generics - Find type parameter of method return type in Java 6 annotation processor

Question

I'm writing a tool that uses the annotation processor to generate source code depending on the return type of methods of an annotated class. The return type is always some subtype (interface or class) of an interface A that defines a type variable T.

interface A<T>{T m();};

I would like to find the type parameter for the method m() return value type variable T.

The return type is represented by the annotation processor as a javax.lang.model.type.TypeMirror instance. The simplest case is to return A<T> directly.

@SomeAnnotation
class SomeClass{
    A<T> x();
}

The processor code to find out T is quite simple. (I'll cast instead of using the visitor API here to keep the code simple.)

DeclaredType type = (DeclaredType) typeMirror;
TypeMirror t = type.getTypeArguments().get(0);

The TypeMirror of the return type is a javax.lang.model.type.DeclaredType and T is the first type argument. The result t is a javax.lang.model.type.TypeVariable for T. The same works for a concrete return type A<B> (B is some type: interface B{}). The result for t is a DeclaredType representing B.

Things start to get complicated with other result types:

interface Subtype<T> extends A<T>{}
interface Concrete extends A<B>{};
interface Multiple<B,T> extends A<T>{}
interface Bounds<T extends B> extends A<T>{}
interface Hierarchy extends Concrete{}

Subtype<B>          -> DeclaredType B
Subtype<T>          -> TypeVariable T
Concrete            -> DeclaredType B
Multiple<B,T>       -> TypeVariable T or DeclaredType B depeding on Multiple
Multiple<B,B>       -> TypeVariable B
<T extends B> A<T>  -> TypeVariable T with super class bound B
Bound<B>            -> DeclaredType B
Bound<C>            -> DeclaredType C (subtype of B)
Hierarchy           -> TypeVariable T

Is there a way to find the correct type parameter for T without mirroring the whole java type system?

Answer 1

Have a look at http://docs.oracle.com/javase/6/docs/api/javax/lang/model/util/Types.html#asMemberOf%28javax.lang.model.type.DeclaredType,%20javax.lang.model.element.Element%29

I used it to solve this problem and contributed the solution to the WsDoc project in this pull request: https://github.com/versly/wsdoc/pull/7

I did something like this:

      Type.MethodType methodType = (Type.MethodType) processingEnv.getTypeUtils().asMemberOf(declaredTypeThatExtendsSomeGenericParent, methodToGetReturnTypeForAsExecutableElement);
      TypeMirror type = methodType.getReturnType();