Java动态代理实现原理浅析

代码编写

1. 接口编写
   public interface TargetInterface {
void targetMethod();
}
2. 实现类编写
   public class Target implements TargetInterface {
@Override
public void targetMethod() {
System.out.println("target method invoking ...");
}
}
3. 增强类编写
   public class ProxyHandler implements InvocationHandler {
private Object proxyTarget;
public ProxyHandler(Object proxyTarget) {
this.proxyTarget = proxyTarget;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("do somethings before target method invoke");
Object result = method.invoke(proxyTarget, args);
System.out.println("do somethings after target method invoke");
return result;
}
}
4. 测试类编写
   public class ProxyTest {
public static void main(String[] args) {
System.out.println(Thread.currentThread().getContextClassLoader());
TargetInterface target = new Target();
ClassLoader loader = target.getClass().getClassLoader();
Class[] interfaces = target.getClass().getInterfaces();
InvocationHandler proxyHandler = new ProxyHandler(target);
TargetInterface proxyTarget = (TargetInterface) Proxy.newProxyInstance(loader, interfaces, proxyHandler);
proxyTarget.targetMethod();
}
}

源码分析(源码篇幅过多,省略部分由“…”表示)

代码中proxyTarget增强类是由Proxy.newProxyInstance(…)生成的，查看该方法源码：
``public static Object newProxyInstance(ClassLoader loader,Class[] interfaces,InvocationHandler h) throws IllegalArgumentException{
...
// step1:从缓存中查找代理类,若未缓存则生成后缓存,否则直接从缓存中获取.
Class cl = getProxyClass0(loader, intfs);
try {
...
//step2:反射生成代理类实例
return newInstance(cons, ih);
...
} catch (NoSuchMethodException e) {
...
}
}
代码中可以看出，代理类的生成主要分为两步：
- 生成代理类
- 反射生成代理类实例

step1:

1. Proxy.getProxyClass0
   private static Class getProxyClass0(ClassLoader loader,Class... interfaces) {
...
// Proxy类中维护了一个WeakCache对象,用于缓存动态生成的类,WeakCache.get方法不仅执行查找操作,当对象不存在时也会执行生成操作,下面来看下该方法的源码
return proxyClassCache.get(loader, interfaces);
}
2. WeakCache.get
   public V get(K key, P parameter) {
...
while (true) {
if (supplier != null) {
//已从缓存中查找到对象,直接返回
V value = supplier.get();
if (value != null) {
return value;
}
}
if (factory == null) {
//已从缓存中查找到对象,且调用该对象的get方法生成代理类后直接返回,get方法实现下文详述
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
//将新生成的对象缓存
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
supplier = factory;
}
} else {
...
}
}
}
}
   此方法会不断的循环以及重复判断，均是在考虑线程安全问题。此处未做深究。
3. Faotory.get
   public synchronized V get() {
...
try {
//实现生成代理类对象,此方法在内部类ProxyClassFactory中实现
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
...
}
...
return value;
}

4. ProxyClassFactory.apply
   public Class apply(ClassLoader loader, Class[] interfaces) {
//校验interfaces
...
String proxyPkg = null;
...
//生成packagename,默认为com.sun.proxy
if (proxyPkg == null) {
// if no non-public proxy interfaces, use package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
//生成classname,采用AutomicLong自增方式
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
// 生成字节码文件
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);
// 调用native方法,加载生成的字节码文件,并返回代理类的引用
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
...
}
}

   将生成的proxyClassFile字节码写入到文件并反编译后，信息如下：

package com.sun.proxy.$Proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import x.constant.proxy.TargetInterface;
public final class 1 extends Proxy implements TargetInterface {
private static Method m3;
private static Method m1;
private static Method m0;
private static Method m2;
public 1(InvocationHandler paramInvocationHandler) throws {
super(paramInvocationHandler);
}
public final void targetMethod() throws {
try {
this.h.invoke(this, m3, null);
return;
}catch (Error|RuntimeException localError) {
throw localError;
}catch (Throwable localThrowable){
throw new UndeclaredThrowableException(localThrowable);
}
}
public final boolean equals(Object paramObject) {
...
}
public final int hashCode() {
...
}
public final String toString() throws {
...
}
static {
try {
m3 = Class.forName("x.constant.proxy.TargetInterface").getMethod("targetMethod", new Class[0]);
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]);
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，该类位于com.sun.proxy.$Proxy包下，继承Proxy对象且实现我们自定义的TargetInterface接口；构造方法需要一个InvocationHandler对象做为参数；增强后的targetMethod方法会调用InvocationHandler对象的invoke方法。

step2

增强后的类信息已经加载到JVM中并得到了该类的引用，通过反射机制调用该类的构造方法并传入我们自定义的InvocationHandler对象做为参数，即可生成该代理类对象，此处不再做解析。

通过源代码的阅读可以发现，java动态代理的实现过程：
1. 根据Interface动态生成一个该接口的代理类，并通过调用InvocationHandler中的invoke方法的方式来实现Interface中定义的方法
2. 动态加载该代理类，并通过反射的方式获取该代理类的对象。
归根结底，其实现也是通过字节码生成ASM技术来实现的。