[java] Use reflection to add annotation to the entity class(为实体类添加 Annotation 注解)

AnnotationUtils.java

public class AnnotationUtils {

    /**
     * Add annotation to the entity class
     * @param clazz Entity's class
     * @param annotationClazz Annotation's class
     * @param annotation New Annotation
     */
    public static void addAnnotation(Class clazz, Class annotationClazz, Annotation annotation) {
        clazz.getAnnotations();

        try {
            Field field = Class.class.getDeclaredField("annotationData");
            field.setAccessible(true);
            Object object = field.get(clazz);
            field = object.getClass().getDeclaredField("annotations");
            field.setAccessible(true);

            Map<Class<? extends Annotation>, Annotation> annotations = (Map<Class<? extends Annotation>, Annotation>) field.get(object);
            annotations.put(annotationClazz, annotation);
        } catch (IllegalAccessException | NoSuchFieldException e) {
            System.out.println("AnnotationUtils: add annotation failed!");
        }
    }
}

TestAnnotation.java

public class TestAnnotation {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        Annotation newAnnotation = new Something() {

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

            @Override
            public short replicas() {
                return 0;
            }

            @Override
            public Class<? extends Annotation> annotationType() {
                return null;
            }
        };

        AnnotationUtils.addAnnotation(Foobar.class, Something.class, newAnnotation);
        Something something = (Something) Foobar.class.getAnnotations()[0];
        System.out.println(something.someProperty());
    }
    
    @Something(someProperty = "some value")
    public static class Foobar {
    }

    @Retention(RetentionPolicy.RUNTIME)
    @interface Something {

        String someProperty();
    }
}