How do different retention policies affect my annotations?
Can anyone explain in a clear way the practical differences between the java.lang.annotation.RetentionPolicy
constants SOURCE
, CLASS
, and RUNTIME
?
I'm also not exactly sure what the phrase "retaining annotation" means.
RetentionPolicy.SOURCE
: Discard during the compile. These annotations don't make any sense after the compile has completed, so they aren't written to the bytecode.
Example:@Override
,@SuppressWarnings
RetentionPolicy.CLASS
: Discard during class load. Useful when doing bytecode-level post-processing. Somewhat surprisingly, this is the default.
RetentionPolicy.RUNTIME
: Do not discard. The annotation should be available for reflection at runtime. Example:@Deprecated
Source:
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According to your comments about class decompilation, here is how I think it should work:
RetentionPolicy.SOURCE
: Won't appear in the decompiled classRetentionPolicy.CLASS
: Appear in the decompiled class, but can't be inspected at run-time with reflection withgetAnnotations()
RetentionPolicy.RUNTIME
: Appear in the decompiled class, and can be inspected at run-time with reflection withgetAnnotations()
Minimal runnable example
Language level:
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
@Retention(RetentionPolicy.SOURCE)
@interface RetentionSource {}
@Retention(RetentionPolicy.CLASS)
@interface RetentionClass {}
@Retention(RetentionPolicy.RUNTIME)
@interface RetentionRuntime {}
public static void main(String[] args) {
@RetentionSource
class B {}
assert B.class.getAnnotations().length == 0;
@RetentionClass
class C {}
assert C.class.getAnnotations().length == 0;
@RetentionRuntime
class D {}
assert D.class.getAnnotations().length == 1;
}
Bytecode level: using javap
we observe 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()
and the Runtime.SOURCE
annotated .class
does not get any annotation.
Examples on GitHub for you to play with.
Retention Policy: A retention policy determines at what point an annotation is discarded. It is s specified using Java's built-in annotations: @Retention
[About]
1.SOURCE: annotation retained only in the source file and is discarded
during compilation.
2.CLASS: annotation stored in the .class file during compilation,
not available in the run time.
3.RUNTIME: annotation stored in the .class file and available in the run time.