From c4135e68ed8330fbc5180abd828e35b30f606ece Mon Sep 17 00:00:00 2001
From: "jude.niroshan11@gmail.com" <jude.niroshan11@gmail.com>
Date: Fri, 19 Feb 2021 23:06:42 +0100
Subject: [PATCH] #2339 Support throwing an exception as an Enum Mapping option

---
 .../java/org/mapstruct/MappingConstants.java  |  8 ++
 .../main/java/org/mapstruct/ValueMapping.java | 27 +++++-
 .../ap/internal/gem/MappingConstantsGem.java  |  2 +
 .../ap/internal/model/ValueMappingMethod.java | 82 +++++++++++++---
 .../model/source/ValueMappingOptions.java     |  3 +-
 .../mapstruct/ap/internal/util/Message.java   |  3 +-
 .../ap/internal/model/ValueMappingMethod.ftl  | 10 +-
 .../DefaultOrderThrowExceptionMapper.java     | 26 ++++++
 .../enum2enum/EnumToEnumMappingTest.java      | 30 +++---
 .../EnumToEnumThrowExceptionMappingTest.java  | 93 +++++++++++++++++++
 ...OrderMapperThrowExceptionAsSourceType.java | 32 +++++++
 .../enum2enum/OrderThrowExceptionMapper.java  | 35 +++++++
 .../value/enum2enum/SpecialOrderMapper.java   |  8 +-
 .../SpecialThrowExceptionMapper.java          | 47 ++++++++++
 .../enum2enum/SpecialOrderMapperImpl.java     |  4 +-
 15 files changed, 368 insertions(+), 42 deletions(-)
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/DefaultOrderThrowExceptionMapper.java
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumThrowExceptionMappingTest.java
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/ErroneousOrderMapperThrowExceptionAsSourceType.java
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/OrderThrowExceptionMapper.java
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialThrowExceptionMapper.java

diff --git a/core/src/main/java/org/mapstruct/MappingConstants.java b/core/src/main/java/org/mapstruct/MappingConstants.java
index 6e30f08c2..53528f559 100644
--- a/core/src/main/java/org/mapstruct/MappingConstants.java
+++ b/core/src/main/java/org/mapstruct/MappingConstants.java
@@ -36,6 +36,14 @@ public final class MappingConstants {
      */
     public static final String ANY_UNMAPPED = "<ANY_UNMAPPED>";
 
+    /**
+     * In an {@link ValueMapping} this represents any target that will be mapped to an
+     * {@link java.lang.IllegalArgumentException} which will be thrown at runtime.
+     * <p>
+     * NOTE: The value is only applicable to {@link ValueMapping#target()} and not to {@link ValueMapping#source()}.
+     */
+    public static final String THROW_EXCEPTION = "<THROW_EXCEPTION>";
+
     /**
      * In an {@link EnumMapping} this represent the enum transformation strategy that adds a suffix to the source enum.
      *
diff --git a/core/src/main/java/org/mapstruct/ValueMapping.java b/core/src/main/java/org/mapstruct/ValueMapping.java
index f8f2d3b18..2d4a58636 100644
--- a/core/src/main/java/org/mapstruct/ValueMapping.java
+++ b/core/src/main/java/org/mapstruct/ValueMapping.java
@@ -23,7 +23,8 @@ import java.lang.annotation.Target;
  *
  * <pre>
  * <code>
- * public enum OrderType { RETAIL, B2B, EXTRA, STANDARD, NORMAL }
+ *
+ * public enum OrderType { RETAIL, B2B, C2C, EXTRA, STANDARD, NORMAL }
  *
  * public enum ExternalOrderType { RETAIL, B2B, SPECIAL, DEFAULT }
  *
@@ -45,13 +46,12 @@ import java.lang.annotation.Target;
  * +---------------------+----------------------------+
  * </pre>
  *
- * MapStruct will <B>WARN</B> on incomplete mappings. However, if for some reason no match is found an
- * {@link java.lang.IllegalStateException} will be thrown.
  * <p>
  * <B>Example 2:</B>
  *
  * <pre>
  * <code>
+ *
  * &#64;ValueMapping( source = MappingConstants.NULL, target = "DEFAULT" ),
  * &#64;ValueMapping( source = "STANDARD", target = MappingConstants.NULL ),
  * &#64;ValueMapping( source = MappingConstants.ANY_REMAINING, target = "SPECIAL" )
@@ -70,6 +70,26 @@ import java.lang.annotation.Target;
  * +---------------------+----------------------------+
  * </pre>
  *
+ * <p>
+ * <B>Example 3:</B>
+ * </p>
+ *
+ * <p></p>MapStruct will <B>WARN</B> on incomplete mappings. However, if for some reason no match is found, an
+ * {@link java.lang.IllegalStateException} will be thrown. This compile-time error can be avoided by
+ * using {@link MappingConstants#THROW_EXCEPTION} for {@link ValueMapping#target()}. It will result an
+ * {@link java.lang.IllegalArgumentException} at runtime.
+ * <pre>
+ * <code>
+ *
+ * &#64;ValueMapping( source = "STANDARD", target = "DEFAULT" ),
+ * &#64;ValueMapping( source = "C2C", target = MappingConstants.THROW_EXCEPTION )
+ * ExternalOrderType orderTypeToExternalOrderType(OrderType orderType);
+ * </code>
+ * Mapping result:
+ * {@link java.lang.IllegalArgumentException} with the error message:
+ * Unexpected enum constant: C2C
+ * </pre>
+ *
  * @author Sjaak Derksen
  */
 @Repeatable(ValueMappings.class)
@@ -104,6 +124,7 @@ public @interface ValueMapping {
      * <ol>
      * <li>enum constant name</li>
      * <li>{@link MappingConstants#NULL}</li>
+     * <li>{@link MappingConstants#THROW_EXCEPTION}</li>
      * </ol>
      *
      * @return The target value.
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/gem/MappingConstantsGem.java b/processor/src/main/java/org/mapstruct/ap/internal/gem/MappingConstantsGem.java
index 5df7fd3f9..be8f23b76 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/gem/MappingConstantsGem.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/gem/MappingConstantsGem.java
@@ -21,6 +21,8 @@ public final class MappingConstantsGem {
 
