How to retrieve class type of annotated during annotation processing for code generation?

Question

An annotation processor will provide you with two TypeElement, one for the annotated object and one the annotation itself.

Let's assume one needs to know the Class type of the annotated object during annotation processing to generate code. One can extract the qualified name with TypeElement.getQualifiedName(). However, any attempt to use Class.forName(fullyQualifiedName) throws a ClassNotFoundException.

It probably means the class is not on the path of the annotation processing code. Is it ever possible to retrieve such a class during annotation processing without having to put all annotated code in the classpath of the processing library?

A workaround is to generate something like:

Class c = Class.forName("thefullyqualifiedname");

and use c in the generated code instead, but it is not clean.

Pierre-Yves Ricau · Accepted Answer · 2011-09-12 07:35:10Z

4

It seems quite normal that the "Class" object of an annotated class is not available when annotation processing occurs, because it happens at compile time. That's why TypeElements and TypeMirrors are there, they are the strict equivalent of the Class object.

As you pointed out, an annotation can only access class objects that are on its classpath. And it's not the case of the annotated class, since it's being compiled.

I understand that you think using string qualified names instead of Class is not clean, and you are right. In fact, you should use TypeElement & TypeMirror ;-) .

For what reason do you need a Class object ? I'm quite sure you can do anything with TypeElement & TypeMirror.

answered Sep 12, 2011 at 7:35

Pierre-Yves Ricau

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2 Comments

Jérôme Verstrynge Over a year ago

I need to register the annotated class type in a structure (i.e., a set) via generated code. It must be automated. Hence, I need to retrieve/access it somehow during annotation processing.

Dandre Allison Over a year ago

I am trying to retrieve the classes that implement an annotated interface, for this purpose theres Reflections, but that requires Class objects: getSubTypesOf(). I haven't found another way to find sub types, or maybe find the annotation in the implemented interface.

Jérôme Verstrynge · Accepted Answer · 2011-09-11 16:29:56Z

The best workaround I could come up with is the following:

    JClass annotatedType = jcmSource.ref(Class.class);

    JInvocation m =  annotatedType.staticInvoke("forName");
    m.arg(fa.getAnnotated().toString());

    JFieldVar field = sourceClass.field(
            JMod.PRIVATE | JMod.STATIC, annotatedType, "c");

    JBlock staticInit = sourceClass.init();

    JTryBlock tb = staticInit._try();
    JBlock jbtb = tb.body();
        jbtb.assign(field, m);
    tb._catch(jcmSource.ref(ClassNotFoundException.class));

which produces:

private static Class c;

static {
    try {
        c = Class.forName("net.codegentest.CodeGenTest");
    } catch (ClassNotFoundException _x) {
    }
}

It does not give explicit access to the class type of the annotated class, but at least, it can be generated in the code.

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