mapstruct · tangyang9464 · May 27, 2025
diff --git a/...truct/ap/internal/model/assignment/ExistingInstanceSetterWrapperForCollectionsAndMaps.ftl b/...truct/ap/internal/model/assignment/ExistingInstanceSetterWrapperForCollectionsAndMaps.ftl
@@ -30,6 +30,10 @@
   <@lib.handleLocalVarNullCheck needs_explicit_local_var=directAssignment>
     ${ext.targetBeanName}.${ext.targetWriteAccessorName}<@lib.handleWrite><#if directAssignment><@wrapLocalVarInCollectionInitializer/><#else><@lib.handleWithAssignmentOrNullCheckVar/></#if></@lib.handleWrite>;
   </@lib.handleLocalVarNullCheck>
+  <#if !ext.defaultValueAssignment?? && !sourcePresenceCheckerReference?? && mapNullToDefault>else {
+    ${ext.targetBeanName}.${ext.targetWriteAccessorName}<@lib.handleWrite><@lib.initTargetObject/></@lib.handleWrite>;
+  }
+  </#if>
 </#macro>
 <#--
   wraps the local variable in a collection initializer (new collection, or EnumSet.copyOf)

diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/DestinationType.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/DestinationType.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.bugs._3865;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Destination type interface for testing null value property mapping strategy with Map properties.
+ */
+public interface DestinationType {
+    Map<String, String> getAttributes();
+
+    void setAttributes(Map<String, String> attributes);
+
+    Map<String, String> getInitializedAttributes();
+
+    void setInitializedAttributes(Map<String, String> attributes);
+
+    List<String> getItems();
+
+    void setItems(List<String> items);
+
+    List<String> getInitializedItems();
+
+    void setInitializedItems(List<String> items);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/Issue3865Mapper.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/Issue3865Mapper.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.bugs._3865;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.MappingTarget;
+import org.mapstruct.NullValuePropertyMappingStrategy;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * Mapper for testing null value property mapping strategy with Map properties.
+ */
+@Mapper(nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.SET_TO_DEFAULT)
+public interface Issue3865Mapper {
+    Issue3865Mapper INSTANCE = Mappers.getMapper( Issue3865Mapper.class );
+
+    void update(@MappingTarget DestinationType destination, SourceType source);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/Issue3865Test.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/Issue3865Test.java
@@ -0,0 +1,140 @@
+/*
+ * 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._3865;
+
+import java.util.HashMap;
+import java.util.ArrayList;
+import java.util.Map;
+import java.util.List;
+
+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;
+
+/**
+ * Test for issue 3865: NullValuePropertyMappingStrategy.SET_TO_DEFAULT should set target Map/Collection to default
+ * when source and target are all null.
+ */
+@IssueKey("3865")
+@WithClasses({
+    DestinationType.class,
+    SourceType.class,
+    Issue3865Mapper.class
+})
+public class Issue3865Test {
+
+    @ProcessorTest
+    public void shouldSetTargetToDefaultWhenBothSourceAndTargetAreNull() {
+        DestinationType target = new SourceType();
+        SourceType source = new SourceType();
+
+        assertThat( source.getAttributes() ).isNull();
+        assertThat( target.getAttributes() ).isNull();
+        assertThat( source.getItems() ).isNull();
+        assertThat( target.getItems() ).isNull();
+
+        Issue3865Mapper.INSTANCE.update( target, source );
+
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).isEmpty();
+        assertThat( target.getItems() ).isNotNull();
+        assertThat( target.getItems() ).isEmpty();
+
+        assertThat( target.getInitializedAttributes() ).containsKey( "key1" );