     public static final String ANY_UNMAPPED = "<ANY_UNMAPPED>";
 
+    public static final String THROW_EXCEPTION = "<THROW_EXCEPTION>";
+
     public static final String SUFFIX_TRANSFORMATION = "suffix";
 
     public static final String STRIP_SUFFIX_TRANSFORMATION = "stripSuffix";
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/model/ValueMappingMethod.java b/processor/src/main/java/org/mapstruct/ap/internal/model/ValueMappingMethod.java
index 627a4b728..e292d38e3 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/model/ValueMappingMethod.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/model/ValueMappingMethod.java
@@ -14,7 +14,6 @@ import java.util.Map;
 import java.util.Set;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.type.TypeMirror;
-import org.mapstruct.ap.internal.util.TypeUtils;
 
 import org.mapstruct.ap.internal.gem.BeanMappingGem;
 import org.mapstruct.ap.internal.model.common.Parameter;
@@ -25,11 +24,13 @@ import org.mapstruct.ap.internal.model.source.SelectionParameters;
 import org.mapstruct.ap.internal.model.source.ValueMappingOptions;
 import org.mapstruct.ap.internal.util.Message;
 import org.mapstruct.ap.internal.util.Strings;
+import org.mapstruct.ap.internal.util.TypeUtils;
 import org.mapstruct.ap.spi.EnumTransformationStrategy;
 
 import static org.mapstruct.ap.internal.gem.MappingConstantsGem.ANY_REMAINING;
 import static org.mapstruct.ap.internal.gem.MappingConstantsGem.ANY_UNMAPPED;
 import static org.mapstruct.ap.internal.gem.MappingConstantsGem.NULL;
+import static org.mapstruct.ap.internal.gem.MappingConstantsGem.THROW_EXCEPTION;
 import static org.mapstruct.ap.internal.util.Collections.first;
 
 /**
@@ -43,6 +44,8 @@ public class ValueMappingMethod extends MappingMethod {
     private final List<MappingEntry> valueMappings;
     private final String defaultTarget;
     private final String nullTarget;
+    private boolean nullAsException;
+    private boolean defaultAsException;
 
     private final Type unexpectedValueMappingException;
 
@@ -119,10 +122,12 @@ public class ValueMappingMethod extends MappingMethod {
             return new ValueMappingMethod( method,
                 mappingEntries,
                 valueMappings.nullValueTarget,
+                valueMappings.hasNullValueAsException,
                 valueMappings.defaultTargetValue,
                 determineUnexpectedValueMappingException(),
                 beforeMappingMethods,
-                afterMappingMethods
+                afterMappingMethods,
+                determineExceptionMappingForDefaultCase()
             );
         }
 
