carlspring carlspring - 6 months ago 20
Java Question

How to create an instance of an annotation

I am trying to do some Java annotation magic. I must say I am still catching up on annotation tricks and that certain things are still not quite clear to me.

So... I have some annotated classes, methods and fields. I have a method, which uses reflection to run some checks on the classes and inject some values into a class. This all works fine.

However, I am now facing a case where I need an instance (so to say) of an annotation. So... annotations aren't like regular interfaces and you can't do an anonymous implementation of a class. I get it. I have looked around some posts here regarding similar problems, but I can't seem to be able to find the answer to what I am looking for.

I would basically like to get and instance of an annotation and be able to set some of it's fields using reflection (I suppose). Is there at all a way to do this?

Answer

Well, it's apparently nothing all that complicated. Really!

As pointed out by a colleague, you can simply create an anonymous instance of the annotation (like any interface) like this:

MyAnnotation:

public @interface MyAnnotation
{

    String foo();

}

Invoking code:

class MyApp
{
    MyAnnotation getInstanceOfAnnotation(final String foo)
    {
        MyAnnotation annotation = new MyAnnotation()
        {
            @Override
            public String foo()
            {
                return foo;
            }

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

        return annotation;
    }
}

Credits to Martin Grigorov.

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