mapstruct · kalayciburak · Jul 19, 2026
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/model/Decorator.java b/processor/src/main/java/org/mapstruct/ap/internal/model/Decorator.java
@@ -5,7 +5,7 @@
  */
 package org.mapstruct.ap.internal.model;
 
-import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 import java.util.SortedSet;
@@ -80,10 +80,12 @@ public Decorator build() {
                 Mapper.getFlatName( mapperElement ) );
 
             Type decoratorType = typeFactory.getType( decorator.value().get() );
+            boolean delegateFieldNeeded = !methods.isEmpty();
             DecoratorConstructor decoratorConstructor = new DecoratorConstructor(
                 implementationName,
                 implementationName + "_",
-                hasDelegateConstructor );
+                hasDelegateConstructor,
+                delegateFieldNeeded );
 
 
             Type mapperType = typeFactory.getType( mapperElement );
@@ -97,6 +99,7 @@ public Decorator build() {
                 decoratorType,
                 mapperType,
                 methods,
+                delegateFieldNeeded,
                 options,
                 versionInformation,
                 suppressGeneratorTimestamp,
@@ -110,11 +113,12 @@ public Decorator build() {
 
     private final Type decoratorType;
     private final Type mapperType;
+    private final boolean delegateFieldNeeded;
 
     @SuppressWarnings( "checkstyle:parameternumber" )
     private Decorator(TypeFactory typeFactory, String packageName, String name, Type decoratorType,
                       Type mapperType,
-                      List<MappingMethod> methods,
+                      List<MappingMethod> methods, boolean delegateFieldNeeded,
                       Options options, VersionInformation versionInformation,
                       boolean suppressGeneratorTimestamp,
                       Accessibility accessibility, SortedSet<Type> extraImports,
@@ -126,7 +130,9 @@ private Decorator(TypeFactory typeFactory, String packageName, String name, Type
             name,
             decoratorType,
             methods,
-            Arrays.asList( new Field( mapperType, "delegate", true ) ),
+            delegateFieldNeeded
+                ? Collections.singletonList( new Field( mapperType, "delegate", true ) )
+                : Collections.emptyList(),
             options,
             versionInformation,
             suppressGeneratorTimestamp,
@@ -137,6 +143,7 @@ private Decorator(TypeFactory typeFactory, String packageName, String name, Type
 
         this.decoratorType = decoratorType;
         this.mapperType = mapperType;
+        this.delegateFieldNeeded = delegateFieldNeeded;
 
         // Add custom annotations
         if ( customAnnotations != null ) {
@@ -163,9 +170,24 @@ public SortedSet<Type> getImportTypes() {
         else {
             importTypes.add( decoratorType );
         }
+
+        if ( !delegateFieldNeeded && decoratorType.getTypeElement() != null &&
+            decoratorType.getTypeElement().getNestingKind().isNested() && !isMapperTypeUsedInAnnotations() ) {
+            importTypes.remove( mapperType );
+        }
+
         return importTypes;
     }
 
+    private boolean isMapperTypeUsedInAnnotations() {
+        for ( Annotation annotation : getAnnotations() ) {
+            if ( annotation.getImportTypes().contains( mapperType ) ) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     @Override
     protected String getTemplateName() {
         return getTemplateNameForClass( GeneratedType.class );

diff --git a/processor/src/main/java/org/mapstruct/ap/internal/model/DecoratorConstructor.java b/processor/src/main/java/org/mapstruct/ap/internal/model/DecoratorConstructor.java
@@ -21,11 +21,14 @@ public class DecoratorConstructor extends ModelElement implements Constructor {
     private final String name;
     private final String delegateName;
     private final boolean invokeSuperConstructor;
+    private final boolean delegateFieldNeeded;
 
-    public DecoratorConstructor(String name, String delegateName, boolean invokeSuperConstructor) {
+    public DecoratorConstructor(String name, String delegateName, boolean invokeSuperConstructor,
+                                boolean delegateFieldNeeded) {
         this.name = name;
         this.delegateName = delegateName;
         this.invokeSuperConstructor = invokeSuperConstructor;
+        this.delegateFieldNeeded = delegateFieldNeeded;
     }
 
     @Override
@@ -45,4 +48,8 @@ public String getDelegateName() {
     public boolean isInvokeSuperConstructor() {
         return invokeSuperConstructor;
     }
+
+    public boolean isDelegateFieldNeeded() {
+        return delegateFieldNeeded;
+    }
 }
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/processor/MapperCreationProcessor.java b/processor/src/main/java/org/mapstruct/ap/internal/processor/MapperCreationProcessor.java
@@ -304,25 +304,29 @@ else if ( constructor.getParameters().size() == 1 ) {
             .versionInformation( versionInformation )
             .implName( mapperOptions.implementationName() )
             .implPackage( mapperOptions.implementationPackage() )
-            .extraImports( getExtraImports( element, mapperOptions ) )
+            .extraImports( getExtraImports( element, mapperOptions, !mappingMethods.isEmpty() ) )
             .suppressGeneratorTimestamp( mapperOptions.suppressTimestampInGenerated() )
             .additionalAnnotations( decoratorAnnotations )
             .build();
 
         return decorator;
     }
 
-    private SortedSet<Type> getExtraImports(TypeElement element,  MapperOptions mapperOptions) {
-        SortedSet<Type> extraImports = new TreeSet<>();
+    private SortedSet<Type> getExtraImports(TypeElement element, MapperOptions mapperOptions) {
+        return getExtraImports( element, mapperOptions, true );
+    }
 