@@ -313,7 +318,17 @@ public class ValueMappingMethod extends MappingMethod {
 
             for ( ValueMappingOptions mappedConstant : valueMappings.regularValueMappings ) {
 
-                if ( !sourceEnumConstants.contains( mappedConstant.getSource() ) ) {
+                if ( !enumMapping.isInverse() && THROW_EXCEPTION.equals( mappedConstant.getSource() ) ) {
+                    ctx.getMessager().printMessage(
+                        method.getExecutable(),
+                        mappedConstant.getMirror(),
+                        mappedConstant.getSourceAnnotationValue(),
+                        Message.VALUEMAPPING_THROW_EXCEPTION_SOURCE
+                    );
+                    foundIncorrectMapping = true;
+                }
+                else if ( !sourceEnumConstants.contains( mappedConstant.getSource() ) ) {
+
                     ctx.getMessager().printMessage(
                         method.getExecutable(),
                         mappedConstant.getMirror(),
@@ -361,6 +376,7 @@ public class ValueMappingMethod extends MappingMethod {
 
             for ( ValueMappingOptions mappedConstant : valueMappings.regularValueMappings ) {
                 if ( !NULL.equals( mappedConstant.getTarget() )
+                    && !THROW_EXCEPTION.equals( mappedConstant.getTarget() )
                     && !targetEnumConstants.contains( mappedConstant.getTarget() ) ) {
                     ctx.getMessager().printMessage(
                         method.getExecutable(),
@@ -374,7 +390,9 @@ public class ValueMappingMethod extends MappingMethod {
                 }
             }
 
-            if ( valueMappings.defaultTarget != null && !NULL.equals( valueMappings.defaultTarget.getTarget() )
+            if ( valueMappings.defaultTarget != null
+                && !THROW_EXCEPTION.equals( valueMappings.defaultTarget.getTarget() )
+                && !NULL.equals( valueMappings.defaultTarget.getTarget() )
                 && !targetEnumConstants.contains( valueMappings.defaultTarget.getTarget() ) ) {
                 ctx.getMessager().printMessage(
                     method.getExecutable(),
@@ -415,7 +433,9 @@ public class ValueMappingMethod extends MappingMethod {
         }
 
         private Type determineUnexpectedValueMappingException() {
-            if ( !valueMappings.hasDefaultValue ) {
+            boolean noDefaultValueForSwitchCase = !valueMappings.hasDefaultValue;
+            if ( noDefaultValueForSwitchCase || valueMappings.hasAtLeastOneExceptionValue
+                || valueMappings.hasNullValueAsException ) {
                 TypeMirror unexpectedValueMappingException = enumMapping.getUnexpectedValueMappingException();
                 if ( unexpectedValueMappingException != null ) {
                     return ctx.getTypeFactory().getType( unexpectedValueMappingException );
@@ -427,6 +447,15 @@ public class ValueMappingMethod extends MappingMethod {
 
             return null;
         }
+
+        private boolean determineExceptionMappingForDefaultCase() {
+            if ( valueMappings.hasDefaultValue ) {
+                return THROW_EXCEPTION.equals( valueMappings.defaultTargetValue );
+            }
+            else {
+                return true;
+            }
+        }
     }
 
     private static class EnumTransformationStrategyInvoker {
@@ -464,7 +493,8 @@ public class ValueMappingMethod extends MappingMethod {
         boolean hasMapAnyUnmapped = false;
         boolean hasMapAnyRemaining = false;
         boolean hasDefaultValue = false;
-        boolean hasNullValue = false;
+        boolean hasNullValueAsException = false;
+        boolean hasAtLeastOneExceptionValue = false;
 
         ValueMappings(List<ValueMappingOptions> valueMappings) {
 
@@ -484,11 +514,17 @@ public class ValueMappingMethod extends MappingMethod {
                 else if ( NULL.equals( valueMapping.getSource() ) ) {
                     nullTarget = valueMapping;
                     nullValueTarget = getValue( nullTarget );
-                    hasNullValue = true;
+                    if ( THROW_EXCEPTION.equals( nullValueTarget ) ) {
+                        hasNullValueAsException = true;
+                    }
                 }
                 else {
                     regularValueMappings.add( valueMapping );
                 }
+
+                if ( THROW_EXCEPTION.equals( valueMapping.getTarget() ) ) {
+                    hasAtLeastOneExceptionValue = true;
+                }
             }
         }
 
@@ -497,14 +533,20 @@ public class ValueMappingMethod extends MappingMethod {
         }
     }
 
