jdk动态代理实现原理

朴灿烈づ我的快乐病毒、 2021-12-14 00:13 373阅读 0赞

写在前面:

大神和diao炸天的亲请绕道..

关于代理模式的概念这里省去,大家可以放鸟尽情搜..

关于为什么叫动态代理,个人理解是代理的类是在运行时动态生成的,大家也可以参考网上的理解..

文笔很差,所以文字较少,都在代码和注释中..

=======一点不华丽的分割线-------------------------

开门见山,lets go..

java中可以通过jdk提供的 Proxy.newProxyInstance静态方法来创建动态代理对象,下面先来看看这个方法的实现

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
                throws IllegalArgumentException {
            //InvocationHandler不能为空,因为对代理对象的所有方法调用实际上都会委托到InvocationHandler的invoke方法,
            //这个我们后面通过查看产生的代理类的源代码便会一目了然
            if (h == null) {
                throw new NullPointerException();
            }
    
            //这个是核心的地方,通过提供的ClassLoader和interface列表来产生代理类,具体的实现可以参考getProxyClass这个方法的实现,
            //真正的工作是由sun.misc.ProxyGenerator这个类来完成的,可以google查看具体的逻辑.在我们的程序中通过设置
            //System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true")可以查看产生的类文件
            Class cl = getProxyClass(loader, interfaces);
    
            //因为代理类继承了Proxy类.而Proxy中定义了构造函数protected Proxy(InvocationHandler h),所以可以反射得到Constructer实例
            //创建代理对象
            try {
                Constructor cons = cl.getConstructor(constructorParams);
                return (Object) cons.newInstance(new Object[] { h });
            } catch (NoSuchMethodException e) {
                throw new InternalError(e.toString());
            } catch (IllegalAccessException e) {
                throw new InternalError(e.toString());
            } catch (InstantiationException e) {
                throw new InternalError(e.toString());
            } catch (InvocationTargetException e) {
                throw new InternalError(e.toString());
            }
        }

下面通过个例子来说明下:

先来定义一个接口,jdk的动态代理基于接口来创建代理类,不能基于类的原因是java不支持多重继承,而代理类都会继承Proxy类(个人理解).

/**
     * Subject
     * 
     * @author Kevin Fan
     * @since 2013-9-13 下午2:43:33
     */
    public interface Subject {
        void pub(String key, String content);
    
        String sub(String key);
    }

再来一个具体的实现,在代理模式中可以叫它的实例可以叫target,这个是真正执行操作的对象

/**
     * SimpleSubject
     * 
     * @author Kevin Fan
     * @since 2013-9-13 下午2:45:03
     */
    public class SimpleSubject implements Subject {
        private Map<String, String> msg = new ConcurrentHashMap<String, String>();
    
        public void pub(String key, String content) {
            System.out.println("pub msg: key is " + key + ", content is " + content);
            msg.put(key, content);
        }
    
        public String sub(String key) {
            if (msg.containsKey(key)) {
                String ret = msg.get(key);
                System.out.println("sub msg: key is " + key + ", result is " + ret);
                return ret;
            }
    
            return null;
        }
    
    }

好,接下来我们来写个动态代理工厂,根据 不同的target来创建动态代理对象

/**
     * SubjectProxyFactory
     * 
     * @author Kevin Fan
     * @since 2013-9-13 下午2:47:24
     */
    public class SubjectProxyFactory {
        //TODO: cache 
        public static Subject getSubject(final Subject realSubject) {
            return (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(), new Class[] { Subject.class },
                    new InvocationHandler() {
    
                        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                            System.out.println("\naction before method invocation....");
                            Object retVal = method.invoke(realSubject, args);
                            System.out.println("action after method invocation....\n");
                            return retVal;
                        }
                    });
        }
    }

可以看到这是一个简单的实现,只是在真实对象执行前后各打一句信息..

接下来用一个 main函数来把这些结合起来

/**
     * Demo
     * 
     * @author Kevin Fan
     * @since 2013-9-13 下午2:50:28
     */
    public class Demo {
        public static void main(String[] args) {
            //设置此系统属性,以查看代理类文件
            System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
            
            //创建真实对象
            Subject subj = new SimpleSubject();
            subj.pub("name", "kevin.fan");
            subj.sub("name");
            
            //创建代理对象
            Subject proxy = SubjectProxyFactory.getSubject(subj);
            proxy.pub("hobby", "r&b music");
            proxy.sub("name");
        }
    }

ok,小手抖一下,走你,看下执行结果

pub msg: key is name, content is kevin.fan
    sub msg: key is name, result is kevin.fan
    
    
    action before method invocation....
    pub msg: key is hobby, content is r&b music
    action after method invocation....
    
    
    
    
    action before method invocation....
    sub msg: key is name, result is kevin.fan
    action after method invocation....

可以看到在调用代理对象的方法时,添加的额外动作已经生效,接下来我们看下生成的代理类的代码..

import com.aliyun.demo.kevin.coder.lang.proxy.Subject;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    import java.lang.reflect.UndeclaredThrowableException;
    
    //这里很清楚了,代理类继承了Proxy类,并且实现了Proxy.newProxyInstance这个方法中传入的接口
    public final class $Proxy0 extends Proxy
      implements Subject
    {
      
      //这些方法在下面的static init block中进行初始化
      private static Method m4;
      private static Method m1;
      private static Method m3;
      private static Method m0;
      private static Method m2;
      
      static
      {
        try
        {
          m4 = Class.forName("com.aliyun.demo.kevin.coder.lang.proxy.Subject").getMethod("sub", new Class[] { Class.forName("java.lang.String") });
          m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
          m3 = Class.forName("com.aliyun.demo.kevin.coder.lang.proxy.Subject").getMethod("pub", new Class[] { Class.forName("java.lang.String"), Class.forName("java.lang.String") });
          m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
          m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
          return;
        }
        catch (NoSuchMethodException localNoSuchMethodException)
        {
          throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException localClassNotFoundException)
        {
          throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
        }
      }
    
      //构造函数,接收一个 InvocationHandler作为参数,这就是为什么Proxy.newProxyInstance方法里可以
      //通过InvocationHandler实例作为参数来反射获取Constructer实例 
      public $Proxy0 paramInvocationHandler)
        throws 
      {
        super(paramInvocationHandler);
      }
    
      //下面通过这个来看下代理对象中方法是怎样调用的
      public final String sub(String paramString)
        throws 
      {
        try
        {
           //全部是通过调用InvocationHandler的invoke方法,传入对应的方法和参数
          return (String)this.h.invoke(this, m4, new Object[] { paramString });
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final boolean equals(Object paramObject)
        throws 
      {
        try
        {
          return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final void pub(String paramString1, String paramString2)
        throws 
      {
        try
        {
          this.h.invoke(this, m3, new Object[] { paramString1, paramString2 });
          return;
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final int hashCode()
        throws 
      {
        try
        {
          return ((Integer)this.h.invoke(this, m0, null)).intValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final String toString()
        throws 
      {
        try
        {
          return (String)this.h.invoke(this, m2, null);
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
    }

\-------------END

by Kevin Fan

转载于:https://www.cnblogs.com/james1207/p/3320311.html