Question

From the Java doc:

CLASS:
Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.

RUNTIME:
Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.

SOURCE:
Annotations are to be discarded by the compiler.

I understand the usages of RUNTIME (in order to use annotation with reflection) and CLASS (for the compiler) but I don’t understand when it can be usefull to use

@Retention(RetentionPolicy.SOURCE)

Can you explain?

Answer 1

Things like @SuppressWarnings, @Override are annotations used by the compiler – not needed at runtime. For those RetentionPolicy.SOURCE would make sense. Also annotations can be used to generate code (look at Spring ROO) – such annotation are also not required at run time.