourceType;

processor/src/main/java/org/mapstruct/ap/internal/model/presence/AllPresenceChecksPresenceCheck.java

@@ -6,7 +6,6 @@
 package org.mapstruct.ap.internal.model.presence;
 
 import java.util.Collection;
-import java.util.Collections;
 import java.util.HashSet;
 import java.util.Objects;
 import java.util.Set;

processor/src/main/java/org/mapstruct/ap/internal/model/presence/JavaExpressionPresenceCheck.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.internal.model.presence;
+
+import java.util.Collections;
+import java.util.Objects;
+import java.util.Set;
+
+import org.mapstruct.ap.internal.model.common.ModelElement;
+import org.mapstruct.ap.internal.model.common.PresenceCheck;
+import org.mapstruct.ap.internal.model.common.Type;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class JavaExpressionPresenceCheck extends ModelElement implements PresenceCheck {
+
+    private final String javaExpression;
+
+    public JavaExpressionPresenceCheck(String javaExpression) {
+        this.javaExpression = javaExpression;
+    }
+
+    public String getJavaExpression() {
+        return javaExpression;
+    }
+
+    @Override
+    public Set<Type> getImportTypes() {
+        return Collections.emptySet();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if ( this == o ) {
+            return true;
+        }
+        if ( o == null || getClass() != o.getClass() ) {
+            return false;
+        }
+        JavaExpressionPresenceCheck that = (JavaExpressionPresenceCheck) o;
+        return Objects.equals( javaExpression, that.javaExpression );
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash( javaExpression );
+    }
+}

processor/src/main/java/org/mapstruct/ap/internal/model/source/MappingOptions.java

@@ -43,6 +43,7 @@ public class MappingOptions extends DelegatingOptions {
     private final String constant;
     private final String javaExpression;
     private final String defaultJavaExpression;
+    private final String conditionJavaExpression;
     private final String targetName;
     private final String defaultValue;
     private final FormattingParameters formattingParameters;
@@ -114,6 +115,7 @@ public class MappingOptions extends DelegatingOptions {
         String constant = mapping.constant().getValue();
         String expression = getExpression( mapping, method, messager );
         String defaultExpression = getDefaultExpression( mapping, method, messager );
+        String conditionExpression = getConditionExpression( mapping, method, messager );
         String dateFormat = mapping.dateFormat().getValue();
         String numberFormat = mapping.numberFormat().getValue();
         String defaultValue = mapping.defaultValue().getValue();
@@ -132,6 +134,8 @@ public class MappingOptions extends DelegatingOptions {
         SelectionParameters selectionParams = new SelectionParameters(
             mapping.qualifiedBy().get(),
             mapping.qualifiedByName().get(),
+            mapping.conditionQualifiedBy().get(),
+            mapping.conditionQualifiedByName().get(),
             mapping.resultType().getValue(),
             typeUtils
         );
@@ -145,6 +149,7 @@ public class MappingOptions extends DelegatingOptions {
             constant,
             expression,
             defaultExpression,
+            conditionExpression,
             defaultValue,
             mapping.ignore().get(),
             formattingParam,
@@ -174,6 +179,7 @@ public class MappingOptions extends DelegatingOptions {
             null,
             null,
             null,
+            null,
             true,
             null,
             null,
@@ -261,6 +267,7 @@ public class MappingOptions extends DelegatingOptions {
                            String constant,
                            String javaExpression,
                            String defaultJavaExpression,
+                           String conditionJavaExpression,
                            String defaultValue,
                            boolean isIgnored,
                            FormattingParameters formattingParameters,
@@ -279,6 +286,7 @@ public class MappingOptions extends DelegatingOptions {
         this.constant = constant;
         this.javaExpression = javaExpression;
         this.defaultJavaExpression = defaultJavaExpression;
+        this.conditionJavaExpression = conditionJavaExpression;
         this.defaultValue = defaultValue;
         this.isIgnored = isIgnored;
         this.formattingParameters = formattingParameters;
@@ -330,6 +338,27 @@ public class MappingOptions extends DelegatingOptions {
         return javaExpressionMatcher.group( 1 ).trim();
     }
 
+    private static String getConditionExpression(MappingGem mapping, ExecutableElement element,
+                                        FormattingMessager messager) {
+        if ( !mapping.conditionExpression().hasValue() ) {
+            return null;
+        }
+
+        Matcher javaExpressionMatcher = JAVA_EXPRESSION.matcher( mapping.conditionExpression().get() );
+
+        if ( !javaExpressionMatcher.matches() ) {
+            messager.printMessage(
+                element,
+                mapping.mirror(),
+                mapping.conditionExpression().getAnnotationValue(),
+                Message.PROPERTYMAPPING_INVALID_CONDITION_EXPRESSION
+            );
+            return null;
+        }
+
+        return javaExpressionMatcher.group( 1 ).trim();
+    }
+
     public String getTargetName() {
         return targetName;
     }
@@ -364,6 +393,10 @@ public class MappingOptions extends DelegatingOptions {
         return defaultJavaExpression;
     }
 
+    public String getConditionJavaExpression() {
+        return conditionJavaExpression;
+    }
+
     public String getDefaultValue() {
         return defaultValue;
     }
@@ -452,6 +485,7 @@ public class MappingOptions extends DelegatingOptions {
             null, // constant
             null, // expression
             null, // defaultExpression
+            null, // conditionExpression
             null,
             isIgnored,
             formattingParameters,
@@ -481,6 +515,7 @@ public class MappingOptions extends DelegatingOptions {
             constant,
             javaExpression,
             defaultJavaExpression,
+            conditionJavaExpression,
             defaultValue,
             isIgnored,
             formattingParameters,

processor/src/main/java/org/mapstruct/ap/internal/model/source/Method.java

@@ -90,6 +90,14 @@ public interface Method {
      */
     boolean isObjectFactory();
 
+    /**
+     * Returns whether the method is designated as a presence check method
+     * @return {@code true} if it is a presence check method
+     */
+    default boolean isPresenceCheck() {
+        return false;
+    }
+
     /**
      * Returns the parameter designated as target type (if present) {@link org.mapstruct.TargetType }
      *

processor/src/main/java/org/mapstruct/ap/internal/model/source/SelectionParameters.java

