Why Spring-proxy uses delegate pattern instead of inheritance+super?

Question

As we all know, the self-invokation of bean's method is not working in Spring without AspectJ.

See this question for example.

I think this is because the Spring-created proxy calls the target object's methods using delagate pattern. Like this:

class MyClass {

    @Autowired
    private MyClass self; // actually a MyProxy instance

    @Transactional // or any other proxy magic
    public void myMethod() {}

    public void myOtherMethod() {
        this.myMethod(); // or self.myMethod() to avoid self-invokation problem
    }
}

class MyProxy extends MyClass { // or implements MyInterface if proxyMode is not TARGET_CLASS and MyClass also implements MyInterface

    private final MyClass delegate;

    @Override
    public void myMethod() {
        // some proxy magic: caching, transaction management etc
        delegate.myMethod();
        // some proxy magic: caching, transaction management etc
    }

    @Override
    public void myOtherMethod() {
        delegate.myOtherMethod();
    }
}

Am I right?

With this code:

public void myOtherMethod() {
    this.myMethod();
}

this.myMethod() will bypass the proxy (so all @Transactional or @Cacheable magic) because it is just internal delegate's call... So we should inject a MyClass bean (which is actually is MyProxy instance) inside MyClass and call self.myMethod() instead. It is understandable.

But why the proxy is implemented this way? Why it is not just extends the target class, overriding all public methods and calling super instead of delegate? Like this:

class MyProxy extends MyClass {
    // private final MyClass delegate; // no delegate
    @Override
    public void myMethod() {
        // some proxy magic: caching, transaction management etc
        super.myMethod();
        // some proxy magic: caching, transaction management etc
    }
    @Override
    public void myOtherMethod() {
        super.myOtherMethod();
    }
}

It should solve the self-invokation problem, where this.myMethod() bypasses the proxy, because in this case this.myMethod(), invoked from MyClass.myOtherMethod() (we remember that MyClass bean actually is MyProxy instance), will invoke overriden child's method (MyProxy.myMethod()).

So, my main question is why it is not implemented this way?

Answer 1

Your assumption that Spring AOP uses delegation for its proxies is correct. This is also documented.

Using CGLIB, you can theoretically use proxy.invokeSuper() in order to achieve the effect you want, i.e. that self-invocation is registered by the aspect implemented by the proxy's method interceptor (I am using Spring's embedded version of CGLIB here, thus the package names):

package spring.aop;

import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

class SampleClass {
  public void x() {
    System.out.println("x");
    y();
  }

  public void y() {
    System.out.println("y");
  }

  public static void main(String[] args) {
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(SampleClass.class);
    enhancer.setCallback(new MethodInterceptor() {
      @Override
      public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy)
        throws Throwable {
        if(method.getDeclaringClass() == Object.class)
          return proxy.invokeSuper(obj, args);
        System.out.println("Before proxy.invokeSuper " + method.getName());
        Object result = proxy.invokeSuper(obj, args);
        System.out.println("After proxy.invokeSuper " + method.getName());
        return result;
      }
    });
    SampleClass proxy = (SampleClass) enhancer.create();
    proxy.x();
  }
}

Console log:

Before proxy.invokeSuper x
x
Before proxy.invokeSuper y
y
After proxy.invokeSuper y
After proxy.invokeSuper x

This is exactly what you want. The problem starts, however, when you have several aspects: transactions, logging, whatever else. How do you make sure that they all work together?

• Option 1: Each aspect gets its own proxy. This obviously will not work unless you nest the proxies into each other according to aspect precedence. But nesting them into each other means inheritance, i.e. one proxy would have to inherit from the other outside-in. Try proxying a CGLIB proxy, it does not work, you get exceptions. Furthermore, CGLIB proxies are quite expensive and use perm-gen memory, see descriptions in this CGLIB primer.

• Option 2: Use composition instead of inheritance. Composition is more flexible. Having one proxy to which you can register aspects as needed solves the inheritance problem, but also means delegation: The proxy registers the aspects and calls their methods during runtime in the right order before/after the actual real object's code is executed (or not, if an @Around advice never calls proceed()). See this example from the Spring manual about manually registering aspects to a proxy:

// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);

// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);

// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
factory.addAspect(usageTracker);

// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();

As to why the Spring developers chose this approach and whether it might have been possible to use the one-proxy approach but still make sure that self-invocation works like in my little CGLIB sample "logging aspect" above, I can only speculate. You can maybe ask them on the developers mailing list or look into the source code. Maybe the reason was that CGLIB proxies should behave similarly to the default Java dynamic proxies so as to make switching between the two for interface types seamless. Maybe the reason is another one.

I did not mean to be rude in my comments, only straightforward, because your question is really not suited to StackOverflow because it is not a technical problem to which someone can find a solution. It is a historical design question and rather philosophic in nature because with AspectJ a solution to your technical problem (self-invocation) beneath the actual question already exists. But maybe you still want to dive into the Spring source code, change the Spring AOP implementation from delegation to proxy.invokeSuper() and file a pull request. I am not sure such a breaking change would be accepted, though.

Answer 2

In addition, you will not able to use Inheritance + super in the following cases:

• What about if the RealSubject is final, so the proxy will can NOT extends it
• What about if the Proxy needs to extend something other than the RealSubject
• What about if you need to hide some functionality (methods) inside the RealSubject
• Prefer Composition over Inheritance (recommended by many developers)