+        assertThat( target.getInitializedItems() ).contains( "item1" );
+    }
+
+    @ProcessorTest
+    public void shouldClearTargetWhenSourceIsNullAndTargetIsInitialized() {
+        DestinationType target = new SourceType();
+        SourceType source = new SourceType();
+
+        Map<String, String> targetAttributes = new HashMap<>();
+        targetAttributes.put( "targetKey", "targetValue" );
+        target.setAttributes( targetAttributes );
+
+        List<String> targetItems = new ArrayList<>();
+        targetItems.add( "targetItem" );
+        target.setItems( targetItems );
+
+        assertThat( source.getAttributes() ).isNull();
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( source.getItems() ).isNull();
+        assertThat( target.getItems() ).isNotNull();
+
+        Issue3865Mapper.INSTANCE.update( target, source );
+
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).isEmpty();
+        assertThat( target.getItems() ).isNotNull();
+        assertThat( target.getItems() ).isEmpty();
+    }
+
+    @ProcessorTest
+    public void shouldCopySourceToTargetWhenSourceIsInitializedAndTargetIsNull() {
+        DestinationType target = new SourceType();
+        SourceType source = new SourceType();
+
+        Map<String, String> sourceAttributes = new HashMap<>();
+        sourceAttributes.put( "sourceKey", "sourceValue" );
+        source.setAttributes( sourceAttributes );
+
+        List<String> sourceItems = new ArrayList<>();
+        sourceItems.add( "sourceItem" );
+        source.setItems( sourceItems );
+
+        assertThat( source.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).isNull();
+        assertThat( source.getItems() ).isNotNull();
+        assertThat( target.getItems() ).isNull();
+
+        Issue3865Mapper.INSTANCE.update( target, source );
+
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).containsEntry( "sourceKey", "sourceValue" );
+        assertThat( target.getItems() ).isNotNull();
+        assertThat( target.getItems() ).contains( "sourceItem" );
+    }
+
+    @ProcessorTest
+    public void shouldCopySourceToTargetWhenBothSourceAndTargetAreInitialized() {
+        DestinationType target = new SourceType();
+        SourceType source = new SourceType();
+
+        Map<String, String> sourceAttributes = new HashMap<>();
+        sourceAttributes.put( "sourceKey", "sourceValue" );
+        source.setAttributes( sourceAttributes );
+
+        Map<String, String> targetAttributes = new HashMap<>();
+        targetAttributes.put( "targetKey", "targetValue" );
+        target.setAttributes( targetAttributes );
+
+        List<String> sourceItems = new ArrayList<>();
+        sourceItems.add( "sourceItem" );
+        source.setItems( sourceItems );
+
+        List<String> targetItems = new ArrayList<>();
+        targetItems.add( "targetItem" );
+        target.setItems( targetItems );
+
+        assertThat( source.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( source.getItems() ).isNotNull();
+        assertThat( target.getItems() ).isNotNull();
+
+        Issue3865Mapper.INSTANCE.update( target, source );
+
+        assertThat( target.getAttributes() ).isNotNull();
+        assertThat( target.getAttributes() ).containsEntry( "sourceKey", "sourceValue" );
+        assertThat( target.getAttributes() ).doesNotContainKey( "targetKey" );
+
+        assertThat( target.getItems() ).isNotNull();
+        assertThat( target.getItems() ).contains( "sourceItem" );
+        assertThat( target.getItems() ).doesNotContain( "targetItem" );
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/SourceType.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3865/SourceType.java
@@ -0,0 +1,67 @@
+/*
+ * 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._3865;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Source type class implementing DestinationType for testing null value property mapping strategy with Map properties.