@@ -23,6 +23,8 @@ public class SelectionParameters {
 
     private final List<TypeMirror> qualifiers;
     private final List<String> qualifyingNames;
+    private final List<TypeMirror> conditionQualifiers;
+    private final List<String> conditionQualifyingNames;
     private final TypeMirror resultType;
     private final TypeUtils typeUtils;
     private final SourceRHS sourceRHS;
@@ -39,6 +41,8 @@ public class SelectionParameters {
         return new SelectionParameters(
             selectionParameters.qualifiers,
             selectionParameters.qualifyingNames,
+            selectionParameters.conditionQualifiers,
+            selectionParameters.conditionQualifyingNames,
             null,
             selectionParameters.typeUtils
         );
@@ -46,13 +50,32 @@ public class SelectionParameters {
 
     public SelectionParameters(List<TypeMirror> qualifiers, List<String> qualifyingNames, TypeMirror resultType,
         TypeUtils typeUtils) {
-        this( qualifiers, qualifyingNames, resultType, typeUtils, null );
+        this(
+            qualifiers,
+            qualifyingNames,
+            Collections.emptyList(),
+            Collections.emptyList(),
+            resultType,
+            typeUtils,
+            null
+        );
     }
 
-    private SelectionParameters(List<TypeMirror> qualifiers, List<String> qualifyingNames, TypeMirror resultType,
+    public SelectionParameters(List<TypeMirror> qualifiers, List<String> qualifyingNames,
+                               List<TypeMirror> conditionQualifiers, List<String> conditionQualifyingNames,
+                               TypeMirror resultType,
+                               TypeUtils typeUtils) {
+        this( qualifiers, qualifyingNames, conditionQualifiers, conditionQualifyingNames, resultType, typeUtils, null );
+    }
+
+    private SelectionParameters(List<TypeMirror> qualifiers, List<String> qualifyingNames,
+                                List<TypeMirror> conditionQualifiers, List<String> conditionQualifyingNames,
+                                TypeMirror resultType,
                                 TypeUtils typeUtils, SourceRHS sourceRHS) {
         this.qualifiers = qualifiers;
         this.qualifyingNames = qualifyingNames;
+        this.conditionQualifiers = conditionQualifiers;
+        this.conditionQualifyingNames = conditionQualifyingNames;
         this.resultType = resultType;
         this.typeUtils = typeUtils;
         this.sourceRHS = sourceRHS;
@@ -74,6 +97,21 @@ public class SelectionParameters {
         return qualifyingNames;
     }
 
+    /**
+     * @return qualifiers used for further select the appropriate presence check method based on class and name
+     */
+    public List<TypeMirror> getConditionQualifiers() {
+        return conditionQualifiers;
+    }
+
+    /**
+     * @return qualifyingNames, used in combination with with @Named
+     * @see #getConditionQualifiers()
+     */
+    public List<String> getConditionQualifyingNames() {
+        return conditionQualifyingNames;
+    }
+
     /**
      *
      * @return resultType used for further select the appropriate mapping method based on resultType (bean mapping)
@@ -119,6 +157,14 @@ public class SelectionParameters {
             return false;
         }
 
+        if ( !Objects.equals( this.conditionQualifiers, other.conditionQualifiers ) ) {
+            return false;
+        }
+
+        if ( !Objects.equals( this.conditionQualifyingNames, other.conditionQualifyingNames ) ) {
+            return false;
+        }
+
         if ( !Objects.equals( this.sourceRHS, other.sourceRHS ) ) {
             return false;
         }
@@ -151,8 +197,22 @@ public class SelectionParameters {
         }
     }
 
+    public SelectionParameters withSourceRHS(SourceRHS sourceRHS) {
+        return new SelectionParameters(
+            this.qualifiers,
+            this.qualifyingNames,
+            this.conditionQualifiers,
+            this.conditionQualifyingNames,
+            null,
+            this.typeUtils,
+            sourceRHS
+        );
+    }
+
     public static SelectionParameters forSourceRHS(SourceRHS sourceRHS) {
         return new SelectionParameters(
+            Collections.emptyList(),
+            Collections.emptyList(),
             Collections.emptyList(),
             Collections.emptyList(),
             null,

processor/src/main/java/org/mapstruct/ap/internal/model/source/SourceMethod.java

@@ -14,6 +14,8 @@ import java.util.stream.Collectors;
 import javax.lang.model.element.Element;
 import javax.lang.model.element.ExecutableElement;
 import javax.lang.model.element.Modifier;
+
+import org.mapstruct.ap.internal.gem.ConditionGem;
 import org.mapstruct.ap.internal.util.TypeUtils;
 
 import org.mapstruct.ap.internal.model.common.Accessibility;
@@ -47,6 +49,7 @@ public class SourceMethod implements Method {
     private final Parameter mappingTargetParameter;
     private final Parameter targetTypeParameter;
     private final boolean isObjectFactory;
+    private final boolean isPresenceCheck;
     private final Type returnType;
     private final Accessibility accessibility;
     private final List<Type> exceptionTypes;
@@ -230,6 +233,7 @@ public class SourceMethod implements Method {
         this.targetTypeParameter = Parameter.getTargetTypeParameter( parameters );
         this.hasObjectFactoryAnnotation = ObjectFactoryGem.instanceOn( executable ) != null;
         this.isObjectFactory = determineIfIsObjectFactory();
+        this.isPresenceCheck = determineIfIsPresenceCheck();
 
         this.typeUtils = builder.typeUtils;
         this.typeFactory = builder.typeFactory;
@@ -247,6 +251,19 @@ public class SourceMethod implements Method {
             && ( hasObjectFactoryAnnotation || hasNoSourceParameters );
     }
 
+    private boolean determineIfIsPresenceCheck() {
+        if ( returnType.isPrimitive() ) {
+            if ( !returnType.getName().equals( "boolean" ) ) {
+                return false;
+            }
+        }
+        else if ( !returnType.getFullyQualifiedName().equals( Boolean.class.getCanonicalName() ) ) {
+            return false;
+        }
+
+        return ConditionGem.instanceOn( executable ) != null;
+    }
+
     @Override
     public Type getDeclaringMapper() {
         return declaringMapper;
@@ -519,6 +536,11 @@ public class SourceMethod implements Method {
         return Executables.isLifecycleCallbackMethod( getExecutable() );
     }
 
+    @Override
+    public boolean isPresenceCheck() {
+        return isPresenceCheck;
+    }
+
     public boolean isAfterMappingMethod() {
         return Executables.isAfterMappingMethod( getExecutable() );
     }

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/CreateOrUpdateSelector.java

@@ -36,7 +36,8 @@ public class CreateOrUpdateSelector implements MethodSelector {
                                                                          Type returnType,
                                                                          SelectionCriteria criteria) {
 
