Since Java 9, the way retention works for annotations which have as a target LOCAL_VARIABLE has changed. Now the bytecode representation will hold information about annotated local variables in each method. Also, the specification states that If m has an element whose value is java.lang.annotation.RetentionPolicy.RUNTIME, the reflection libraries of the Java SE Platform must make a available at run time., which, if I understand correctly, confirms that at least the annotations on the local variables of the method should be available through reflection.
However, I have been trying to use reflection to find the annotations (or local variables annotated, or their values) with no luck.
Therefore, does anyone have a clue on how to do this or if it is possible? I am using java 11.
As an example, this is my Main class where I try to get the @Parallel annotations from the test method.
public class Main {
public static void main(String[] args) {
try {
System.out.println("Main.class.getDeclaredMethod(\"test\").getAnnotations().length = " +
Main.class.getDeclaredMethod("test").getAnnotations().length);
System.out.println("Main.class.getDeclaredMethod(\"test\").getDeclaredAnnotations().length = " +
Main.class.getDeclaredMethod("test").getDeclaredAnnotations().length);
Annotation annotation = Main.class.getDeclaredMethod("test").getAnnotation(Parallel.class);
System.out.println("annotation = " + annotation);
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
}
void test() {
@Parallel int a = 2;
@Parallel int b = 4;
}
}
And this is my annotation implementation. I've just added everything as a target since I am only testing it:
@Target({ ElementType.LOCAL_VARIABLE, ElementType.TYPE, ElementType.TYPE_USE, ElementType.TYPE_PARAMETER,
ElementType.FIELD, ElementType.PARAMETER, ElementType.PACKAGE, ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.ANNOTATION_TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface Parallel {
}
Running javap -l -v -c -s Main.class will show this:
void test();
descriptor: ()V
flags: (0x0000)
Code:
stack=1, locals=3, args_size=1
0: iconst_2
1: istore_1
2: iconst_4
3: istore_2
4: return
LineNumberTable:
line 22: 0
line 24: 2
line 25: 4
RuntimeVisibleTypeAnnotations:
0: #27(): LOCAL_VARIABLE, {start_pc=2, length=3, index=1}
Parallel
1: #27(): LOCAL_VARIABLE, {start_pc=4, length=1, index=2}
Parallel
It's clear that information about the annotations on the local variables is there, I just don't know if reflection is able retrieve it (or if there's any other way to get it at runtime)