Question

Defining simple getters and setters is easy using Asm (and fortunately it is even explained in their FAQ). But one thing that is not mentioned, and for which I have been unable to find documentation, is how to implement these using generic type information.

I am actually able to determine generic type information itself quite easily (since code will take existing fields and/or methods and full generic type handling and resolution exists). I just need to generate generics version for types that have generic type included.

I hope this is something as easy as modifying signature Asm ClassWriter/MethodVisitor calls take, but some comments in documentation indicate it might not be that easy (as generics information is stored in bit different place than regular info).

EDIT: looks like entry point is “ClassWriter.visitField/Method(…., String signature) — note that it’s “description” that contains normal non-generic class information, but term “signature” (in JLS) specifically refers to generics-including type information.

Answer 1

You can build the signature using ASM’s SignatureWriter class.

For example, suppose you wish to write the signature for this method:

public <K> void doSomething(K thing)

You could use this code:

SignatureWriter signature = new SignatureWriter();
signature.visitFormalTypeParameter("K");

// Ensure that <K> extends java.lang.Object
{
    SignatureVisitor classBound = signature.visitClassBound();
    classBound.visitClassType(Type.getInternalName(Object.class));
    classBound.visitEnd();
}

// The parameter uses the <K> type variable
signature.visitParameterType().visitTypeVariable("K");

// The return type uses the void primitive ('V')
signature.visitReturnType().visitBaseType('V');

signature.visitEnd();

String signatureString = signature.toString();

Which is equivalent to:

String signatureString = "<K:Ljava/lang/Object;>(TK;)V;"