+    private SortedSet<Type> getExtraImports(TypeElement element, MapperOptions mapperOptions,
+                                            boolean includeMapperType) {
+        SortedSet<Type> extraImports = new TreeSet<>();
 
         for ( TypeMirror extraImport : mapperOptions.imports() ) {
             Type type = typeFactory.getAlwaysImportedType( extraImport );
             extraImports.add( type );
         }
 
-        // Add original package if a dest package has been set
-        if ( !"default".equals( mapperOptions.implementationPackage() ) ) {
+        // Add the original mapper type if a destination package has been set
+        if ( includeMapperType && !"default".equals( mapperOptions.implementationPackage() ) ) {
             extraImports.add( typeFactory.getType( element ) );
         }
 

diff --git a/processor/src/main/resources/org/mapstruct/ap/internal/model/DecoratorConstructor.ftl b/processor/src/main/resources/org/mapstruct/ap/internal/model/DecoratorConstructor.ftl
@@ -6,6 +6,7 @@
 
 -->
 <#-- @ftlvariable name="" type="org.mapstruct.ap.internal.model.DecoratorConstructor" -->
+<#if invokeSuperConstructor || delegateFieldNeeded>
 public ${name}() {
     this( new ${delegateName}() );
 }
@@ -14,5 +15,11 @@ private ${name}(${delegateName} delegate) {
     <#if invokeSuperConstructor>
     super( delegate );
     </#if>
+    <#if delegateFieldNeeded>
     this.delegate = delegate;
-}
+    </#if>
+}
+<#else>
+public ${name}() {
+}
+</#if>
diff --git a/...sor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519DefaultConstructorMapper.java b/...sor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519DefaultConstructorMapper.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.test.bugs._1519;
+
+import org.mapstruct.DecoratedWith;
+import org.mapstruct.Mapper;
+
+@Mapper(implementationPackage = "<PACKAGE_NAME>.dest")
+@DecoratedWith(Issue1519DefaultConstructorMapper.Decorator.class)
+public interface Issue1519DefaultConstructorMapper {
+
+    Target map(Source source);
+
+    class Source {
+
+        private String value;
+
+        public String getValue() {
+            return value;
+        }
+
+        public void setValue(String value) {
+            this.value = value;
+        }
+    }
+
+    class Target {
+
+        private String value;
+
+        public String getValue() {
+            return value;
+        }
+
+        public void setValue(String value) {
+            this.value = value;
+        }
+    }
+
+    abstract class Decorator implements Issue1519DefaultConstructorMapper {
+
+        @Override
+        public Target map(Source source) {
+            Target target = new Target();
+            target.setValue( source.getValue() );
+            return target;
+        }
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519Mapper.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519Mapper.java
@@ -0,0 +1,56 @@
+/*
+ * 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._1519;
+
+import org.mapstruct.DecoratedWith;
+import org.mapstruct.Mapper;
+
+@Mapper
+@DecoratedWith(Issue1519Mapper.Decorator.class)
+public interface Issue1519Mapper {
+
+    Target map(Source source);
+
+    class Source {
+
+        private String value;
+
+        public String getValue() {
+            return value;
+        }
+
+        public void setValue(String value) {
+            this.value = value;
+        }
+    }
+
+    class Target {
+
+        private String value;
+
+        public String getValue() {
+            return value;
+        }
+
+        public void setValue(String value) {
+            this.value = value;
+        }
+    }
+
+    abstract class Decorator implements Issue1519Mapper {
+
+        private final Issue1519Mapper delegate;
+
+        Decorator(Issue1519Mapper delegate) {
+            this.delegate = delegate;
+        }
+
+        @Override
+        public Target map(Source source) {
+            return delegate.map( source );
+        }
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519Test.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_1519/Issue1519Test.java
@@ -0,0 +1,42 @@
+/*
+ * 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._1519;
+
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.ProcessorTest;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.runner.GeneratedSource;
+
+@IssueKey("1519")
+@WithClasses({
+    Issue1519Mapper.class,
+    Issue1519DefaultConstructorMapper.class
+})
+class Issue1519Test {
+
+    @RegisterExtension
+    final GeneratedSource generatedSource = new GeneratedSource();
+
+    @ProcessorTest
+    void shouldNotGenerateUnusedDelegateField() {
+        generatedSource.forMapper( Issue1519Mapper.class ).content()
+            .contains( "super( delegate );" )
+            .doesNotContain( "Issue1519Mapper delegate;" )
+            .doesNotContain( "this.delegate = delegate;" );
+    }
+
+    @ProcessorTest
+    void shouldNotCreateDelegateForDefaultConstructorDecorator() {
+        generatedSource.forJavaFile(
+            "org/mapstruct/ap/test/bugs/_1519/dest/Issue1519DefaultConstructorMapperImpl.java"
+        ).content()
+            .contains( "public Issue1519DefaultConstructorMapperImpl() {\n    }" )
+            .doesNotContain( "delegate" )
+            .doesNotContain( "Issue1519DefaultConstructorMapperImpl_" )
+            .doesNotContain( "import org.mapstruct.ap.test.bugs._1519.Issue1519DefaultConstructorMapper;" );
+    }
+}