-        if ( criteria.isLifecycleCallbackRequired() || criteria.isObjectFactoryRequired() ) {
+        if ( criteria.isLifecycleCallbackRequired() || criteria.isObjectFactoryRequired()
+            || criteria.isPresenceCheckRequired() ) {
             return methods;
         }
 

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/MethodFamilySelector.java

@@ -30,7 +30,9 @@ public class MethodFamilySelector implements MethodSelector {
         List<SelectedMethod<T>> result = new ArrayList<>( methods.size() );
         for ( SelectedMethod<T> method : methods ) {
             if ( method.getMethod().isObjectFactory() == criteria.isObjectFactoryRequired()
-                && method.getMethod().isLifecycleCallbackMethod() == criteria.isLifecycleCallbackRequired() ) {
+                && method.getMethod().isLifecycleCallbackMethod() == criteria.isLifecycleCallbackRequired()
+                && method.getMethod().isPresenceCheck() == criteria.isPresenceCheckRequired()
+            ) {
 
                 result.add( method );
             }

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/MethodSelectors.java

@@ -35,6 +35,7 @@ public class MethodSelectors {
             new XmlElementDeclSelector( typeUtils ),
             new InheritanceSelector(),
             new CreateOrUpdateSelector(),
+            new SourceRhsSelector(),
             new FactoryParameterSelector() );
     }
 

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/SelectionCriteria.java

@@ -26,20 +26,23 @@ public class SelectionCriteria {
     private final String targetPropertyName;
     private final TypeMirror qualifyingResultType;
     private final SourceRHS sourceRHS;
-    private boolean preferUpdateMapping;
-    private final boolean objectFactoryRequired;
-    private final boolean lifecycleCallbackRequired;
+    private Type type;
     private final boolean allowDirect;
     private final boolean allowConversion;
     private final boolean allowMappingMethod;
     private final boolean allow2Steps;
 
     public SelectionCriteria(SelectionParameters selectionParameters, MappingControl mappingControl,
-                             String targetPropertyName, boolean preferUpdateMapping, boolean objectFactoryRequired,
-                             boolean lifecycleCallbackRequired) {
+                             String targetPropertyName, Type type) {
         if ( selectionParameters != null ) {
+            if ( type == Type.PRESENCE_CHECK ) {
+                qualifiers.addAll( selectionParameters.getConditionQualifiers() );
+                qualifiedByNames.addAll( selectionParameters.getConditionQualifyingNames() );
+            }
+            else {
                 qualifiers.addAll( selectionParameters.getQualifiers() );
                 qualifiedByNames.addAll( selectionParameters.getQualifyingNames() );
+            }
             qualifyingResultType = selectionParameters.getResultType();
             sourceRHS = selectionParameters.getSourceRHS();
         }
@@ -60,23 +63,28 @@ public class SelectionCriteria {
             this.allow2Steps = true;
         }
         this.targetPropertyName = targetPropertyName;
-        this.preferUpdateMapping = preferUpdateMapping;
-        this.objectFactoryRequired = objectFactoryRequired;
-        this.lifecycleCallbackRequired = lifecycleCallbackRequired;
+        this.type = type;
     }
 
     /**
      * @return true if factory methods should be selected, false otherwise.
      */
     public boolean isObjectFactoryRequired() {
-        return objectFactoryRequired;
+        return type == Type.OBJECT_FACTORY;
     }
 
     /**
      * @return true if lifecycle callback methods should be selected, false otherwise.
      */
     public boolean isLifecycleCallbackRequired() {
-        return lifecycleCallbackRequired;
+        return type == Type.LIFECYCLE_CALLBACK;
+    }
+
+    /**
+     * @return {@code true} if presence check methods should be selected, {@code false} otherwise
+     */
+    public boolean isPresenceCheckRequired() {
+        return type == Type.PRESENCE_CHECK;
     }
 
     public List<TypeMirror> getQualifiers() {
@@ -96,7 +104,7 @@ public class SelectionCriteria {
     }
 
     public boolean isPreferUpdateMapping() {
-        return preferUpdateMapping;
+        return type == Type.PREFER_UPDATE_MAPPING;
     }
 
     public SourceRHS getSourceRHS() {
@@ -104,7 +112,7 @@ public class SelectionCriteria {
     }
 
     public void setPreferUpdateMapping(boolean preferUpdateMapping) {
-        this.preferUpdateMapping = preferUpdateMapping;
+        this.type = preferUpdateMapping ? Type.PREFER_UPDATE_MAPPING : null;
     }
 
     public boolean hasQualfiers() {
@@ -135,17 +143,26 @@ public class SelectionCriteria {
             selectionParameters,
             mappingControl,
             targetPropertyName,
-            preferUpdateMapping,
-            false,
-            false
+            preferUpdateMapping ? Type.PREFER_UPDATE_MAPPING : null
         );
     }
 
     public static SelectionCriteria forFactoryMethods(SelectionParameters selectionParameters) {
-        return new SelectionCriteria( selectionParameters, null, null, false, true, false );
+        return new SelectionCriteria( selectionParameters, null, null, Type.OBJECT_FACTORY );
     }
 
     public static SelectionCriteria forLifecycleMethods(SelectionParameters selectionParameters) {
-        return new SelectionCriteria( selectionParameters, null, null, false, false, true );
+        return new SelectionCriteria( selectionParameters, null, null, Type.LIFECYCLE_CALLBACK );
+    }
+
+    public static SelectionCriteria forPresenceCheckMethods(SelectionParameters selectionParameters) {
+        return new SelectionCriteria( selectionParameters, null, null, Type.PRESENCE_CHECK );
+    }
+
+    public enum Type {
+        PREFER_UPDATE_MAPPING,
+        OBJECT_FACTORY,
+        LIFECYCLE_CALLBACK,
+        PRESENCE_CHECK,
     }
 }

