动态代理[JDK]机制解析

代理是一种常用的设计模式，其目的就是为其他对象提供一个代理以控制对某个对象的访问。代理类负责为委托类预处理消息，过滤消息并转发消息，以及进行消息被委托类执行后的后续处理。

动态代理是一种比较常用的代理方式，也许你已经很熟悉它的使用了，但是它的实现原理你是否搞懂？不得不说搞懂动态代理的机制是很有必要的。为什么？当前十分火爆的Retrofit你应该很熟悉，没错，Retrofit就使用了动态代理。这么牛逼的框架都在使用的技术，你还不来学学它的原理么。

注意：JDK动态代理要求被代理类必须实现接口，而且对于private方法JDK动态代理也是无能无力的。当然，你会说那还玩毛线，直接用CGLIB不就行了，没错，CGLIB直接支持类，但是很遗憾，CGLIB不能再Android中使用，Android虚拟机还是与JVM有不同之处 的。

动态代理的特点是编译阶段没有代理类在运行时才生成代理类。

以下分析基于JDK1.7

示例下载（包含生成的代理类字节码文件，可以反编译查看）：http://download.csdn.net/download/json_it/10148805

1、使用示例

定义接口：

public interface ITestDynamicProxy {void doSomething();}

实现类：

public class TestDynamicProxy implements ITestDynamicProxy {    @Override    public void doSomething() {        System.out.println("doSomething()");    }}

自定义InvocationHandler：

public class MyInvoxationHandler implements InvocationHandler {    private Object proxied;    public MyInvoxationHandler(Object proxied) {        this.proxied = proxied;    }    @Override    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {    System.out.println("你可以在这里做一些其他的工作=====");        return method.invoke(proxied, args);    }}

动态代理使用：

public class MyDynamicProxy {public static void main(String[] args) {TestDynamicProxy testDynamicProxy = new TestDynamicProxy();ITestDynamicProxy proxy = (ITestDynamicProxy) Proxy.newProxyInstance(ITestDynamicProxy.class.getClassLoader(),new Class[] { ITestDynamicProxy.class },new MyInvoxationHandler(testDynamicProxy));proxy.doSomething();}}

----> 你可以在这里做一些其他的工作=====

doSomething()

可以看到，代码量并不是很大。而且看起来也不是很复杂。说实话，以前初次接触动态代理的时候，我还是很疑惑的。这简短的几行代码背后到底发生了什么？动态代理给开发者带来了哪些惊喜呢？

2、原理分析

Proxy.newProxyInstance（...）是动态代理的入口，它为我们生成了代理类：

public static Object newProxyInstance(ClassLoader loader,                                          Class<?>[] interfaces,                                          InvocationHandler h)

该方法有三个参数：

ClassLoader：被代理类的类加载器；

Class<?> interfaces：被代理类实现的接口数组；
InvocationHandler：与被代理类关联的handler;

 @CallerSensitive    public static Object newProxyInstance(ClassLoader loader,                                          Class<?>[] interfaces,                                          InvocationHandler h)        throws IllegalArgumentException    {        if (h == null) {            throw new NullPointerException();        }        final Class<?>[] intfs = interfaces.clone();        final SecurityManager sm = System.getSecurityManager();        if (sm != null) {            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);        }        /*         * Look up or generate the designated proxy class.         */        Class<?> cl = getProxyClass0(loader, intfs);//关键方法，获取代理类        /*         * Invoke its constructor with the designated invocation handler.         */        try {            final Constructor<?> cons = cl.getConstructor(constructorParams);            final InvocationHandler ih = h;            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() {                        return newInstance(cons, ih);                    }                });            } else {                return newInstance(cons, ih);//生成代理类的实例            }        } catch (NoSuchMethodException e) {            throw new InternalError(e.toString());        }    }   private static Object newInstance(Constructor<?> cons, InvocationHandler h) {        try {            return cons.newInstance(new Object[] {h} );//生成代理类的实例        } catch (IllegalAccessException | InstantiationException e) {            throw new InternalError(e.toString());        } catch (InvocationTargetException e) {            Throwable t = e.getCause();            if (t instanceof RuntimeException) {                throw (RuntimeException) t;            } else {                throw new InternalError(t.toString());            }        }    }