-    private ValueMappingMethod(Method method, List<MappingEntry> enumMappings, String nullTarget, String defaultTarget,
-        Type unexpectedValueMappingException,
-        List<LifecycleCallbackMethodReference> beforeMappingMethods,
-        List<LifecycleCallbackMethodReference> afterMappingMethods) {
+    private ValueMappingMethod(Method method,
+                               List<MappingEntry> enumMappings,
+                               String nullTarget,
+                               boolean hasNullTargetAsException,
+                               String defaultTarget,
+                               Type unexpectedValueMappingException,
+                               List<LifecycleCallbackMethodReference> beforeMappingMethods,
+                               List<LifecycleCallbackMethodReference> afterMappingMethods, boolean defaultAsException) {
         super( method, beforeMappingMethods, afterMappingMethods );
         this.valueMappings = enumMappings;
         this.nullTarget = nullTarget;
+        this.nullAsException = hasNullTargetAsException;
         this.defaultTarget = defaultTarget;
+        this.defaultAsException = defaultAsException;
         this.unexpectedValueMappingException = unexpectedValueMappingException;
         this.overridden = method.overridesMethod();
     }
@@ -532,10 +574,18 @@ public class ValueMappingMethod extends MappingMethod {
         return nullTarget;
     }
 
+    public boolean isNullAsException() {
+        return nullAsException;
+    }
+
     public Type getUnexpectedValueMappingException() {
         return unexpectedValueMappingException;
     }
 
+    public boolean isDefaultAsException() {
+        return defaultAsException;
+    }
+
     public Parameter getSourceParameter() {
         return first( getSourceParameters() );
     }
@@ -547,17 +597,25 @@ public class ValueMappingMethod extends MappingMethod {
     public static class MappingEntry {
         private final String source;
         private final String target;
+        private boolean targetAsException = false;
 
-        MappingEntry( String source, String target ) {
+        MappingEntry(String source, String target) {
             this.source = source;
             if ( !NULL.equals( target ) ) {
                 this.target = target;
+                if ( THROW_EXCEPTION.equals( target ) ) {
+                    this.targetAsException = true;
+                }
             }
             else {
                 this.target = null;
             }
         }
 
+        public boolean isTargetAsException() {
+            return targetAsException;
+        }
+
         public String getSource() {
             return source;
         }
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/model/source/ValueMappingOptions.java b/processor/src/main/java/org/mapstruct/ap/internal/model/source/ValueMappingOptions.java
index 10aef4bfa..963683b5c 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/model/source/ValueMappingOptions.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/model/source/ValueMappingOptions.java
@@ -18,6 +18,7 @@ import org.mapstruct.ap.internal.util.Message;
 
 import static org.mapstruct.ap.internal.gem.MappingConstantsGem.ANY_REMAINING;
 import static org.mapstruct.ap.internal.gem.MappingConstantsGem.ANY_UNMAPPED;
+import static org.mapstruct.ap.internal.gem.MappingConstantsGem.THROW_EXCEPTION;
 
 /**
  * Represents the mapping between one value constant and another.
@@ -112,7 +113,7 @@ public class ValueMappingOptions {
 
     public ValueMappingOptions inverse() {
         ValueMappingOptions result;
-        if ( !(ANY_REMAINING.equals( source ) || ANY_UNMAPPED.equals( source ) ) ) {
+        if ( !(ANY_REMAINING.equals( source ) || ANY_UNMAPPED.equals( source ) || THROW_EXCEPTION.equals( target ) ) ) {
             result = new ValueMappingOptions(
                 target,
                 source,
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java b/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
index 378395d4d..e30407ea2 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
@@ -175,7 +175,8 @@ public enum Message {
     VALUEMAPPING_ANY_REMAINING_FOR_NON_ENUM( "Source = \"<ANY_REMAINING>\" can only be used on targets of type enum and not for %s." ),
     VALUEMAPPING_ANY_REMAINING_OR_UNMAPPED_MISSING( "Source = \"<ANY_REMAINING>\" or \"<ANY_UNMAPPED>\" is advisable for mapping of type String to an enum type.", Diagnostic.Kind.WARNING  ),
     VALUEMAPPING_NON_EXISTING_CONSTANT_FROM_SPI( "Constant %s doesn't exist in enum type %s. Constant was returned from EnumMappingStrategy: %s"),
-    VALUEMAPPING_NON_EXISTING_CONSTANT( "Constant %s doesn't exist in enum type %s." );
+    VALUEMAPPING_NON_EXISTING_CONSTANT( "Constant %s doesn't exist in enum type %s." ),
+    VALUEMAPPING_THROW_EXCEPTION_SOURCE( "Source = \"<THROW_EXCEPTION>\" is not allowed. Target = \"<THROW_EXCEPTION>\" can only be used." );
     // CHECKSTYLE:ON
 