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/SourceRhsSelector.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.internal.model.source.selector;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.mapstruct.ap.internal.model.common.ParameterBinding;
+import org.mapstruct.ap.internal.model.common.Type;
+import org.mapstruct.ap.internal.model.source.Method;
+
+/**
+ * Selector that tries to resolve an ambiquity between methods that contain source parameters and
+ * {@link org.mapstruct.ap.internal.model.common.SourceRHS SourceRHS} type parameters.
+ * @author Filip Hrisafov
+ */
+public class SourceRhsSelector implements MethodSelector {
+
+    @Override
+    public <T extends Method> List<SelectedMethod<T>> getMatchingMethods(Method mappingMethod,
+                                                                         List<SelectedMethod<T>> candidates,
+                                                                         List<Type> sourceTypes, Type mappingTargetType,
+                                                                         Type returnType, SelectionCriteria criteria) {
+        if ( candidates.size() < 2 || criteria.getSourceRHS() == null ) {
+            return candidates;
+        }
+
+        List<SelectedMethod<T>> sourceRHSFavoringCandidates = new ArrayList<>();
+
+        for ( SelectedMethod<T> candidate : candidates ) {
+            for ( ParameterBinding parameterBinding : candidate.getParameterBindings() ) {
+                if ( parameterBinding.getSourceRHS() != null ) {
+                    sourceRHSFavoringCandidates.add( candidate );
+                    break;
+                }
+            }
+
+        }
+
+        if ( !sourceRHSFavoringCandidates.isEmpty() ) {
+            return sourceRHSFavoringCandidates;
+        }
+
+        return candidates;
+    }
+}

processor/src/main/java/org/mapstruct/ap/internal/model/source/selector/TypeSelector.java

@@ -89,7 +89,7 @@ public class TypeSelector implements MethodSelector {
         List<ParameterBinding> availableParams = new ArrayList<>( method.getParameters().size() + 3 );
 
         if ( sourceRHS != null ) {
-            availableParams.addAll( ParameterBinding.fromParameters( method.getContextParameters() ) );
+            availableParams.addAll( ParameterBinding.fromParameters( method.getParameters() ) );
             availableParams.add( ParameterBinding.fromSourceRHS( sourceRHS ) );
         }
         else {

processor/src/main/java/org/mapstruct/ap/internal/processor/MethodRetrievalProcessor.java

@@ -23,6 +23,7 @@ import javax.lang.model.type.ExecutableType;
 import javax.lang.model.type.TypeKind;
 
 import org.mapstruct.ap.internal.gem.BeanMappingGem;
+import org.mapstruct.ap.internal.gem.ConditionGem;
 import org.mapstruct.ap.internal.gem.IterableMappingGem;
 import org.mapstruct.ap.internal.gem.MapMappingGem;
 import org.mapstruct.ap.internal.gem.MappingGem;
@@ -225,7 +226,8 @@ public class MethodRetrievalProcessor implements ModelElementProcessor<Void, Lis
         }
         // otherwise add reference to existing mapper method
         else if ( isValidReferencedMethod( parameters ) || isValidFactoryMethod( method, parameters, returnType )
-            || isValidLifecycleCallbackMethod( method ) ) {
+            || isValidLifecycleCallbackMethod( method )
+            || isValidPresenceCheckMethod( method, returnType ) ) {
             return getReferencedMethod( usedMapper, methodType, method, mapperToImplement, parameters );
         }
         else {
@@ -333,7 +335,8 @@ public class MethodRetrievalProcessor implements ModelElementProcessor<Void, Lis
 
             List<SourceMethod> contextProvidedMethods = new ArrayList<>( contextParamMethods.size() );
             for ( SourceMethod sourceMethod : contextParamMethods ) {
-                if ( sourceMethod.isLifecycleCallbackMethod() || sourceMethod.isObjectFactory() ) {
+                if ( sourceMethod.isLifecycleCallbackMethod() || sourceMethod.isObjectFactory()
+                    || sourceMethod.isPresenceCheck() ) {
                     contextProvidedMethods.add( sourceMethod );
                 }
             }
@@ -389,10 +392,22 @@ public class MethodRetrievalProcessor implements ModelElementProcessor<Void, Lis
         return ObjectFactoryGem.instanceOn( method ) != null;
     }
 
+    private boolean isValidPresenceCheckMethod(ExecutableElement method, Type returnType) {
+        return isBoolean( returnType ) && hasConditionAnnotation( method );
+    }
+
+    private boolean hasConditionAnnotation(ExecutableElement method) {
+        return ConditionGem.instanceOn( method ) != null;
+    }
+
     private boolean isVoid(Type returnType) {
         return returnType.getTypeMirror().getKind() == TypeKind.VOID;
     }
 
+    private boolean isBoolean(Type returnType) {
+        return Boolean.class.getCanonicalName().equals( returnType.getBoxedEquivalent().getFullyQualifiedName() );
+    }
+
     private boolean isValidReferencedOrFactoryMethod(int sourceParamCount, int targetParamCount,
                                                      List<Parameter> parameters) {
         int validSourceParameters = 0;

processor/src/main/java/org/mapstruct/ap/internal/util/Message.java

@@ -66,6 +66,7 @@ public enum Message {
     PROPERTYMAPPING_EXPRESSION_AND_QUALIFIER_BOTH_DEFINED("Expression and a qualifier both defined in @Mapping, either define an expression or a qualifier."),
     PROPERTYMAPPING_INVALID_EXPRESSION( "Value for expression must be given in the form \"java(<EXPRESSION>)\"." ),
     PROPERTYMAPPING_INVALID_DEFAULT_EXPRESSION( "Value for default expression must be given in the form \"java(<EXPRESSION>)\"." ),
+    PROPERTYMAPPING_INVALID_CONDITION_EXPRESSION( "Value for condition expression must be given in the form \"java(<EXPRESSION>)\"." ),
     PROPERTYMAPPING_INVALID_PARAMETER_NAME( "Method has no source parameter named \"%s\". Method source parameters are: \"%s\"." ),
     PROPERTYMAPPING_NO_PROPERTY_IN_PARAMETER( "The type of parameter \"%s\" has no property named \"%s\"." ),
     PROPERTYMAPPING_INVALID_PROPERTY_NAME( "No property named \"%s\" exists in source parameter(s). Did you mean \"%s\"?" ),
@@ -120,6 +121,7 @@ public enum Message {
     GENERAL_ABSTRACT_RETURN_TYPE( "The return type %s is an abstract class or interface. Provide a non abstract / non interface result type or a factory method." ),
     GENERAL_AMBIGUOUS_MAPPING_METHOD( "Ambiguous mapping methods found for mapping %s to %s: %s. See " + FAQ_AMBIGUOUS_URL + " for more info." ),
     GENERAL_AMBIGUOUS_FACTORY_METHOD( "Ambiguous factory methods found for creating %s: %s. See " + FAQ_AMBIGUOUS_URL + " for more info." ),
+    GENERAL_AMBIGUOUS_PRESENCE_CHECK_METHOD( "Ambiguous presence check methods found for checking %s: %s. See " + FAQ_AMBIGUOUS_URL + " for more info." ),
     GENERAL_AMBIGUOUS_CONSTRUCTORS( "Ambiguous constructors found for creating %s. Either declare parameterless constructor or annotate the default constructor with an annotation named @Default." ),
     GENERAL_CONSTRUCTOR_PROPERTIES_NOT_MATCHING_PARAMETERS( "Incorrect @ConstructorProperties for %s. The size of the @ConstructorProperties does not match the number of constructor parameters" ),
     GENERAL_UNSUPPORTED_DATE_FORMAT_CHECK( "No dateFormat check is supported for types %s, %s" ),

processor/src/main/resources/org/mapstruct/ap/internal/model/MethodReferencePresenceCheck.ftl

