java反射机制与动态代理

1 java反射机制：运行时的类信息

Class类与java.lang.reflect类库一起对反射概念进行了支持，该类库包含了Field，Method以及Constructor类。这些类型的对象是由JVM在运行时创建的，用以表示未知类里对应的成员。这样就可以使用Constructor创建新的对象，用get（）与set（）方法读取与修改与Field对象相关的字段，用invoke（）方法调用与Method对象相关的方法等等。

2 动态代理：

代理模式：为了提供额外的或不同的操作，而插入的用来代替“实际”对象。这些操作通常涉及与“实际”对象的通讯，因此代理通常充当着中间人的角色。

动态代理：所有的调用都会被重定向到单一的调用处理器上，他的工作是揭示调用的类型并确定相应的对策。

java反射机制实现动态代理的源码：

interface Interface {
  void doSomething();
  void somethingElse(String arg);
}

class RealObject implements Interface {
  public void doSomething() { print("doSomething"); }
  public void somethingElse(String arg) {
    print("somethingElse " + arg);
  }
}

import java.lang.reflect.*;

class DynamicProxyHandler implements InvocationHandler {
	private Object proxied;

	public DynamicProxyHandler(Object proxied) {
		this.proxied = proxied;
	}

	public Object invoke(Object proxy, Method method, Object[] args)
			throws Throwable {
		System.out.println("**** proxy: " + proxy.getClass() + ", method: "
				+ method + ", args: " + args);
		if (args != null)
			for (Object arg : args)
				System.out.println("  " + arg);
		return method.invoke(proxied, args);
	}
}

public class SimpleDynamicProxy {
	public static void consumer(Interface iface) {
		iface.doSomething();
		iface.somethingElse("bonobo");
	}

	public static void main(String[] args) {
		RealObject real = new RealObject();
		consumer(real);
		// Insert a proxy and call again:
		Interface proxy = (Interface) Proxy.newProxyInstance(Interface.class
				.getClassLoader(), new Class[] { Interface.class },
				new DynamicProxyHandler(real));
		consumer(proxy);
	}
}

 当我们查看java.lang.reflect.Proxy源码，我们发现起核心作用的是ProxyGenerator.generateProxyClass(String paramString, Class[] paramArrayOfClass)，最令人疑惑的问题是，代理对象和如何调用DynamicProxyHandler的invoke方法的，从源码里面我们很难发现如何处理的，对于ProxyGenerator写了一个测试类ProxyClassFile

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import sun.misc.ProxyGenerator;

public class ProxyClassFile {

	public static void main(String[] args) {

		String proxyName = "SimpleDynamicProxy";

		RealObject t = new RealObject();

		Class[] interfaces = t.getClass().getInterfaces();

		byte[] proxyClassFile = ProxyGenerator.generateProxyClass(

		proxyName, interfaces);

		File f = new File("E:/workspace/TIJ4/bin/typeinfo/SimpleDynamicProxy.class");

		try {

			FileOutputStream fos = new FileOutputStream(f);

			fos.write(proxyClassFile);

			fos.flush();

			fos.close();

		} catch (FileNotFoundException e) {

			e.printStackTrace(); // To change body of catch statement use File |
									// Settings | File Templates.

		} catch (IOException e) {

			e.printStackTrace(); // To change body of catch statement use File |
									// Settings | File Templates.

		}

	}

}

反编译SimpleDynamicProxy.class，代码初看起来有点复杂，仔细观察还是很有规律的，将SimpleDynamicProxy5个方法都重定向到invoke（）方法，equals（），hashCode（）和toString（）都是父类Object方法，doSomething（）和somethingElse（）为接口方法。

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import typeinfo.Interface;

public final class SimpleDynamicProxy extends Proxy
  implements Interface
{
  private static Method m1;
  private static Method m0;
  private static Method m3;
  private static Method m4;
  private static Method m2;

  public SimpleDynamicProxy(InvocationHandler paramInvocationHandler)
    throws 
  {
    super(paramInvocationHandler);
  }

  public final boolean equals(Object paramObject)
    throws 
  {
    try
    {
      return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final int hashCode()
    throws 
  {
    try
    {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final void doSomething()
    throws 
  {
    try
    {
      this.h.invoke(this, m3, null);
      return;
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final void somethingElse(String paramString)
    throws 
  {
    try
    {
      this.h.invoke(this, m4, new Object[] { paramString });
      return;
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final String toString()
    throws 
  {
    try
    {
      return (String)this.h.invoke(this, m2, null);
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  static
  {
    try
    {
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      m3 = Class.forName("typeinfo.Interface").getMethod("doSomething", new Class[0]);
      m4 = Class.forName("typeinfo.Interface").getMethod("somethingElse", new Class[] { Class.forName("java.lang.String") });
      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());
    }
  }
}

 通过源代码我们不难看出，代理类是如何调用invoke方法的了。