 
diff --git a/processor/src/main/resources/org/mapstruct/ap/internal/model/ValueMappingMethod.ftl b/processor/src/main/resources/org/mapstruct/ap/internal/model/ValueMappingMethod.ftl
index 530d5643e..2844b43e3 100644
--- a/processor/src/main/resources/org/mapstruct/ap/internal/model/ValueMappingMethod.ftl
+++ b/processor/src/main/resources/org/mapstruct/ap/internal/model/ValueMappingMethod.ftl
@@ -15,17 +15,17 @@
         </#if>
     </#list>
     if ( ${sourceParameter.name} == null ) {
-        return <@writeTarget target=nullTarget/>;
+        <#if nullAsException >throw new <@includeModel object=unexpectedValueMappingException />( "Unexpected enum constant: " + ${sourceParameter.name} );<#else>return <@writeTarget target=nullTarget/>;</#if>
     }
 
     <@includeModel object=resultType/> ${resultName};
 
     switch ( ${sourceParameter.name} ) {
     <#list valueMappings as valueMapping>
-        case <@writeSource source=valueMapping.source/>: ${resultName} = <@writeTarget target=valueMapping.target/>;
-        break;
+        case <@writeSource source=valueMapping.source/>: <#if valueMapping.targetAsException >throw new <@includeModel object=unexpectedValueMappingException />( "Unexpected enum constant: " + ${sourceParameter.name} );<#else>${resultName} = <@writeTarget target=valueMapping.target/>;
+        break;</#if>
     </#list>
-    default: <#if unexpectedValueMappingException??>throw new <@includeModel object=unexpectedValueMappingException />( "Unexpected enum constant: " + ${sourceParameter.name} )<#else>${resultName} = <@writeTarget target=defaultTarget/></#if>;
+    default: <#if defaultAsException >throw new <@includeModel object=unexpectedValueMappingException />( "Unexpected enum constant: " + ${sourceParameter.name} )<#else>${resultName} = <@writeTarget target=defaultTarget/></#if>;
     }
     <#list beforeMappingReferencesWithMappingTarget as callback>
         <#if callback_index = 0>
@@ -65,4 +65,4 @@
             null
         </#if>
     </@compress>
-</#macro>
\ No newline at end of file
+</#macro>
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/DefaultOrderThrowExceptionMapper.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/DefaultOrderThrowExceptionMapper.java
new file mode 100644
index 000000000..ab1ca9782
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/DefaultOrderThrowExceptionMapper.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.value.enum2enum;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.MappingConstants;
+import org.mapstruct.Named;
+import org.mapstruct.ValueMapping;
+import org.mapstruct.ap.test.value.ExternalOrderType;
+import org.mapstruct.ap.test.value.OrderType;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Jude Niroshan
+ */
+@Mapper
+public interface DefaultOrderThrowExceptionMapper {
+    DefaultOrderThrowExceptionMapper INSTANCE = Mappers.getMapper( DefaultOrderThrowExceptionMapper.class );
+
+    @Named("orderTypeToExternalOrderTypeAnyUnmappedToException")
+    @ValueMapping(source = MappingConstants.ANY_UNMAPPED, target = MappingConstants.THROW_EXCEPTION)
+    ExternalOrderType orderTypeToExternalOrderTypeAnyUnmappedToException(OrderType orderType);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumMappingTest.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumMappingTest.java
index 43b621f8c..1e93f4b00 100644
--- a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumMappingTest.java
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumMappingTest.java
@@ -5,8 +5,6 @@
  */
 package org.mapstruct.ap.test.value.enum2enum;
 
-import static org.assertj.core.api.Assertions.assertThat;
-
 import javax.tools.Diagnostic.Kind;
 
 import org.junit.Rule;
@@ -22,14 +20,18 @@ import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutco
 import org.mapstruct.ap.testutil.runner.AnnotationProcessorTestRunner;
 import org.mapstruct.ap.testutil.runner.GeneratedSource;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * Test for the generation and invocation of enum mapping methods.
  *
  * @author Gunnar Morling, Sjaak Derksen
  */
 @IssueKey("128")
-@WithClasses({  OrderMapper.class, SpecialOrderMapper.class, DefaultOrderMapper.class, OrderEntity.class,
-    OrderType.class, OrderDto.class, ExternalOrderType.class })
+@WithClasses({
+    OrderMapper.class, SpecialOrderMapper.class, DefaultOrderMapper.class, OrderEntity.class,
+    OrderType.class, OrderDto.class, ExternalOrderType.class
+})
 @RunWith(AnnotationProcessorTestRunner.class)
 public class EnumToEnumMappingTest {
 