@@ -0,0 +1,9 @@
+<#--
+
+    Copyright MapStruct Authors.
+
+    Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+
+-->
+<#-- @ftlvariable name="" type="org.mapstruct.ap.internal.model.MethodReferencePresenceCheck" -->
+<@includeModel object=methodReference/>

processor/src/main/resources/org/mapstruct/ap/internal/model/presence/JavaExpressionPresenceCheck.ftl

@@ -0,0 +1,9 @@
+<#--
+
+    Copyright MapStruct Authors.
+
+    Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+
+-->
+<#-- @ftlvariable name="" type="org.mapstruct.ap.internal.model.presence.JavaExpressionPresenceCheck" -->
+${javaExpression}

processor/src/test/java/org/mapstruct/ap/test/conditional/Employee.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional;
+
+public class Employee {
+    private String name;
+    private String ssid;
+    private String nin;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getSsid() {
+        return ssid;
+    }
+
+    public void setSsid(String ssid) {
+        this.ssid = ssid;
+    }
+
+    public String getNin() {
+        return nin;
+    }
+
+    public void setNin(String nin) {
+        this.nin = nin;
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/EmployeeDto.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional;
+
+public class EmployeeDto {
+    private String name;
+    private String country;
+    private String uniqueIdNumber;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getCountry() {
+        return country;
+    }
+
+    public void setCountry(String country) {
+        this.country = country;
+    }
+
+    public String getUniqueIdNumber() {
+        return uniqueIdNumber;
+    }
+
+    public void setUniqueIdNumber(String uniqueIdNumber) {
+        this.uniqueIdNumber = uniqueIdNumber;
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/BasicEmployee.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class BasicEmployee {
+
+    private String name;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/BasicEmployeeDto.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class BasicEmployeeDto {
+
+    private final String name;
+    private final String strategy;
+
+    public BasicEmployeeDto(String name) {
+        this( name, "default" );
+    }
+
+    public BasicEmployeeDto(String name, String strategy) {
+        this.name = name;
+        this.strategy = strategy;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getStrategy() {
+        return strategy;
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMappingTest.java

