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
d140
, 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。