@@ -74,16 +76,16 @@ public class EnumToEnumMappingTest {
     @Test
     public void shouldApplyReverseMappings() {
 
-        OrderType result =  OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.SPECIAL );
+        OrderType result = OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.SPECIAL );
         assertThat( result ).isEqualTo( OrderType.EXTRA );
 
-        result =  OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.DEFAULT );
+        result = OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.DEFAULT );
         assertThat( result ).isEqualTo( OrderType.STANDARD );
 
-        result =  OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.RETAIL );
+        result = OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.RETAIL );
         assertThat( result ).isEqualTo( OrderType.RETAIL );
 
-        result =  OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.B2B );
+        result = OrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.B2B );
         assertThat( result ).isEqualTo( OrderType.B2B );
 
     }
@@ -141,16 +143,16 @@ public class EnumToEnumMappingTest {
     @Test
     public void shouldApplyDefaultReverseMappings() {
 
-        OrderType result =  SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.SPECIAL );
+        OrderType result = SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.SPECIAL );
         assertThat( result ).isEqualTo( OrderType.EXTRA );
 
-        result =  SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.DEFAULT );
+        result = SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.DEFAULT );
         assertThat( result ).isNull();
 
-        result =  SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.RETAIL );
+        result = SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.RETAIL );
         assertThat( result ).isEqualTo( OrderType.RETAIL );
 
-        result =  SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.B2B );
+        result = SpecialOrderMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.B2B );
         assertThat( result ).isEqualTo( OrderType.B2B );
 