@@ -0,0 +1,229 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import java.util.Arrays;
+import java.util.Collections;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.compilation.annotation.CompilationResult;
+import org.mapstruct.ap.testutil.compilation.annotation.Diagnostic;
+import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutcome;
+import org.mapstruct.ap.testutil.runner.AnnotationProcessorTestRunner;
+import org.mapstruct.ap.testutil.runner.GeneratedSource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Filip Hrisafov
+ */
+@IssueKey("2051")
+@WithClasses({
+    BasicEmployee.class,
+    BasicEmployeeDto.class
+})
+@RunWith(AnnotationProcessorTestRunner.class)
+public class ConditionalMappingTest {
+
+    @Rule
+    public final GeneratedSource generatedSource = new GeneratedSource();
+
+    @Test
+    @WithClasses({
+        ConditionalMethodInMapper.class
+    })
+    public void conditionalMethodInMapper() {
+        generatedSource.addComparisonToFixtureFor( ConditionalMethodInMapper.class );
+        ConditionalMethodInMapper mapper = ConditionalMethodInMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "Tester" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+
+        employee = mapper.map( new BasicEmployeeDto( "" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "    " ) );
+        assertThat( employee.getName() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodAndBeanPresenceCheckMapper.class
+    })
+    public void conditionalMethodAndBeanPresenceCheckMapper() {
+        ConditionalMethodAndBeanPresenceCheckMapper mapper = ConditionalMethodAndBeanPresenceCheckMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new ConditionalMethodAndBeanPresenceCheckMapper.EmployeeDto( "Tester" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+
+        employee = mapper.map( new ConditionalMethodAndBeanPresenceCheckMapper.EmployeeDto( "" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new ConditionalMethodAndBeanPresenceCheckMapper.EmployeeDto( "    " ) );
+        assertThat( employee.getName() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodInUsesMapper.class
+    })
+    public void conditionalMethodInUsesMapper() {
+        ConditionalMethodInUsesMapper mapper = ConditionalMethodInUsesMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "Tester" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+
+        employee = mapper.map( new BasicEmployeeDto( "" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "    " ) );
+        assertThat( employee.getName() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodInUsesStaticMapper.class
+    })
+    public void conditionalMethodInUsesStaticMapper() {
+        ConditionalMethodInUsesStaticMapper mapper = ConditionalMethodInUsesStaticMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "Tester" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+
+        employee = mapper.map( new BasicEmployeeDto( "" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "    " ) );
+        assertThat( employee.getName() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodInContextMapper.class
+    })
+    public void conditionalMethodInUsesContextMapper() {
+        ConditionalMethodInContextMapper mapper = ConditionalMethodInContextMapper.INSTANCE;
+
+        ConditionalMethodInContextMapper.PresenceUtils utils = new ConditionalMethodInContextMapper.PresenceUtils();
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "Tester" ), utils );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+
+        employee = mapper.map( new BasicEmployeeDto( "" ), utils );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "    " ), utils );
+        assertThat( employee.getName() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodWithSourceParameterMapper.class
+    })
+    public void conditionalMethodWithSourceParameter() {
+        ConditionalMethodWithSourceParameterMapper mapper = ConditionalMethodWithSourceParameterMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "Tester" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "Tester", "map" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodWithSourceParameterAndValueMapper.class
+    })
+    public void conditionalMethodWithSourceParameterAndValue() {
+        generatedSource.addComparisonToFixtureFor( ConditionalMethodWithSourceParameterAndValueMapper.class );
+        ConditionalMethodWithSourceParameterAndValueMapper mapper =
+            ConditionalMethodWithSourceParameterAndValueMapper.INSTANCE;
+
+        BasicEmployee employee = mapper.map( new BasicEmployeeDto( "    ", "empty" ) );
+        assertThat( employee.getName() ).isEqualTo( "    " );
+
+        employee = mapper.map( new BasicEmployeeDto( "    ", "blank" ) );
+        assertThat( employee.getName() ).isNull();
+
+        employee = mapper.map( new BasicEmployeeDto( "Tester", "blank" ) );
+        assertThat( employee.getName() ).isEqualTo( "Tester" );
+    }
+
+    @Test
+    @WithClasses({
+        ErroneousAmbiguousConditionalMethodMapper.class
+    })
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(
+                kind = javax.tools.Diagnostic.Kind.ERROR,
+                type = ErroneousAmbiguousConditionalMethodMapper.class,
+                line = 17,
+                message = "Ambiguous presence check methods found for checking String: " +
+                    "boolean isNotBlank(String value), " +
+                    "boolean isNotEmpty(String value). " +
+                    "See https://mapstruct.org/faq/#ambiguous for more info."
+            )
+        }
+    )
+    public void ambiguousConditionalMethod() {
+
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodForCollectionMapper.class
+    })
+    public void conditionalMethodForCollection() {
+        ConditionalMethodForCollectionMapper mapper = ConditionalMethodForCollectionMapper.INSTANCE;
+
+        ConditionalMethodForCollectionMapper.Author author = new ConditionalMethodForCollectionMapper.Author();
+        ConditionalMethodForCollectionMapper.AuthorDto dto = mapper.map( author );
+
+        assertThat( dto.getBooks() ).isNull();
+
+        author.setBooks( Collections.emptyList() );
+        dto = mapper.map( author );
+
+        assertThat( dto.getBooks() ).isNull();
+
+        author.setBooks( Arrays.asList(
+            new ConditionalMethodForCollectionMapper.Book( "Test" ),
+            new ConditionalMethodForCollectionMapper.Book( "Test Vol. 2" )
+        ) );
+        dto = mapper.map( author );
+
+        assertThat( dto.getBooks() )
+            .extracting( ConditionalMethodForCollectionMapper.BookDto::getName )
+            .containsExactly( "Test", "Test Vol. 2" );
+    }
+
+    @Test
+    @WithClasses({
+        OptionalLikeConditionalMapper.class
+    })
+    @IssueKey("2084")
+    public void optionalLikeConditional() {
+        OptionalLikeConditionalMapper mapper = OptionalLikeConditionalMapper.INSTANCE;
+
+        OptionalLikeConditionalMapper.Target target = mapper.map( new OptionalLikeConditionalMapper.Source(
+            OptionalLikeConditionalMapper.Nullable.ofNullable( "test" ) ) );
+
+        assertThat( target.getValue() ).isEqualTo( "test" );
+
+        target = mapper.map(
+            new OptionalLikeConditionalMapper.Source( OptionalLikeConditionalMapper.Nullable.undefined() )
+        );
+
+        assertThat( target.getValue() ).isEqualTo( "initial" );
+
+    }
+
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodAndBeanPresenceCheckMapper.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodAndBeanPresenceCheckMapper {
+
+    ConditionalMethodAndBeanPresenceCheckMapper INSTANCE = Mappers.getMapper(
+        ConditionalMethodAndBeanPresenceCheckMapper.class );
+
+    BasicEmployee map(EmployeeDto employee);
+
+    @Condition
+    default boolean isNotBlank(String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+
+    class EmployeeDto {
+
+        private final String name;
+
+        public EmployeeDto(String name) {
+            this.name = name;
+        }
+
+        public boolean hasName() {
+            return false;
+        }
+
+        public String getName() {
+            return name;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodForCollectionMapper.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import java.util.Collection;
+import java.util.List;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodForCollectionMapper {
+
+    ConditionalMethodForCollectionMapper INSTANCE = Mappers.getMapper( ConditionalMethodForCollectionMapper.class );
+
+    AuthorDto map(Author author);
+
+    @Condition
+    default <T> boolean isNotEmpty(Collection<T> collection) {
+        return collection != null && !collection.isEmpty();
+    }
+
+    class Author {
+        private List<Book> books;
+
+        public List<Book> getBooks() {
+            return books;
+        }
+
+        public boolean hasBooks() {
+            return false;
+        }
+
+        public void setBooks(List<Book> books) {
+            this.books = books;
+        }
+    }
+
+    class Book {
+        private final String name;
+
+        public Book(String name) {
+            this.name = name;
+        }
+
+        public String getName() {
+            return name;
+        }
+    }
+
+    class AuthorDto {
+        private List<BookDto> books;
+
+        public List<BookDto> getBooks() {
+            return books;
+        }
+
+        public void setBooks(List<BookDto> books) {
+            this.books = books;
+        }
+    }
+
+    class BookDto {
+        private String name;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodInContextMapper.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Context;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodInContextMapper {
+
+    ConditionalMethodInContextMapper INSTANCE = Mappers.getMapper( ConditionalMethodInContextMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee, @Context PresenceUtils utils);
+
+    class PresenceUtils {
+        @Condition
+        public boolean isNotBlank(String value) {
+            return value != null && !value.trim().isEmpty();
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodInMapper.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodInMapper {
+
+    ConditionalMethodInMapper INSTANCE = Mappers.getMapper( ConditionalMethodInMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    @Condition
+    default boolean isNotBlank(String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodInUsesMapper.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper(uses = ConditionalMethodInUsesMapper.PresenceUtils.class)
+public interface ConditionalMethodInUsesMapper {
+
+    ConditionalMethodInUsesMapper INSTANCE = Mappers.getMapper( ConditionalMethodInUsesMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    class PresenceUtils {
+
+        @Condition
+        public boolean isNotBlank(String value) {
+            return value != null && !value.trim().isEmpty();
+        }
+
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodInUsesStaticMapper.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper(uses = ConditionalMethodInUsesStaticMapper.PresenceUtils.class)
+public interface ConditionalMethodInUsesStaticMapper {
+
+    ConditionalMethodInUsesStaticMapper INSTANCE = Mappers.getMapper( ConditionalMethodInUsesStaticMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    interface PresenceUtils {
+
+        @Condition
+        static boolean isNotBlank(String value) {
+            return value != null && !value.trim().isEmpty();
+        }
+
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodWithSourceParameterAndValueMapper.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodWithSourceParameterAndValueMapper {
+
+    ConditionalMethodWithSourceParameterAndValueMapper INSTANCE = Mappers.getMapper(
+        ConditionalMethodWithSourceParameterAndValueMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    @Condition
+    default boolean isPresent(BasicEmployeeDto source, String value) {
+        if ( value == null ) {
+            return false;
+        }
+        switch ( source.getStrategy() ) {
+            case "blank":
+                return !value.trim().isEmpty();
+            case "empty":
+                return !value.isEmpty();
+            default:
+                return true;
+        }
+    }
+
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ConditionalMethodWithSourceParameterMapper.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ConditionalMethodWithSourceParameterMapper {
+
+    ConditionalMethodWithSourceParameterMapper INSTANCE =
