#3159 Do null value check for collections with default expression

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

@@ -510,7 +510,7 @@ public class PropertyMapping extends ModelElement {
                 return true;
             }
 
-            if ( defaultValue != null || defaultJavaExpression != null ) {
+            if ( hasDefaultValueOrDefaultExpression() ) {
                 // If there is default value defined then a check is needed
                 return true;
             }
@@ -518,6 +518,10 @@ public class PropertyMapping extends ModelElement {
             return false;
         }
 
+        private boolean hasDefaultValueOrDefaultExpression() {
+            return defaultValue != null || defaultJavaExpression != null;
+        }
+
         private Assignment assignToPlainViaAdder( Assignment rightHandSide) {
 
             Assignment result = rightHandSide;
@@ -555,7 +559,7 @@ public class PropertyMapping extends ModelElement {
                 .targetAccessorType( targetAccessorType )
                 .rightHandSide( rightHandSide )
                 .assignment( rhs )
-                .nullValueCheckStrategy( nvcs )
+                .nullValueCheckStrategy( hasDefaultValueOrDefaultExpression() ? ALWAYS : nvcs )
                 .nullValuePropertyMappingStrategy( nvpms )
                 .build();
         }

processor/src/test/java/org/mapstruct/ap/test/bugs/_3159/Issue3159Mapper.java

@@ -0,0 +1,64 @@
+/*
+ * 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.bugs._3159;
+
+import java.util.Collection;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface Issue3159Mapper {
+
+    Issue3159Mapper INSTANCE = Mappers.getMapper( Issue3159Mapper.class );
+
+    @Mapping(target = "elements", defaultExpression = "java(new ArrayList<>())")
+    Target map(Source source);
+
+    default String elementName(Element element) {
+        return element != null ? element.getName() : null;
+    }
+
+    class Target {
+        private final Collection<String> elements;
+
+        public Target(Collection<String> elements) {
+            this.elements = elements;
+        }
+
+        public Collection<String> getElements() {
+            return elements;
+        }
+    }
+
+    class Source {
+        private final Collection<Element> elements;
+
+        public Source(Collection<Element> elements) {
+            this.elements = elements;
+        }
+
+        public Collection<Element> getElements() {
+            return elements;
+        }
+    }
+
+    class Element {
+        private final String name;
+
+        public Element(String name) {
+            this.name = name;
+        }
+
+        public String getName() {
+            return name;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_3159/Issue3159Test.java

@@ -0,0 +1,27 @@
+/*
+ * 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.bugs._3159;
+
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.ProcessorTest;
+import org.mapstruct.ap.testutil.WithClasses;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Filip Hrisafov
+ */
+@IssueKey("3159")
+@WithClasses(Issue3159Mapper.class)
+class Issue3159Test {
+
+    @ProcessorTest
+    void shouldUseDefaultExpressionForCollection() {
+        Issue3159Mapper.Target target = Issue3159Mapper.INSTANCE.map( new Issue3159Mapper.Source( null ) );
+
+        assertThat( target.getElements() ).isEmpty();
+    }
+}