     }
@@ -186,7 +188,7 @@ public class EnumToEnumMappingTest {
 
     }
 
-    @IssueKey( "1091" )
+    @IssueKey("1091")
     @Test
     public void shouldMapAnyRemainingToNullCorrectly() {
         ExternalOrderType externalOrderType = SpecialOrderMapper.INSTANCE.anyRemainingToNull( OrderType.RETAIL );
@@ -265,7 +267,7 @@ public class EnumToEnumMappingTest {
             @Diagnostic(type = ErroneousOrderMapperDuplicateANY.class,
                 kind = Kind.ERROR,
                 line = 28,
-                message = "Source = \"<ANY_REMAINING>\" or \"<ANY_UNMAPPED>\" can only be used once." )
+                message = "Source = \"<ANY_REMAINING>\" or \"<ANY_UNMAPPED>\" can only be used once.")
         }
     )
     public void shouldRaiseErrorIfMappingsContainDuplicateANY() {
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumThrowExceptionMappingTest.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumThrowExceptionMappingTest.java
new file mode 100644
index 000000000..15f0d7964
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/EnumToEnumThrowExceptionMappingTest.java
@@ -0,0 +1,93 @@
+/*
+ * 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.value.enum2enum;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mapstruct.ap.test.value.ExternalOrderType;
+import org.mapstruct.ap.test.value.OrderType;
+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;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+/**
+ * @author Jude Niroshan
+ */
+@IssueKey("2339")
+@WithClasses({
+    OrderEntity.class,
+    OrderType.class, ExternalOrderType.class
+})
+@RunWith(AnnotationProcessorTestRunner.class)
+public class EnumToEnumThrowExceptionMappingTest {
+
+    @IssueKey("2339")
+    @Test
+    @WithClasses(DefaultOrderThrowExceptionMapper.class)
+    public void shouldBeAbleToMapAnyUnmappedToThrowException() {
+
+        assertThatThrownBy( () ->
+            DefaultOrderThrowExceptionMapper.INSTANCE
+                .orderTypeToExternalOrderTypeAnyUnmappedToException( OrderType.EXTRA ) )
+            .isInstanceOf( IllegalArgumentException.class )
+            .hasMessage( "Unexpected enum constant: EXTRA" );
+    }
+
+    @IssueKey("2339")
+    @Test
+    @WithClasses({ DefaultOrderThrowExceptionMapper.class, ErroneousOrderMapperThrowExceptionAsSourceType.class })
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(type = ErroneousOrderMapperThrowExceptionAsSourceType.class,
+                kind = javax.tools.Diagnostic.Kind.ERROR,
+                line = 29,
+                message = "Source = \"<THROW_EXCEPTION>\" is not allowed. " +
+                    "Target = \"<THROW_EXCEPTION>\" can only be used.")
+        }
+    )
+    public void shouldRaiseErrorWhenThrowExceptionUsedAsSourceType() {
+    }
+
+    @IssueKey("2339")
+    @Test
+    @WithClasses({OrderThrowExceptionMapper.class, OrderDto.class})
+    public void shouldIgnoreThrowExceptionWhenInverseValueMappings() {
+
+        OrderType target = OrderThrowExceptionMapper.INSTANCE.externalOrderTypeToOrderType( ExternalOrderType.B2B );
+        assertThat( target ).isEqualTo( OrderType.B2B );
+    }
+
+    @IssueKey("2339")
+    @Test
+    @WithClasses({SpecialThrowExceptionMapper.class, OrderDto.class})
+    public void shouldBeAbleToMapAnyRemainingToThrowException() {
+
+        assertThatThrownBy( () ->
+            SpecialThrowExceptionMapper.INSTANCE
+                .orderTypeToExternalOrderType( OrderType.EXTRA ) )
+            .isInstanceOf( IllegalArgumentException.class )
+            .hasMessage( "Unexpected enum constant: EXTRA" );
+    }
+
+    @IssueKey("2339")
+    @Test
+    @WithClasses({SpecialThrowExceptionMapper.class, OrderDto.class})
+    public void shouldBeAbleToMapNullToThrowException() {
+
+        assertThatThrownBy( () ->
+            SpecialThrowExceptionMapper.INSTANCE
+                .anyRemainingToNullToException( null ) )
+            .isInstanceOf( IllegalArgumentException.class )
+            .hasMessage( "Unexpected enum constant: null" );
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/ErroneousOrderMapperThrowExceptionAsSourceType.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/ErroneousOrderMapperThrowExceptionAsSourceType.java
new file mode 100644
index 000000000..5c872c2de
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/ErroneousOrderMapperThrowExceptionAsSourceType.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.value.enum2enum;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.ValueMapping;
+import org.mapstruct.ValueMappings;
+import org.mapstruct.ap.test.value.ExternalOrderType;
+import org.mapstruct.ap.test.value.OrderType;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Jude Niroshan
+ */
+@Mapper
+public interface ErroneousOrderMapperThrowExceptionAsSourceType {
+
+    ErroneousOrderMapperThrowExceptionAsSourceType INSTANCE = Mappers.getMapper(
+        ErroneousOrderMapperThrowExceptionAsSourceType.class );
+
+    @ValueMappings({
+        @ValueMapping(source = "EXTRA", target = "SPECIAL"),
+        @ValueMapping(source = "STANDARD", target = "DEFAULT"),
+        @ValueMapping(source = "NORMAL", target = "DEFAULT"),
+        @ValueMapping(source = "<ANY_REMAINING>", target = "DEFAULT"),
+        @ValueMapping(source = "<THROW_EXCEPTION>", target = "DEFAULT")
+    })
+    ExternalOrderType orderTypeToExternalOrderTypeWithErroneousSourceMapping(OrderType orderType);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/OrderThrowExceptionMapper.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/OrderThrowExceptionMapper.java
new file mode 100644
index 000000000..c189b22d5
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/OrderThrowExceptionMapper.java
@@ -0,0 +1,35 @@
+/*
+ * 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.value.enum2enum;
+
+import org.mapstruct.InheritInverseConfiguration;
+import org.mapstruct.Mapper;
+import org.mapstruct.MappingConstants;
+import org.mapstruct.ValueMapping;
+import org.mapstruct.ValueMappings;
+import org.mapstruct.ap.test.value.ExternalOrderType;
+import org.mapstruct.ap.test.value.OrderType;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Jude Niroshan
+ */
+@Mapper
+public interface OrderThrowExceptionMapper {
+    OrderThrowExceptionMapper INSTANCE = Mappers.getMapper( OrderThrowExceptionMapper.class );
+
+    OrderDto orderEntityToDto(OrderEntity order);
+
+    @ValueMappings({
+        @ValueMapping(source = "EXTRA", target = "SPECIAL"),
+        @ValueMapping(source = "STANDARD", target = "DEFAULT"),
+        @ValueMapping(source = "NORMAL", target = MappingConstants.THROW_EXCEPTION)
+    })
+    ExternalOrderType orderTypeToExternalOrderType(OrderType orderType);
+
+    @InheritInverseConfiguration
+    OrderType externalOrderTypeToOrderType(ExternalOrderType orderType);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapper.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapper.java
index 46ffb8b1c..60d2d5f67 100644
--- a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapper.java
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapper.java
@@ -29,14 +29,14 @@ public interface SpecialOrderMapper {
 
