如何扫描类以获取自定义注释？

Question

I have my custom annotation and I want to scan all the classes for this annotation at runtime. What is the best way to do this? I'm not using Spring.

Answer 1

You could use the Reflections Library to determine the class names first and then use getAnnotations to check for the annotation:

Reflections reflections = new Reflections("org.package.foo");

Set<Class<? extends Object>> allClasses = 
                 reflections.getSubTypesOf(Object.class);


for (Class clazz : allClasses) {
   Annotation[] annotations = clazz.getAnnotations();

   for (Annotation annotation : annotations) {
     if (annotation instanceof MyAnnotation) {
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("value: " + myAnnotation.value());
     }
   }
}     

Answer 2

You might get annotations from the class using getClass().getAnnotations() or ask for a particular annotation if you don't want to loop over the results of the previous. In order for the annotation to appear on the result it's retention must be RUNTIME. For example (not strictly correct):

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyAnnotation {}

Check the Javadoc: Class#getAnnotation(Class)

And after that your class should be annotated like this:

@MyAnnotation public class MyClass {}