[JAVA] Annotation instance is Dynamic Proxy

The instance of the annotation is the Dynamic Proxy.

I will check

Define an annotation like this,

package com.example;

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

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

Write the code to write Class from the annotation instance like this,

package com.example;

@FooBar
public class Main {

    public static void main(final String[] args) {
        final FooBar a = Main.class.getAnnotation(FooBar.class);
        System.out.println(a.getClass());
    }
}

When I ran it, I got the following output:

`output`


class com.sun.proxy.$Proxy1

As you can see (?) Dynamic Proxy.

Where is the Dynamic Proxy made?

If you follow Class.getAnnotation, you will reach ʻAnnotationParser.annotationForMap`.

Looking at the code, you can see that the Dynamic Proxy is created using the annotation Class.

    /**
     * Returns an annotation of the given type backed by the given
     * member {@literal ->} value map.
     */
    public static Annotation annotationForMap(final Class<? extends Annotation> type,
                                              final Map<String, Object> memberValues)
    {
        return AccessController.doPrivileged(new PrivilegedAction<Annotation>() {
            public Annotation run() {
                return (Annotation) Proxy.newProxyInstance(
                    type.getClassLoader(), new Class<?>[] { type },
                    new AnnotationInvocationHandler(type, memberValues));
            }});
    }

AnnotationParser.annotationForMap

The ʻInvocationHandler implementation passed when creating a Dynamic Proxy is ʻAnnotationInvocationHandler. All method calls to annotations are handled by ʻAnnotationInvocationHandler`.

AnnotationInvocationHandler

If you want to get ʻAnnotationInvocationHandler from an instance of Annotation, use Proxy.getInvocationHandler`.

final FooBar a = ...
final InvocationHandler h = Proxy.getInvocationHandler(a);
System.out.println(h.getClass());

`output`


class sun.reflect.annotation.AnnotationInvocationHandler

Supplementary information

What is Dynamic Proxy in the first place? Please refer to the Javadoc of Proxy and ʻInvocationHandler`.