     @Named("orderTypeToExternalOrderType")
     @ValueMappings({
-        @ValueMapping( source = MappingConstants.NULL, target = "DEFAULT" ),
-        @ValueMapping( source = "STANDARD", target = MappingConstants.NULL ),
-        @ValueMapping( source = MappingConstants.ANY_REMAINING, target = "SPECIAL" )
+        @ValueMapping(source = MappingConstants.NULL, target = "DEFAULT"),
+        @ValueMapping(source = "STANDARD", target = MappingConstants.NULL),
+        @ValueMapping(source = MappingConstants.ANY_REMAINING, target = "SPECIAL")
     })
     ExternalOrderType orderTypeToExternalOrderType(OrderType orderType);
 
     @InheritInverseConfiguration(name = "orderTypeToExternalOrderType")
-    @ValueMapping( target = "EXTRA", source = "SPECIAL" )
+    @ValueMapping(target = "EXTRA", source = "SPECIAL")
     OrderType externalOrderTypeToOrderType(ExternalOrderType orderType);
 
     @ValueMappings({
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialThrowExceptionMapper.java b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialThrowExceptionMapper.java
new file mode 100644
index 000000000..0c6d4740c
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/enum2enum/SpecialThrowExceptionMapper.java
@@ -0,0 +1,47 @@
+/*
+ * 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.value.enum2enum;
+
+import org.mapstruct.InheritInverseConfiguration;
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.MappingConstants;
+import org.mapstruct.Named;
+import org.mapstruct.ValueMapping;
+import org.mapstruct.ValueMappings;
+import org.mapstruct.ap.test.value.ExternalOrderType;
+import org.mapstruct.ap.test.value.OrderType;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Jude Niroshan
+ */
+@Mapper
+public interface SpecialThrowExceptionMapper {
+    SpecialThrowExceptionMapper INSTANCE = Mappers.getMapper( SpecialThrowExceptionMapper.class );
+
+    @Mapping(target = "orderType", source = "orderType", qualifiedByName = "orderTypeToExternalOrderType")
+    OrderDto orderEntityToDto(OrderEntity order);
+
+    @Named("orderTypeToExternalOrderType")
+    @ValueMappings({
+        @ValueMapping(source = MappingConstants.NULL, target = "DEFAULT"),
+        @ValueMapping(source = "STANDARD", target = MappingConstants.NULL),
+        @ValueMapping(source = MappingConstants.ANY_REMAINING, target = MappingConstants.THROW_EXCEPTION)
+    })
+    ExternalOrderType orderTypeToExternalOrderType(OrderType orderType);
+
+    @InheritInverseConfiguration(name = "orderTypeToExternalOrderType")
+    @ValueMapping(target = "EXTRA", source = "SPECIAL")
+    OrderType externalOrderTypeToOrderType(ExternalOrderType orderType);
+
+    @ValueMappings({
+        @ValueMapping(source = MappingConstants.NULL, target = MappingConstants.THROW_EXCEPTION),
+        @ValueMapping(source = "STANDARD", target = MappingConstants.NULL),
+        @ValueMapping(source = MappingConstants.ANY_REMAINING, target = MappingConstants.NULL)
+    })
+    ExternalOrderType anyRemainingToNullToException(OrderType orderType);
+}
diff --git a/processor/src/test/resources/fixtures/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapperImpl.java b/processor/src/test/resources/fixtures/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapperImpl.java
index f420a63ba..7b5f6f288 100644
--- a/processor/src/test/resources/fixtures/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapperImpl.java
+++ b/processor/src/test/resources/fixtures/org/mapstruct/ap/test/value/enum2enum/SpecialOrderMapperImpl.java
@@ -11,8 +11,8 @@ import org.mapstruct.ap.test.value.OrderType;
 
 @Generated(
     value = "org.mapstruct.ap.MappingProcessor",
-    date = "2017-02-20T21:25:45+0100",
-    comments = "version: , compiler: javac, environment: Java 1.8.0_112 (Oracle Corporation)"
+    date = "2021-02-19T21:20:19+0100",
+    comments = "version: , compiler: javac, environment: Java 1.8.0_191 (Oracle Corporation)"
 )
 public class SpecialOrderMapperImpl implements SpecialOrderMapper {