mapstruct · BK202503 · Jun 21, 2026
diff --git a/NEXT_RELEASE_CHANGELOG.md b/NEXT_RELEASE_CHANGELOG.md
@@ -24,6 +24,7 @@
 
 * Prevent mapper generation from a type with a generic super bound to a type with a generic extends bound (#3994)
 * Fix location for Javadoc when generating the distribution zip
+* Fix property name resolution for fluent setters whose second character is upper case (#4000)
 
 ### Documentation
 

diff --git a/processor/src/main/java/org/mapstruct/ap/spi/DefaultAccessorNamingStrategy.java b/processor/src/main/java/org/mapstruct/ap/spi/DefaultAccessorNamingStrategy.java
@@ -242,7 +242,14 @@ public String getPropertyName(ExecutableElement getterOrSetterMethod) {
                 return IntrospectorUtils.decapitalize( methodName.substring( 3 ) );
             }
             else {
-                return methodName;
+                // [#4000] Apply the same JavaBean-style decapitalization as for `getXyz` /
+                // `setXyz` accessors so that a fluent setter `xNameField(...)` resolves to
+                // the same property name as a JavaBean accessor `getXNameField()` /
+                // `setXNameField(...)`. Without this, the two are recognised as different
+                // properties when the second character is upper case (see Introspector spec).
+                return IntrospectorUtils.decapitalize(
+                    Character.toUpperCase( methodName.charAt( 0 ) ) + methodName.substring( 1 )
+                );
             }
         }
         return IntrospectorUtils.decapitalize( methodName.substring( methodName.startsWith( "is" ) ? 2 : 3 ) );

diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Issue4000Mapper.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Issue4000Mapper.java
@@ -0,0 +1,17 @@
+/*
+ * 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._4000;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface Issue4000Mapper {
+
+    Issue4000Mapper INSTANCE = Mappers.getMapper( Issue4000Mapper.class );
+
+    Target map(Source source);
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Issue4000Test.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Issue4000Test.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.bugs._4000;
+
+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;
+
+@WithClasses({
+    Issue4000Mapper.class,
+    Source.class,
+    Target.class,
+})
+@IssueKey("4000")
+public class Issue4000Test {
+
+    @ProcessorTest
+    public void fluentSetterWithSecondCharUpperCaseShouldMatchJavaBeanAccessor() {
+        Target target = Issue4000Mapper.INSTANCE.map( new Source( "value" ) );
+
+        assertThat( target.getXNameField() ).isEqualTo( "value" );
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Source.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Source.java
@@ -0,0 +1,19 @@
+/*
+ * 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._4000;
+
+public class Source {
+
+    private final String xNameField;
+
+    public Source(String xNameField) {
+        this.xNameField = xNameField;
+    }
+
+    public String getXNameField() {
+        return xNameField;
+    }
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Target.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_4000/Target.java
@@ -0,0 +1,37 @@
+/*
+ * 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._4000;
+
+public class Target {
+
+    private final String xNameField;
+
+    public Target(Builder builder) {
+        this.xNameField = builder.xNameField;
+    }
+
+    public String getXNameField() {
+        return xNameField;
+    }
+
+    public static Target.Builder builder() {
+        return new Builder();
+    }
+
+    public static class Builder {
+
+        private String xNameField;
+
+        public Builder xNameField(String xNameField) {
+            this.xNameField = xNameField;
+            return this;
+        }
+
+        public Target build() {
+            return new Target( this );
+        }
+    }
+}