Modify a class definition's annotation string parameter at runtime

Question

Imagine there is a class:

@Something(someProperty = \"some value\")
public class Foobar {
    //...
}

Which is already compiled (I cannot c

Answer 1

This code does more or less what you ask for - it is a simple proof of concept:

• a proper implementation needs to also deal with the declaredAnnotations
• if the implementation of annotations in Class.java changes, the code will break (i.e. it can break at any time in the future)
• I have no idea if there are side effects...

Output:

oldAnnotation = some value
modifiedAnnotation = another value

public static void main(String[] args) throws Exception {
    final Something oldAnnotation = (Something) Foobar.class.getAnnotations()[0];
    System.out.println("oldAnnotation = " + oldAnnotation.someProperty());
    Annotation newAnnotation = new Something() {

        @Override
        public String someProperty() {
            return "another value";
        }

        @Override
        public Class annotationType() {
            return oldAnnotation.annotationType();
        }
    };
    Field field = Class.class.getDeclaredField("annotations");
    field.setAccessible(true);
    Map, Annotation> annotations = (Map, Annotation>) field.get(Foobar.class);
    annotations.put(Something.class, newAnnotation);

    Something modifiedAnnotation = (Something) Foobar.class.getAnnotations()[0];
    System.out.println("modifiedAnnotation = " + modifiedAnnotation.someProperty());
}

@Something(someProperty = "some value")
public static class Foobar {
}

@Retention(RetentionPolicy.RUNTIME)
@interface Something {

    String someProperty();
}