How to use annotation in an annotation?

Question

Given:

public @interface MyAnnotation(){
    public SomeType[] value();
}

in Java 7 is it possible to do something like:

@MyAnnotation({
    value1,
    @MyAnnotation({subValue1, subvalue2, ...}) value2,
    ...
    valueN
})
public Object someProperty;

?

Answer 1

You can. This is an example from Jackson library (leaving out the comments):

package com.fasterxml.jackson.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.ANNOTATION_TYPE, ElementType.TYPE, ElementType.FIELD,
    ElementType.METHOD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@JacksonAnnotation
public @interface JsonSubTypes {
    public Type[] value();

    public @interface Type {
        /**
         * Class of the subtype
         */
        public Class<?> value();

        /**
         * Logical type name used as the type identifier for the class
         */
        public String name() default "";
    }
}

And here is an example usage:

@JsonSubTypes({
        @JsonSubTypes.Type(value = TestSystemEvent.class, name = "TestEvent"),
})
public interface SystemEvent {
}

Answer 2

How to use annotation in an annotation?

Maybe like this

public @interface MyAnnotation(){
    public SomeType[] value();

    public MyAnnotation[] refine();
}

@MyAnnotation(
  {value1, value2},
  refine={ @MyAnnotation({valueX, valueY}), @MyAnnotation(valueM) }
)
public Object someProperty;

Also, in Java 8, you can have Repeatable annotations - so you may refine or add to your 'primary' (eg the first) other refinements brought in by subsequent repetitions of the same annotation.