可以看到，比较关键的方法是getProxyClass0（..）方法、

 private static Class<?> getProxyClass0(ClassLoader loader,                                           Class<?>... interfaces) {        if (interfaces.length > 65535) {            throw new IllegalArgumentException("interface limit exceeded");        }        // If the proxy class defined by the given loader implementing        // the given interfaces exists, this will simply return the cached copy;        // otherwise, it will create the proxy class via the ProxyClassFactory        return proxyClassCache.get(loader, interfaces);    }

在这个方法中，我们发现其使用了缓存，通过loader和interfaces来获取代理类。有了缓存，我们很自然的就会想到，如果缓存中存在，则直接使用。否则，生成新的代理类。关于如何缓存，不再介绍。我们只需抓住关键思想，毕竟缓存不是我们分析的重点。

那么，这个proxyClassCache是一个什么东西呢？

private static final WeakCache<ClassLoader, Class<?>[], Class<?>>        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

它的两个参数让我们想到，第一个应该是用于产生Key的工厂，而第二个则是用于生成代理Class的工厂。

 public V get(K key, P parameter) {        Objects.requireNonNull(parameter);        expungeStaleEntries();        Object cacheKey = CacheKey.valueOf(key, refQueue);        // lazily install the 2nd level valuesMap for the particular cacheKey        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);        if (valuesMap == null) {            ConcurrentMap<Object, Supplier<V>> oldValuesMap                = map.putIfAbsent(cacheKey,                                  valuesMap = new ConcurrentHashMap<>());            if (oldValuesMap != null) {                valuesMap = oldValuesMap;            }        }        // create subKey and retrieve the possible Supplier<V> stored by that        // subKey from valuesMap        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));        Supplier<V> supplier = valuesMap.get(subKey);        Factory factory = null;        while (true) {            if (supplier != null) {                // supplier might be a Factory or a CacheValue<V> instance                V value = supplier.get();//关键方法                if (value != null) {                    return value;                }            }            // else no supplier in cache            // or a supplier that returned null (could be a cleared CacheValue            // or a Factory that wasn't successful in installing the CacheValue)            // lazily construct a Factory            if (factory == null) {                factory = new Factory(key, parameter, subKey, valuesMap);            }            if (supplier == null) {                supplier = valuesMap.putIfAbsent(subKey, factory);                if (supplier == null) {                    // successfully installed Factory                    supplier = factory;                }                // else retry with winning supplier            } else {                if (valuesMap.replace(subKey, supplier, factory)) {                    // successfully replaced                    // cleared CacheEntry / unsuccessful Factory                    // with our Factory                    supplier = factory;                } else {                    // retry with current supplier                    supplier = valuesMap.get(subKey);                }            }        }    }

这段代码的主要思想是这样的：

查看缓存中是否存在 --> 如果存在，则直接返回supplier,get（）-->如果不存在，则先生成Supplier（Factory是Supplier的子类），然后通过get（）方法返回代理类。

现在最关键的就是这个get方法：

@Override        public synchronized V get() { // serialize access            // re-check            Supplier<V> supplier = valuesMap.get(subKey);            if (supplier != this) {                // something changed while we were waiting:                // might be that we were replaced by a CacheValue                // or were removed because of failure ->                // return null to signal WeakCache.get() to retry                // the loop                return null;            }            // else still us (supplier == this)            // create new value            V value = null;            try {                value = Objects.requireNonNull(valueFactory.apply(key, parameter));            } finally {                if (value == null) { // remove us on failure                    valuesMap.remove(subKey, this);                }            }            // the only path to reach here is with non-null value            assert value != null;            // wrap value with CacheValue (WeakReference)            CacheValue<V> cacheValue = new CacheValue<>(value);            // try replacing us with CacheValue (this should always succeed)            if (valuesMap.replace(subKey, this, cacheValue)) {                // put also in reverseMap                reverseMap.put(cacheValue, Boolean.TRUE);            } else {                throw new AssertionError("Should not reach here");            }            // successfully replaced us with new CacheValue -> return the value            // wrapped by it            return value;        }    }

