does java annotations uses the java refection.

[Nuwan Arambage]

I have read that java reflection is working behind the scene of annotations.
Q1) is it true?
Q2) how does reflection help to make annotations useful?

[Roberto Perillo]

Howdy, Nuwan!

I have read that java reflection is working behind the scene of annotations.
Q1) is it true?
Q2) how does reflection help to make annotations useful?

Q1: Well, depends on what you mean by "working behind the scene of annotations". An annotation is simply a form of metadata.
Q2: An annotation does not do anything alone. It can't contain code, which means that it needs other code to consume it. This is done with reflection. Here's an example:

public void MyClassWithAnnotations {
    @MyCustomAnnotation(name = "Bob", lastName = "Perillo")
    public void doStuff() {
        // some code here...
    }
}

Now, in order to verify if an annotation annotates one element of your class (the doStuff() method in the example above), you can use reflection:

    public void test() throws NoSuchMethodException {
        Class<?> clazz = MyClassWithAnnotations.class;
        Method doStuffMethod = clazz.getMethod("doStuff", null);
        if (doStuffMethod.isAnnotationPresent(MyCustomAnnotation.class)) {
            MyCustomAnnotation annotation = doStuffMethod.getAnnotation(MyCustomAnnotation.class);
            String name = annotation.name();
            String lastName = annotation.lastName();
            // Here, you can do computations based on these data and the fact that @MyCustomAnnotation annotates a method...
        }
    }

Most of the metadata-based frameworks nowadays use annotations. Spring, EJB, JPA, Hibernate Validator are examples of such frameworks.

[Nuwan Arambage]

Hi Roberto,

I think i got your point.please correct me if i'm wrong.okey for a example, in JPA we can use @Entity to annotate classes as entities.when I apply what you said is to my example,JPA reference implementation uses java reflection to map the annotated class into database table.Am I right?.

Hope you reply.I need to clear my doubts becase I use annotations without knowing what you said early.

Thnaks & regards,

Nuwan Aramabge

[Hebert Coelho]

Yes. JPA, Struts, JSF (...) all of those guys use reflection to search for @annotations.

[Nuwan Arambage]

Hebert Coelho wrote:Yes. JPA, Struts, JSF (...) all of those guys use reflection to search for @annotations.

yes, thanks you again for clearing the doubts.......