原文出处： forever
具体场景
为了使代理类和被代理类对第三方有相同的函数，代理类和被代理类一般实现一个公共的interface，该interface定义如下
public interface Calculator {
public Integer add(Integer num1, Integer num2);
public Integer minus(Integer num1, Integer num2);
}
被代理类定义如下
public class CalculatorImpl implements Calculator {
@Override
public Integer add(Integer num1, Integer num2) {
int ret = num1 + num2;
System.out.println("in calculatorImpl, res: " + ret);
return ret;
}
@Override
public Integer minus(Integer num1, Integer num2) {
int ret = num1 - num2;
System.out.println("int calculatorImpl, res: " + ret);
return ret;
}
}
代理需求：在add函数和minus函数调用前后分别输出before invocation和after invocation字样
静态代理解决方案
代码如下：简单直接，无需赘言，如果calculator里边不仅有add和minus，还有divide，product,log,sin…呢，呵呵哒
public class StaticCalculatorProxy implements Calculator {
Calculator obj;
public StaticCalculatorProxy(Calculator obj) {
this.obj = obj;
}
@Override
public Integer add(Integer num1, Integer num2) {
System.out.println("in StaticCalculatorProxy, before invocation");
Integer ret = obj.add(num1, num2);
System.out.println("in StaticCalculatorProxy, after invocation");
return ret;
}
@Override
public Integer minus(Integer num1, Integer num2) {
System.out.println("in StaticCalculatorProxy, before invocation");
Integer ret = obj.minus(num1, num2);
System.out.println("in StaticCalculatorProxy, after invocation");
return ret;
}
}
动态代理解决方案
首先编写实现InvocationHandler接口的类，用于请求转发，实现如下
public class CalculatorHandler implements InvocationHandler {
private Object obj; //被代理类
public CalculatorHandler(Object obj) {
this.obj = obj;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("in calculatorhandler, before invocation");
Object ret = method.invoke(obj, args); //执行被代理类方法
System.out.println("in calculationhandler, after invocation");
return ret;
}
}
生成动态代理
CalculatorImpl calculatorImpl = new CalculatorImpl();//被代理类 CalculatorHandler calculatorHandler = new CalculatorHandler(calculatorImpl); Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler); System.out.println(calculator.add(1,2)); System.out.println(calculator.minus(1, 2));
无论calculator中包含多少函数，动态代理只需实现一次，实际工程中，System.out.println(“in calculatorhandler, before invocation”)可能是加缓存，打日志等操作
动态代理如何工作的
为了搞清楚动态代理如何工作，首先看看生成的动态代理的代码是什么，借助[1]中ProxyUtil代码
public class ProxyUtils {
/**
* Save proxy class to path
*
* @param path path to save proxy class
* @param proxyClassName name of proxy class
* @param interfaces interfaces of proxy class
* @return
*/
public static boolean saveProxyClass(String path, String proxyClassName, Class[] interfaces) {
if (proxyClassName == null || path == null) {
return false;
}
// get byte of proxy class
byte[] classFile = ProxyGenerator.generateProxyClass(proxyClassName, interfaces);
FileOutputStream out = null;
try {
out = new FileOutputStream(path);
out.write(classFile);
out.flush();
return true;
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return false;
}
}
得到了生成的动态代理代码如下：
public final class $Proxy0 extends Proxy
implements Calculator
{
public $Proxy0(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
public final boolean equals(Object obj)
{
try
{
return ((Boolean)super.h.invoke(this, m1, new Object[] {
obj
})).booleanValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final String toString()
{
try
{
return (String)super.h.invoke(this, m2, null);
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final Integer minus(Integer integer, Integer integer1)
{
try
{
return (Integer)super.h.invoke(this, m4, new Object[] {
integer, integer1
});
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final Integer add(Integer integer, Integer integer1)
{
try
{
return (Integer)super.h.invoke(this, m3, new Object[] {
integer, integer1
});
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final int hashCode()
{
try
{
return ((Integer)super.h.invoke(this, m0, null)).intValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m3;
private static Method m0;
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
Class.forName("java.lang.Object")
});
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus", new Class[] {
Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")
});
m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add", new Class[] {
Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")
});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
}
catch(NoSuchMethodException nosuchmethodexception)
{
throw new NoSuchMethodError(nosuchmethodexception.getMessage());
}
catch(ClassNotFoundException classnotfoundexception)
{
throw new NoClassDefFoundError(classnotfoundexception.getMessage());
}
}
}
有点长，按照初始化顺序慢慢来分析，首先分析静态代码块：
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
Class.forName("java.lang.Object")
});
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus", new Class[] {
Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")
});
m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add", new Class[] {
Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")
});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
得到公共interface中的add函数和minus函数对应的Method方法，同事也得到了equals，toString，hashCode三个函数的Method，所以调用代理类的equals，toString，hashCode也是要执行被代理类的方法的，知道这点很有必要
构造函数
public $Proxy0(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
初始化了内部的InvocationHandler变量，也就是下文的super.h
以add为例看一下请求的转发
public final Integer add(Integer integer, Integer integer1)
{
try
{
return (Integer)super.h.invoke(this, m3, new Object[] {
integer, integer1
});
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
super.h.invoke就是invocationhandler.invoke就是传入的CalculatorHandler中实现的
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("in calculatorhandler, before invocation");
Object ret = method.invoke(obj, args); //执行被代理类方法
System.out.println("in calculationhandler, after invocation");
return ret;
}
最终执行的就是CalculatorHandler对应的invoke函数
总结
Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler);
生成动态代理的过程步骤如下[2]：
// InvocationHandlerImpl 实现了 InvocationHandler 接口，并能实现方法调用从代理类到委托类的分派转发
// 其内部通常包含指向委托类实例的引用，用于真正执行分派转发过来的方法调用
InvocationHandler handler = new InvocationHandlerImpl(..);
// 通过 Proxy 为包括 Interface 接口在内的一组接口动态创建代理类的类对象
Class clazz = Proxy.getProxyClass(classLoader, new Class[] { Interface.class, ... });
// 通过反射从生成的类对象获得构造函数对象
Constructor constructor = clazz.getConstructor(new Class[] { InvocationHandler.class });
// 通过构造函数对象创建动态代理类实例
Interface Proxy = (Interface)constructor.newInstance(new Object[] { handler });
Proxy.newProxyInstance帮我们做了2，3，4步，直接返回给我们一个动态代理对象，代理对象最终执行InvocationHandler中invoke函数。顺便强推文章[2]
References
- https://github.com/android-cn/android-open-project-demo/tree/master/java-dynamic-proxy
- https://www.ibm.com/developerworks/cn/java/j-lo-proxy1/index.html
