JDK动态代理的一些认识

         通过学习静态代理，知道如果接口增加了方法，那么委托类和代理类都必须实现，同时在代理类中还要对新增方法做处理，比如日志记录等，很繁琐。而JDK的动态代理就应运而生了，代理类不需要程序员手动预先写好，而是有Proxy类来帮你生成（运行期生成）。先直接看例子。

package proxy; public interface SingInterface { public void sing(); } package proxy; public interface DanceInterface { public void dance(); } package proxy; public class Person implements SingInterface, DanceInterface { @Override public void dance() { System.out.println("I'm dancing......"); } @Override public void sing() { System.out.println("I'm singging......"); } } package proxy; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; /** * 给目标类(被代理类，委托类)的方法执行前和执行后做些日志 * */ public class LogInvocationHandler implements InvocationHandler{ private Object targer;//目标类 public LogInvocationHandler(Object target){ this.targer=target; } /** * 取得有Proxy产生的代理类 */ public Object getProxy(){ return Proxy.newProxyInstance( /*取得当前线程使用的加载器*/Thread.currentThread().getContextClassLoader(), /*目标类都实现了哪些接口，告诉Proxy，Proxy生成的代理类也将会同样实现*/this.targer.getClass().getInterfaces(), /*将自身传给Proxy,Proxy生成的代理类将会用它调用下面的invoke方法*/this); } @Override /** * proxy就是Proxy类生成的代理类 * 初步学习JDK的动态代理，这里最容不明白，到底是谁调用了它 */ public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println(method.getName()+"准备执行......"); Object result=method.invoke(this.targer, args);//执行的是目标类的方法 System.out.println(method.getName()+"执行结束......"); return result; } } package proxy; /** *测试类 *给Person的sing和dance方法执行前和执行后做些日志 */ public class Client { public static void main(String[] args){ Person person=new Person(); //取得将对person进行加日志的代理类 Object proxy=new LogInvocationHandler(person).getProxy(); //对sing方法加日志 ((SingInterface)proxy).sing(); //对dance方法加日志 ((DanceInterface)proxy).dance();//可知proxy一定同时实现了SingInterface和DanceInterface接口 } } 运行结果 sing准备执行...... I'm singging...... sing执行结束...... dance准备执行...... I'm dancing...... dance执行结束......   下面来看看Proxy.newInstance方法 public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { if (h == null) { throw new NullPointerException(); } /* * Look up or generate the designated proxy class. * 取得生成的代理类的Class类 */ Class<?> cl = getProxyClass0(loader, interfaces); // stack walk magic: // do not refactor /* * Invoke its constructor with the designated invocation handler. */ try { //取得代理类参数类型为InvocationHandler.class的构造方法 final Constructor<?> cons = cl.getConstructor(constructorParams); final InvocationHandler ih = h; SecurityManager sm = System.getSecurityManager(); if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) { // create proxy instance with doPrivilege as the proxy class may // implement non-public interfaces that requires a special // permission return AccessController .doPrivileged(new PrivilegedAction<Object>() { public Object run() { //把实现了InvocationHandler接口的对象给代理类 //看上面的例子中LogInvocationHandler类中getProxy方法 //里面获取代理类时，第三个参数是不是this(就是指logInvocationHandler) return newInstance(cons, ih); } }); } else { return newInstance(cons, ih); } } catch (NoSuchMethodException e) { throw new InternalError(e.toString()); } }       看了上面的源码，得知重点转移到getProxyClass0(loader, interfaces); 这个方法里面了，进去看了 下，源码挺多的，重要的是后面那部分。 /* * Generate the specified proxy class. */ byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces); try { proxyClass = defineClass0(loader, proxyName, proxyClassFile, 0, proxyClassFile.length); } catch (ClassFormatError e) { /* * A ClassFormatError here means that (barring bugs in the * proxy class generation code) there was some other * invalid aspect of the arguments supplied to the proxy * class creation (such as virtual machine limitations * exceeded). */ throw new IllegalArgumentException(e.toString()); } } // add to set of all generated proxy classes, for isProxyClass proxyClasses.put(proxyClass, null); } finally { /* * We must clean up the "pending generation" state of the proxy * class cache entry somehow. If a proxy class was successfully * generated, store it in the cache (with a weak reference); * otherwise, remove the reserved entry. In all cases, notify * all waiters on reserved entries in this cache. */ synchronized (cache) { if (proxyClass != null) { cache.put(key, new WeakReference(proxyClass)); } else { cache.remove(key); } cache.notifyAll(); } } return proxyClass; }      再继续跟踪ProxyGenerator.generateProxyClass(    proxyName, interfaces); public static byte[] generateProxyClass(String paramString, Class[] paramArrayOfClass) { ProxyGenerator localProxyGenerator = new ProxyGenerator(paramString, paramArrayOfClass); byte[] arrayOfByte = localProxyGenerator.generateClassFile(); if (saveGeneratedFiles) AccessController.doPrivileged(new PrivilegedAction(paramString, arrayOfByte) { public Object run() { try { FileOutputStream localFileOutputStream = new FileOutputStream(ProxyGenerator.access$000(this.val$name) + ".class"); localFileOutputStream.write(this.val$classFile); localFileOutputStream.close(); return null; } catch (IOException localIOException) { } throw new InternalError("I/O exception saving generated file: " + localIOException); } }); return arrayOfByte; }       到这就不再跟踪了，有兴趣的同学，可以自己去看源码吧。       现在来看看Proxy给我们生成的代理类是怎样的。 package proxy; import java.io.FileOutputStream; import java.io.IOException; import sun.misc.ProxyGenerator; /** * 这个类来源http://rejoy.iteye.com/blog/1627405 * */ public class ProxyGeneratorUtils { /** * 把代理类的字节码写到硬盘上 * @param path 保存路径 */ public static void writeProxyClassToHardDisk(String path) { // 获取代理类的字节码 byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy11", Person.class.getInterfaces()); FileOutputStream out = null; try { out = new FileOutputStream(path); out.write(classFile); out.flush(); } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } } } public static void main(String[] args){ ProxyGeneratorUtils.writeProxyClassToHardDisk("E:/$Proxy11.class"); } }     用反编译工具来看看生成出来的$Proxy11.class   import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.lang.reflect.UndeclaredThrowableException; import proxy.DanceInterface; import proxy.SingInterface; public final class $Proxy11 extends Proxy implements SingInterface, DanceInterface { private static Method m1; private static Method m4; private static Method m3; private static Method m0; private static Method m2; public $Proxy11(InvocationHandler paramInvocationHandler) throws { super(paramInvocationHandler); } /** *由于 任何类都是Object的子类，所以Person类也拥有Object的方法 *所以这里也进行了处理 */ 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 void dance() throws { try { //看到LogInvocationHandler中的invoke()方法是在这被调用的 this.h.invoke(this, m4, null); return; } catch (RuntimeException localRuntimeException) { throw localRuntimeException; } catch (Throwable localThrowable) { } throw new UndeclaredThrowableException(localThrowable); } public final void sing() throws { try { this.h.invoke(this, m3, null); return; } 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 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") }); m4 = Class.forName("proxy.DanceInterface").getMethod("dance", new Class[0]); m3 = Class.forName("proxy.SingInterface").getMethod("sing", new Class[0]); 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()); } }     可以看到代理类实现了和目标类一样的接口，同时代理类调用了InvocationHandler中的invoke（）方法，然后InvocationHandler中的invoke()方法调用目标类中的真正方法。