Java Annotations - looking for an example of RetentionPolicy.CLASS

Question

according to Java Annotation API:

• RetentionPolicy.CLASS Annotations are to be recorded in the class file by the compiler but nee

Answer 1

CLASS Annotations are used in obfuscator tools like http://proguard.sourceforge.net . For example annotation @KeepName disables name mangling when you need to have your class name unchanged to be able to call methods like Class.forName().

Answer 2

Of all of the large number of libraries I have in my current project. the only examples I can find are in the Google Guava library, for example com.google.common.annotations.GwtCompatible.

I'm not really sure why they chose this retention policy, though - perhaps for tools support, where the tools read the class files themselves, rather than going through the reflection API. I'm not sure that I really see the point of this distinction, though.

Answer 3

RetentionPolicy.CLASS are Useful when doing byte code-level post-processing.

Example :

https://github.com/thesmythgroup/DroidCook/blob/master/src/org/tsg/android/api/Annotations.java

http://retroweaver.sourceforge.net/

Answer 4

Minimal example

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

@Retention(RetentionPolicy.CLASS)
@interface RetentionClass {}

@Retention(RetentionPolicy.RUNTIME)
@interface RetentionRuntime {}

public static void main(String[] args) {
    @RetentionClass
    class C {}
    assert C.class.getAnnotations().length == 0;

    @RetentionRuntime
    class D {}
    assert D.class.getAnnotations().length == 1;
}

If we use javap on the annotated classes, we see that the Retention.CLASS annotated class gets a RuntimeInvisible class attribute:

#14 = Utf8               LRetentionClass;
[...]
RuntimeInvisibleAnnotations:
  0: #14()

while Retention.RUNTIME annotation gets a RuntimeVisible class attribute:

#14 = Utf8               LRetentionRuntime;
[...]
RuntimeVisibleAnnotations:
  0: #14()

So the information is present on both cases in the bytecode.

Therefore, Runtime.CLASS can be used to associate arbitrary metadata to a class which bytecode manipulation tools can use, without interfering with runtime-visible behavior.

Examples on GitHub for you to play with.