17 January 2008

Tags: annotations inheritance java

I’ve read tons of articles about the subject : are annotations inherited or not ? Java 5 annotations are powerful, but you must understand what you do. Annotating a class does not necessarily mean that a subclass will keep the annotation. So, are they inherited or not ? The truth is that if you want your annotations to be inherited, you must specify it. Here’s a simple JUnit 4.4 test case to show you the concept.

package test.annotations;

import org.junit.Test;
import static org.junit.Assert.assertEquals;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Annotation;
import java.lang.annotation.Inherited;

public class AnnotationTest {

 @Retention(RetentionPolicy.RUNTIME)
 public @interface SimpleAnnotation {
  String value() default "test";
 }

 @Retention(RetentionPolicy.RUNTIME)
 @Inherited
 public @interface InheritedAnnotation {
  String value() default "test";
 }

 // two simple classes that will show that the annotation is *not* inherited
 @SimpleAnnotation
 class BaseClass {
 }

 class SubclassDoesNotInheritAnnotation extends BaseClass {
 }

 // two simple classes that will show that the annotation *is* inherited
 @InheritedAnnotation
 class InheritedBase {
 }

 class ChildInheritsAnnotation extends InheritedBase {
 }

 // proof
 @Test
 public void testAnnotations() {
  // not inherited
  Annotation[] annotations = BaseClass.class.getAnnotations();
  assertEquals(1, annotations.length);
  annotations = SubclassDoesNotInheritAnnotation.class.getAnnotations();
  assertEquals(0, annotations.length);

  // inherited
  annotations = InheritedBase.class.getAnnotations();
  assertEquals(1, annotations.length);
  annotations = ChildInheritsAnnotation.class.getAnnotations();
  assertEquals(1, annotations.length);
 }

}

The secret is the @Inherited annotation, which will ensure that subclasses will be annotated too.

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