+        Mappers.getMapper( ConditionalMethodWithSourceParameterMapper.class );
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    @Condition
+    default boolean shouldMap(BasicEmployeeDto source) {
+        return "map".equals( source.getStrategy() );
+    }
+
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/ErroneousAmbiguousConditionalMethodMapper.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ErroneousAmbiguousConditionalMethodMapper {
+
+    BasicEmployee map(BasicEmployeeDto employee);
+
+    @Condition
+    default boolean isNotBlank(String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+
+    @Condition
+    default boolean isNotEmpty(String value) {
+        return value != null && value.isEmpty();
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/basic/OptionalLikeConditionalMapper.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface OptionalLikeConditionalMapper {
+
+    OptionalLikeConditionalMapper INSTANCE = Mappers.getMapper( OptionalLikeConditionalMapper.class );
+
+    Target map(Source source);
+
+    default <T> T map(Nullable<T> nullable) {
+        return nullable.value;
+    }
+
+    @Condition
+    default <T> boolean isPresent(Nullable<T> nullable) {
+        return nullable.isPresent();
+    }
+
+    class Nullable<T> {
+
+        private final T value;
+        private final boolean present;
+
+        private Nullable(T value, boolean present) {
+            this.value = value;
+            this.present = present;
+        }
+
+        public boolean isPresent() {
+            return present;
+        }
+
+        public static <T> Nullable<T> undefined() {
+            return new Nullable<>( null, false );
+        }
+
+        public static <T> Nullable<T> ofNullable(T value) {
+            return new Nullable<>( value, true );
+        }
+    }
+
+    class Source {
+        protected final Nullable<String> value;
+
+        public Source(Nullable<String> value) {
+            this.value = value;
+        }
+
+        public Nullable<String> getValue() {
+            return value;
+        }
+    }
+
+    class Target {
+        protected String value = "initial";
+
+        public String getValue() {
+            return value;
+        }
+
+        public void setValue(String value) {
+            this.value = value;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/expression/ConditionalExpressionTest.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.expression;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.ap.test.conditional.basic.BasicEmployee;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.compilation.annotation.CompilationResult;
+import org.mapstruct.ap.testutil.compilation.annotation.Diagnostic;
+import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutcome;
+import org.mapstruct.ap.testutil.runner.AnnotationProcessorTestRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Filip Hrisafov
+ */
+@IssueKey("2051")
+@WithClasses({
+    Employee.class,
+    EmployeeDto.class
+})
+@RunWith(AnnotationProcessorTestRunner.class)
+public class ConditionalExpressionTest {
+
+    @Test
+    @WithClasses({
+        ConditionalMethodsInUtilClassMapper.class
+    })
+    public void conditionalExpressionInStaticClassMethod() {
+        ConditionalMethodsInUtilClassMapper mapper = ConditionalMethodsInUtilClassMapper.INSTANCE;
+
+        EmployeeDto dto = new EmployeeDto();
+        dto.setName( "Tester" );
+        dto.setUniqueIdNumber( "SSID-001" );
+        dto.setCountry( null );
+
+        Employee employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "UK" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isEqualTo( "SSID-001" );
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "US" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isEqualTo( "SSID-001" );
+
+        dto.setCountry( "CH" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        SimpleConditionalExpressionMapper.class
+    })
+    public void conditionalSimpleExpression() {
+        SimpleConditionalExpressionMapper mapper = SimpleConditionalExpressionMapper.INSTANCE;
+
+        SimpleConditionalExpressionMapper.Target target =
+            mapper.map( new SimpleConditionalExpressionMapper.Source( 50 ) );
+        assertThat( target.getValue() ).isEqualTo( 50 );
+
+        target = mapper.map( new SimpleConditionalExpressionMapper.Source( 101 ) );
+        assertThat( target.getValue() ).isEqualTo( 0 );
+    }
+
+    @Test
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(type = ErroneousConditionExpressionMapper.class,
+                kind = javax.tools.Diagnostic.Kind.ERROR,
+                line = 19,
+                message = "Value for condition expression must be given in the form \"java(<EXPRESSION>)\"."
+            )
+        }
+    )
+    @WithClasses({
+        BasicEmployee.class,
+        ErroneousConditionExpressionMapper.class
+    } )
+    public void invalidJavaConditionExpression() {
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/expression/ConditionalMethodsInUtilClassMapper.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.expression;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper(imports = ConditionalMethodsInUtilClassMapper.StaticUtil.class)
+public interface ConditionalMethodsInUtilClassMapper {
+
+    ConditionalMethodsInUtilClassMapper INSTANCE = Mappers.getMapper( ConditionalMethodsInUtilClassMapper.class );
+
+    @Mapping(target = "ssid", source = "uniqueIdNumber",
+        conditionExpression = "java(StaticUtil.isAmericanCitizen( employee ))")
+    @Mapping(target = "nin", source = "uniqueIdNumber",
+        conditionExpression = "java(StaticUtil.isBritishCitizen( employee ))")
+    Employee map(EmployeeDto employee);
+
+    interface StaticUtil {
+
+        static boolean isAmericanCitizen(EmployeeDto employeeDto) {
+            return "US".equals( employeeDto.getCountry() );
+        }
+
+        static boolean isBritishCitizen(EmployeeDto employeeDto) {
+            return "UK".equals( employeeDto.getCountry() );
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/expression/ErroneousConditionExpressionMapper.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.expression;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.ap.test.conditional.basic.BasicEmployee;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ErroneousConditionExpressionMapper {
+
+    @Mapping(target = "name", conditionExpression = "!employee.getName().isEmpty()")
+    BasicEmployee map(EmployeeDto employee);
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/expression/SimpleConditionalExpressionMapper.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.expression;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface SimpleConditionalExpressionMapper {
+
+    SimpleConditionalExpressionMapper INSTANCE = Mappers.getMapper( SimpleConditionalExpressionMapper.class );
+
+    @Mapping(target = "value", conditionExpression = "java(source.getValue() < 100)")
+    Target map(Source source);
+
+    class Source {
+        private final int value;
+
+        public Source(int value) {
+            this.value = value;
+        }
+
+        public int getValue() {
+            return value;
+        }
+    }
+
+    class Target {
+        private int value;
+
+        public int getValue() {
+            return value;
+        }
+
+        public void setValue(int value) {
+            this.value = value;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/qualifier/ConditionalMethodWithClassQualifiersMapper.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.qualifier;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.Named;
+import org.mapstruct.Qualifier;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper(uses = ConditionalMethodWithClassQualifiersMapper.StaticUtil.class)
+public interface ConditionalMethodWithClassQualifiersMapper {
+
+    ConditionalMethodWithClassQualifiersMapper INSTANCE =
+        Mappers.getMapper( ConditionalMethodWithClassQualifiersMapper.class );
+
+    @Mapping(target = "ssid", source = "uniqueIdNumber",
+        conditionQualifiedBy = UtilConditions.class, conditionQualifiedByName = "american")
+    @Mapping(target = "nin", source = "uniqueIdNumber",
+        conditionQualifiedBy = UtilConditions.class, conditionQualifiedByName = "british")
+    Employee map(EmployeeDto employee);
+
+    @Condition
+    default boolean isNotBlank(String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+
+    @UtilConditions
+    interface StaticUtil {
+
+        @Condition
+        @Named("american")
+        static boolean isAmericanCitizen(EmployeeDto employerDto) {
+            return "US".equals( employerDto.getCountry() );
+        }
+
+        @Condition
+        @Named("british")
+        static boolean isBritishCitizen(EmployeeDto employeeDto) {
+            return "UK".equals( employeeDto.getCountry() );
+        }
+    }
+
+    @Qualifier
+    @Target(ElementType.TYPE)
+    @Retention(RetentionPolicy.CLASS)
+    @interface UtilConditions {
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/qualifier/ConditionalMethodWithSourceParameterMapper.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.qualifier;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.Named;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper(uses = ConditionalMethodWithSourceParameterMapper.StaticUtil.class)
+public interface ConditionalMethodWithSourceParameterMapper {
+
+    ConditionalMethodWithSourceParameterMapper INSTANCE =
+        Mappers.getMapper( ConditionalMethodWithSourceParameterMapper.class );
+
+    @Mapping(target = "ssid", source = "uniqueIdNumber", conditionQualifiedByName = "isAmericanCitizen")
+    @Mapping(target = "nin", source = "uniqueIdNumber", conditionQualifiedByName = "isBritishCitizen")
+    Employee map(EmployeeDto employee);
+
+    @Condition
+    default boolean isNotBlank(String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+
+    @Condition
+    @Named("isAmericanCitizen")
+    default boolean isAmericanCitizen(EmployeeDto employerDto) {
+        return "US".equals( employerDto.getCountry() );
+    }
+
+    interface StaticUtil {
+
+        @Condition
+        @Named("isBritishCitizen")
+        static boolean isBritishCitizen(EmployeeDto employeeDto) {
+            return "UK".equals( employeeDto.getCountry() );
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/qualifier/ConditionalQualifierTest.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.qualifier;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.runner.AnnotationProcessorTestRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Filip Hrisafov
+ */
+@IssueKey("2051")
+@WithClasses({
+    Employee.class,
+    EmployeeDto.class
+})
+@RunWith(AnnotationProcessorTestRunner.class)
+public class ConditionalQualifierTest {
+
+    @Test
+    @WithClasses({
+        ConditionalMethodWithSourceParameterMapper.class
+    })
+    public void conditionalMethodWithSourceParameter() {
+        ConditionalMethodWithSourceParameterMapper mapper = ConditionalMethodWithSourceParameterMapper.INSTANCE;
+
+        EmployeeDto dto = new EmployeeDto();
+        dto.setName( "Tester" );
+        dto.setUniqueIdNumber( "SSID-001" );
+        dto.setCountry( null );
+
+        Employee employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "UK" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isEqualTo( "SSID-001" );
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "US" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isEqualTo( "SSID-001" );
+
+        dto.setCountry( "CH" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+    }
+
+    @Test
+    @WithClasses({
+        ConditionalMethodWithClassQualifiersMapper.class
+    })
+    public void conditionalClassQualifiers() {
+        ConditionalMethodWithClassQualifiersMapper mapper = ConditionalMethodWithClassQualifiersMapper.INSTANCE;
+
+        EmployeeDto dto = new EmployeeDto();
+        dto.setName( "Tester" );
+        dto.setUniqueIdNumber( "SSID-001" );
+        dto.setCountry( null );
+
+        Employee employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "UK" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isEqualTo( "SSID-001" );
+        assertThat( employee.getSsid() ).isNull();
+
+        dto.setCountry( "US" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isEqualTo( "SSID-001" );
+
+        dto.setCountry( "CH" );
+        employee = mapper.map( dto );
+        assertThat( employee.getNin() ).isNull();
+        assertThat( employee.getSsid() ).isNull();
+    }
+}

