Java bytecode generation with generic signature information from reflect.Method

Question

I am trying to bytecode generate a method signature from a java.lang.reflect.Method. The signature(generic type) part of it is tricky as the reflection api to get the type information and transform it into what asm needs is NOT straightforward. Know of any code out there which does this already?

Answer 1

I'd suggest using an existing library like:

• Javassist
• ObjectWebASM
• Byte Code Engineering Library (BCEL)

Answer 2

I am not aware of a tool that does this automatically. I would probably use the org.objectweb.asm.util.ASMifierClassVisitor class to figure out the relationship between the signatures and the ASM API calls.

For a class containing this code:

  public void foo1(Object o1, String s2) {
  }

...the tool will generate:

mv = cw.visitMethod(ACC_PUBLIC, "foo1",
     "(Ljava/lang/Object;Ljava/lang/String;)V", null, null);
mv.visitCode();
mv.visitInsn(RETURN);
mv.visitMaxs(0, 3);
mv.visitEnd();

For this code:

  public static final String[] foo2() {
    return null;
  }

...it will generate:

mv = cw.visitMethod(ACC_PUBLIC + ACC_FINAL + ACC_STATIC,
    "foo2", "()[Ljava/lang/String;", null, null);
mv.visitCode();
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
mv.visitMaxs(1, 0);
mv.visitEnd();

There are notes on method signatures in the FAQ and it helps if you understand Java class nomenclature.

Note that the ASM API can also be used to turn a java.lang.reflect.Method into an org.objectweb.asm.commons.Method. Since you can get the class from the java.lang.reflect.Method, you could use ClassVisitors / MethodVisitors to inspect the methods.

Answer 3

I think I was trying to do something very similar (http://stackoverflow.com/questions/3250701/generating-methods-with-generic-types-with-asm-bytecode-generator-classwriter).

From class visitor's visitMethod, I can see that method like:

public List<String> foobar(List<Integer> args, List<Boolean> args2);

will produce description like:

  (Ljava/util/List;Ljava/util/List;)Ljava/util/List;

(as expected, without generics info)

and signature like:

  (Ljava/util/List<Ljava/lang/Integer;>;Ljava/util/List<Ljava/lang/Boolean;>;)Ljava/util/List<Ljava/lang/String;>;

(with generics added)

so it should be reasonably straight-forward to generate generic signature to augment non-generic description. It's pretty similar to how generics info is added in source, except for oddities introduced by handling of single-character primitive types.

I haven't tried it yet, but given that ClassWriter's visitMethod() also takes signature, I am hoping it will work. :)