Create Annotation instance with defaults, in Java

had the same issue, i solved it as follows.

public static FieldGroup getDefaultFieldGroup() {
    @FieldGroup
    class settring {
    }
    return settring.class.getAnnotation(FieldGroup.class);
}

To create an instance you need to create a class that implements:

• java.lang.annotation.Annotation
• and the annotation you want to "simulate"

For example: public class MySettings implements Annotation, Settings

But you need to pay special attention to the correct implementation of equals and hashCode according to the Annotation interface. http://download.oracle.com/javase/1.5.0/docs/api/java/lang/annotation/Annotation.html

If you do not want to implement this again and again then have a look at the javax.enterprise.util.AnnotationLiteral class. That is part of the CDI(Context Dependency Injection)-API. (@see code)

To get the default values you can use the way that is described by akuhn (former known as: Adrian). Settings.class.getMethod("a").getDefaultValue()

I compile and ran below with satisfactory results.

class GetSettings {
    public static void main (String[] args){
      @Settings final class c { }
      Settings settings = c.class.getAnnotation(Settings.class);
      System.out.println(settings.aaa());
    }
}

You cannot create an instance, but at least get the default values

Settings.class.getMethod("a").getDefaultValue()
Settings.class.getMethod("b").getDefaultValue()
Settings.class.getMethod("c").getDefaultValue()

And then, a dynamic proxy could be used to return the default values. Which is, as far as I can tell, the way annotations are handled by Java itself also.

class Defaults implements InvocationHandler {
  public static <A extends Annotation> A of(Class<A> annotation) {
    return (A) Proxy.newProxyInstance(annotation.getClassLoader(),
        new Class[] {annotation}, new Defaults());
  }
  public Object invoke(Object proxy, Method method, Object[] args)
      throws Throwable {
    return method.getDefaultValue();
  }
}

Settings s = Defaults.of(Settings.class);
System.out.printf("%s\n%s\n%s\n", s.a(), s.b(), s.c());