Java Annotations not working

Question

I'm trying to use Java annotations, but can't seem to get my code to recognize that one exists. What am I doing wrong?

  import java.lang.reflect.*;
  import java.lang.annotation.*;

  @interface MyAnnotation{}


  public class FooTest
  { 
    @MyAnnotation
    public void doFoo()
    {  
    }

    public static void main(String[] args) throws Exception
    {    
     Method method = FooTest.class.getMethod( "doFoo" );

     Annotation[] annotations = method.getAnnotations();
     for( Annotation annotation : method.getAnnotations() )
      System.out.println( "Annotation: " + annotation  );

    }
  }

Answer 1

Short answer: you need to add @Retention(RetentionPolicy.RUNTIME) to your annotation definition.

Explanation:

Annotations are by default not kept by the compiler. They simply don't exist at runtime. This may sound silly at first, but there are lots of annotations that are only used by the compiler (@Override) or various source code analyzers (@Documentation, etc).

If you want to actually USE the annotation via reflection like in your example, you'll need to let Java know that you want it to make a note of that annotation in the class file itself. That note looks like this:

@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation{}

For more information, check out the official docs[1] and especially note the bit about RetentionPolicy.