+ */
+public class SourceType implements DestinationType {
+    private Map<String, String> attributes;
+    private List<String> items;
+
+    private Map<String, String> initializedAttributes = new HashMap<>();
+    private List<String> initializedItems = new ArrayList<>();
+
+    public SourceType() {
+        initializedAttributes.put( "key1", "value1" );
+        initializedItems.add( "item1" );
+    }
+
+    @Override
+    public Map<String, String> getAttributes() {
+        return attributes;
+    }
+
+    @Override
+    public void setAttributes(Map<String, String> attributes) {
+        this.attributes = attributes;
+    }
+
+    @Override
+    public Map<String, String> getInitializedAttributes() {
+        return initializedAttributes;
+    }
+
+    @Override
+    public void setInitializedAttributes(Map<String, String> attributes) {
+        this.initializedAttributes = attributes;
+    }
+
+    @Override
+    public List<String> getItems() {
+        return items;
+    }
+
+    @Override
+    public void setItems(List<String> items) {
+        this.items = items;
+    }
+
+    @Override
+    public List<String> getInitializedItems() {
+        return initializedItems;
+    }
+
+    @Override
+    public void setInitializedItems(List<String> items) {
+        this.initializedItems = items;
+    }
+}
diff --git a/...urces/fixtures/21/org/mapstruct/ap/test/bugs/_913/DomainDtoWithNvmsDefaultMapperImpl.java b/...urces/fixtures/21/org/mapstruct/ap/test/bugs/_913/DomainDtoWithNvmsDefaultMapperImpl.java
@@ -67,6 +67,9 @@ public void update(Dto source, Domain target) {
                 if ( list != null ) {
                     target.setStrings( new LinkedHashSet<String>( list ) );
                 }
+                else {
+                    target.setStrings( new LinkedHashSet<String>() );
+                }
             }
             if ( target.getLongs() != null ) {
                 Set<Long> set = stringListToLongSet( source.getStrings() );
@@ -83,6 +86,9 @@ public void update(Dto source, Domain target) {
                 if ( set != null ) {
                     target.setLongs( set );
                 }
+                else {
+                    target.setLongs( new LinkedHashSet<Long>() );
+                }
             }
             if ( target.getStringsInitialized() != null ) {
                 List<String> list1 = source.getStringsInitialized();
@@ -99,6 +105,9 @@ public void update(Dto source, Domain target) {
                 if ( list1 != null ) {
                     target.setStringsInitialized( new LinkedHashSet<String>( list1 ) );
                 }
+                else {
+                    target.setStringsInitialized( new LinkedHashSet<String>() );
+                }
             }
             if ( target.getLongsInitialized() != null ) {
                 Set<Long> set1 = stringListToLongSet( source.getStringsInitialized() );
@@ -115,6 +124,9 @@ public void update(Dto source, Domain target) {
                 if ( set1 != null ) {
                     target.setLongsInitialized( set1 );
                 }
+                else {
+                    target.setLongsInitialized( new LinkedHashSet<Long>() );
+                }
             }
             if ( target.getStringsWithDefault() != null ) {
                 List<String> list2 = source.getStringsWithDefault();
@@ -157,6 +169,9 @@ public Domain updateWithReturn(Dto source, Domain target) {
                 if ( list != null ) {
                     target.setStrings( new LinkedHashSet<String>( list ) );
                 }
+                else {
+                    target.setStrings( new LinkedHashSet<String>() );
+                }
             }
             if ( target.getLongs() != null ) {
                 Set<Long> set = stringListToLongSet( source.getStrings() );
@@ -173,6 +188,9 @@ public Domain updateWithReturn(Dto source, Domain target) {
                 if ( set != null ) {
                     target.setLongs( set );
                 }
+                else {
+                    target.setLongs( new LinkedHashSet<Long>() );
+                }
             }
             if ( target.getStringsInitialized() != null ) {
                 List<String> list1 = source.getStringsInitialized();
@@ -189,6 +207,9 @@ public Domain updateWithReturn(Dto source, Domain target) {
                 if ( list1 != null ) {
                     target.setStringsInitialized( new LinkedHashSet<String>( list1 ) );
                 }
+                else {
+                    target.setStringsInitialized( new LinkedHashSet<String>() );
+                }
             }
             if ( target.getLongsInitialized() != null ) {
                 Set<Long> set1 = stringListToLongSet( source.getStringsInitialized() );
@@ -205,6 +226,9 @@ public Domain updateWithReturn(Dto source, Domain target) {
                 if ( set1 != null ) {
                     target.setLongsInitialized( set1 );
                 }
+                else {
+                    target.setLongsInitialized( new LinkedHashSet<Long>() );
+                }
             }
             if ( target.getStringsWithDefault() != null ) {
                 List<String> list2 = source.getStringsWithDefault();