generate subclass of class without default constructor

Question

I need to generate sub class using cglib for a class that does not have default constructor. I have following code that works fine for class with default constructor:

    Enhancer enhancer = new Enhancer();
    enhancer.setCallbackType(NoOp.class);
    enhancer.setUseCache(false);

    enhancer.setSuperclass(clazz);
    return enhancer.createClass();

New class should have default constructor which need to call some non default constructor from its super class.

I have searched and found that cglib can not do such things and I need to use asm. However I could not find examples of adding default constructor to class.

If someone has an example how to implement it, that would be great.

michael nesterenko · Accepted Answer · 2013-02-14 00:19:39Z

I solved this problem. It appeared a bit different than I imagined before. Cglib inherits all constructors and not only default one as I thought before.

However it appeared that I can not replace constructor without affecting existing cglib constructor construction code. This is minor implication, so I just moved from constructor injection to method injection. I am adding my method call just before constructor returns. And this works!!! I am so happy about it.

This is what I got:

cglib Enhancer invocation

Enhancer enhancer = new Enhancer();
        enhancer.setNamingPolicy(new IndexedNamingPolicy());
        enhancer.setCallbackType(NoOp.class);
        enhancer.setUseCache(false);
        enhancer.setStrategy(new DefaultGeneratorStrategy() {
            @Override
            protected ClassGenerator transform(ClassGenerator cg) throws Exception {
                return new TransformingClassGenerator(cg, new DefaultConstructorEmitter(key));
            }
        });

        enhancer.setSuperclass(clazz);
        return enhancer.createClass();

and my DefaultConstructorEmitter (huh it is still named for constructor processing, never mind)

private class DefaultConstructorEmitter extends ClassEmitterTransformer {
        private final Signature CALL_SIGNATURE = TypeUtils.parseSignature("void someMethod(Object)");

        private String parametersKey;

        public DefaultConstructorEmitter(final String key) {
            parametersKey = key;
        }

        @Override
        public CodeEmitter begin_method(int access, Signature sig, Type[] exceptions) {
            final CodeEmitter emitter = super.begin_method(access, sig, exceptions);
            if (sig.getName().equals(Constants.CONSTRUCTOR_NAME)) {
                return new CodeEmitter(emitter) {
                    @Override
                    public void visitInsn(int arg0) {
                        if (arg0 == Opcodes.RETURN) {
                            Type classType = ...   
                            emitter.load_this();
                            emitter.push(parametersKey);
                            emitter.invoke_static(classType, CALL_SIGNATURE);
                        }
                        super.visitInsn(arg0);
                    }
                };
            }

            return emitter;
        }
    }

Hope this example will help someone not to spent several hours like I did.

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