Struts2 中出现的Jdk动态署理实例和方法

package com.opensymphony.xwork2.inject;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;
import java.util.List;
import java.util.ArrayList;

/**
 * Context of a dependency construction. Used to manage circular references.
 *
 * @author crazybob@google.com (Bob Lee)
 */
class ConstructionContext<T> {

  T currentReference;
  boolean constructing;

  List<DelegatingInvocationHandler<T>> invocationHandlers;

  T getCurrentReference() {
    return currentReference;
  }

  void removeCurrentReference() {
    this.currentReference = null;
  }

  void setCurrentReference(T currentReference) {
    this.currentReference = currentReference;
  }

  boolean isConstructing() {
    return constructing;
  }

  void startConstruction() {
    this.constructing = true;
  }

  void finishConstruction() {
    this.constructing = false;
    invocationHandlers = null;
  }
   
 //此处应该是该方法的核心
//
  Object createProxy(Class<? super T> expectedType) {
    // TODO: if I create a proxy which implements all the interfaces of
    // the implementation type, I'll be able to get away with one proxy
    // instance (as opposed to one per caller).

    if (!expectedType.isInterface()) {
      throw new DependencyException(
          expectedType.getName() + " is not an interface.");
    }

    if (invocationHandlers == null) {
      invocationHandlers = new ArrayList<DelegatingInvocationHandler<T>>();
    }

    DelegatingInvocationHandler<T> invocationHandler =
        new DelegatingInvocationHandler<T>();
    invocationHandlers.add(invocationHandler);
    
    return Proxy.newProxyInstance(
      expectedType.getClassLoader(), // 加载接口的类加载器
      new Class[] { expectedType }, // 接口集合
      invocationHandler  // handler处理类
    );
  }
 // 注意此处调用方法 中的参数 和值  认真看看希望能看出端倪来。此处很妙
  void setProxyDelegates(T delegate) {
    if (invocationHandlers != null) {
      for (DelegatingInvocationHandler<T> invocationHandler : invocationHandlers) {
        invocationHandler.setDelegate(delegate);
      }
    }
  }
// 内部静态类
  static class DelegatingInvocationHandler<T> implements InvocationHandler {

    T delegate;

    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable {
      if (delegate == null) {
        throw new IllegalStateException(
            "Not finished constructing. Please don't call methods on this"
                + " object until the caller's construction is complete.");
      }

      try {     // method.invoke 中的参数 代理类  和要传入的参数
        return method.invoke(delegate, args); 
                //method方法应该是delegate接口中的
      } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
      } catch (IllegalArgumentException e) {
        throw new RuntimeException(e);
      } catch (InvocationTargetException e) {
        throw e.getTargetException();
      }
    }
    
    // 此处为何留一个接口方法呢？
    void setDelegate(T delegate) {
      this.delegate = delegate;
    }
  }
}