processor/src/test/resources/fixtures/org/mapstruct/ap/test/conditional/basic/ConditionalMethodInMapperImpl.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import javax.annotation.processing.Generated;
+
+@Generated(
+    value = "org.mapstruct.ap.MappingProcessor",
+    date = "2021-04-19T21:10:40+0200",
+    comments = "version: , compiler: javac, environment: Java 11.0.9.1 (AdoptOpenJDK)"
+)
+public class ConditionalMethodInMapperImpl implements ConditionalMethodInMapper {
+
+    @Override
+    public BasicEmployee map(BasicEmployeeDto employee) {
+        if ( employee == null ) {
+            return null;
+        }
+
+        BasicEmployee basicEmployee = new BasicEmployee();
+
+        if ( isNotBlank( employee.getName() ) ) {
+            basicEmployee.setName( employee.getName() );
+        }
+
+        return basicEmployee;
+    }
+}

processor/src/test/resources/fixtures/org/mapstruct/ap/test/conditional/basic/ConditionalMethodWithSourceParameterAndValueMapperImpl.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.basic;
+
+import javax.annotation.processing.Generated;
+
+@Generated(
+    value = "org.mapstruct.ap.MappingProcessor",
+    date = "2021-04-19T21:10:38+0200",
+    comments = "version: , compiler: javac, environment: Java 11.0.9.1 (AdoptOpenJDK)"
+)
+public class ConditionalMethodWithSourceParameterAndValueMapperImpl implements ConditionalMethodWithSourceParameterAndValueMapper {
+
+    @Override
+    public BasicEmployee map(BasicEmployeeDto employee) {
+        if ( employee == null ) {
+            return null;
+        }
+
+        BasicEmployee basicEmployee = new BasicEmployee();
+
+        if ( isPresent( employee, employee.getName() ) ) {
+            basicEmployee.setName( employee.getName() );
+        }
+
+        return basicEmployee;
+    }
+}