我们终于发现了最关键的信息：

value = Objects.requireNonNull(valueFactory.apply(key, parameter));这个valueFactory是什么？就是ProxyClassFactory。

   @Override        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);            for (Class<?> intf : interfaces) {                /*                 * Verify that the class loader resolves the name of this                 * interface to the same Class object.                 */                Class<?> interfaceClass = null;                try {                    interfaceClass = Class.forName(intf.getName(), false, loader);                } catch (ClassNotFoundException e) {                }                if (interfaceClass != intf) {                    throw new IllegalArgumentException(                        intf + " is not visible from class loader");                }                /*                 * Verify that the Class object actually represents an                 * interface.                 */                if (!interfaceClass.isInterface()) {                    throw new IllegalArgumentException(                        interfaceClass.getName() + " is not an interface");                }                /*                 * Verify that this interface is not a duplicate.                 */                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {                    throw new IllegalArgumentException(                        "repeated interface: " + interfaceClass.getName());                }            }            String proxyPkg = null;     // package to define proxy class in            /*             * Record the package of a non-public proxy interface so that the             * proxy class will be defined in the same package.  Verify that             * all non-public proxy interfaces are in the same package.             */            for (Class<?> intf : interfaces) {                int flags = intf.getModifiers();                if (!Modifier.isPublic(flags)) {                    String name = intf.getName();                    int n = name.lastIndexOf('.');                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));                    if (proxyPkg == null) {                        proxyPkg = pkg;                    } else if (!pkg.equals(proxyPkg)) {                        throw new IllegalArgumentException(                            "non-public interfaces from different packages");                    }                }            }            if (proxyPkg == null) {                // if no non-public proxy interfaces, use com.sun.proxy package                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";            }            /*             * Choose a name for the proxy class to generate.             */            long num = nextUniqueNumber.getAndIncrement();            String proxyName = proxyPkg + proxyClassNamePrefix + num;            /*             * Generate the specified proxy class.             */            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(                proxyName, interfaces);            try {                return 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());            }        }    }

关键信息：

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);

由此，我们找到了最终JDK是通过ProxyGenerator的generateProxyClass方法产生了最终的字节码。我们也可以利用这个类的方法将字节码保存到本地，然后反编译看一下代理类庐山真面目。

public class MyDynamicProxy {public static void main(String[] args) {TestDynamicProxy testDynamicProxy = new TestDynamicProxy();ITestDynamicProxy proxy = (ITestDynamicProxy) Proxy.newProxyInstance(ITestDynamicProxy.class.getClassLoader(),new Class[] { ITestDynamicProxy.class },new MyInvoxationHandler(testDynamicProxy));proxy.doSomething();createProxyClassFile();}private static void createProxyClassFile() {String name = "ProxySubject";byte[] data = ProxyGenerator.generateProxyClass(name,new Class[] { ITestDynamicProxy.class });FileOutputStream out = null;try {out = new FileOutputStream(name + ".class");System.out.println((new File("hello")).getAbsolutePath());out.write(data);} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();} finally {if (null != out)try {out.close();} catch (IOException e) {e.printStackTrace();}}}}

找到我们生成的ProxySubject.class，反编译看下：

import com.example.proxy.ITestDynamicProxy;import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.lang.reflect.Proxy;import java.lang.reflect.UndeclaredThrowableException;public final class ProxySubject  extends Proxy  implements ITestDynamicProxy{  private static Method m1;  private static Method m0;  private static Method m3;  private static Method m2;    public ProxySubject(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 (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 void doSomething()    throws   {    try    {      this.h.invoke(this, m3, null);      return;    }    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);    }  }    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("com.example.proxy.ITestDynamicProxy").getMethod("doSomething", 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());    }  }}

代理类是proxy的子类，实现了代理接口。除了生成常用的几个通用的方法外，还生成了代理接口方法：

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

可以看到对代理类的代理接口的调用会被转发为由InvocationHandler这个接口的 invoke（对方法的增强就写在这里面） 方法来进行调用。这也是为什么需要我们来实现InvocationHandler，并在其invoke方法中完成主要工作的原因了。