浅谈模拟java动态代理（自己的理解,里面代码是马士兵老师的代码）

定义一个接口

package com.bjsxt.proxy.test;

public interface UserMgr {
void addUser();

}

实现这个接口

package com.bjsxt.proxy.test;

public class UserMgrImpl implements UserMgr {

@Override
public void addUser() {
System.out.println("1: 插入记录到user表");
System.out.println("2: 做日志在另外一张表");
}

}

创建自己的InvocationHandler 接口

package com.bjsxt.proxy;

import java.lang.reflect.Method;

public interface InvocationHandler {
public void invoke(Object o, Method m);

}

创建一个实现接口的Handler类：

package com.bjsxt.proxy.test;

import java.lang.reflect.Method;

import com.bjsxt.proxy.InvocationHandler;

public class TransactionHandler implements InvocationHandler {

private Object target;

public TransactionHandler(Object target) {
super();
this.target = target;
}

@Override
public void invoke(Object o, Method m) {
System.out.println("Transaction Start");
try {
m.invoke(target);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("Transaction Commit");
}

}

代理类的编写：

package com.bjsxt.proxy;

import java.io.File;

import java.io.FileWriter;

import java.lang.reflect.Constructor;

import java.lang.reflect.Method;

import java.net.URL;

import java.net.URLClassLoader;

import javax.tools.JavaCompiler;

import javax.tools.StandardJavaFileManager;

import javax.tools.ToolProvider;

import javax.tools.JavaCompiler.CompilationTask;

public class Proxy {
public static Object newProxyInstance(Class infce, InvocationHandler h) throws Exception { //JDK6 Complier API, CGLib, ASM
String methodStr = "";
String rt = "\r\n";

Method[] methods = infce.getMethods();
/*
for(Method m : methods) {
methodStr += "@Override" + rt + 
"public void " + m.getName() + "() {" + rt +
"   long start = System.currentTimeMillis();" + rt +
"   System.out.println(\"starttime:\" + start);" + rt +
"   t." + m.getName() + "();" + rt +
"   long end = System.currentTimeMillis();" + rt +
"   System.out.println(\"time:\" + (end-start));" + rt +
"}";
}
*/
for(Method m : methods) {
methodStr += "@Override" + rt + 
"public void " + m.getName() + "() {" + rt +
"    try {" + rt +
"    Method md = " + infce.getName() + ".class.getMethod(\"" + m.getName() + "\");" + rt +
"    h.invoke(this, md);" + rt +
"    }catch(Exception e) {e.printStackTrace();}" + rt +

"}";
}

String src = 
"package com.bjsxt.proxy;" +  rt +
"import java.lang.reflect.Method;" + rt +
"public class $Proxy1 implements " + infce.getName() + "{" + rt +
"    public $Proxy1(InvocationHandler h) {" + rt +
"        this.h = h;" + rt +
"    }" + rt +

"    com.bjsxt.proxy.InvocationHandler h;" + rt +

methodStr +
"}";
String fileName = 
"d:/src/com/bjsxt/proxy/$Proxy1.java";
File f = new File(fileName);
FileWriter fw = new FileWriter(f);
fw.write(src);
fw.flush();
fw.close();

//compile
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMgr = compiler.getStandardFileManager(null, null, null);
Iterable units = fileMgr.getJavaFileObjects(fileName);
CompilationTask t = compiler.getTask(null, fileMgr, null, null, null, units);
t.call();
fileMgr.close();

//load into memory and create an instance
URL[] urls = new URL[] {new URL("file:/" + "d:/src/")};
URLClassLoader ul = new URLClassLoader(urls);
Class c = ul.loadClass("com.bjsxt.proxy.$Proxy1");
System.out.println(c);

Constructor ctr = c.getConstructor(InvocationHandler.class);
Object m = ctr.newInstance(h);
//m.move();

return m;
}

}

测试文件：

package com.bjsxt.proxy.test;

import com.bjsxt.proxy.InvocationHandler;

import com.bjsxt.proxy.Proxy;

import com.bjsxt.proxy.TimeHandler;

public class Client {
public static void main(String[] args) throws Exception {
UserMgr mgr = new UserMgrImpl();
InvocationHandler h = new TransactionHandler(mgr);
//TimeHandler h2 = new TimeHandler(h);
UserMgr u = (UserMgr)Proxy.newProxyInstance(UserMgr.class,h);
u.addUser();
}

}

具体的流程是这样的：

UserMgr u = (UserMgr)Proxy.newProxyInstance(UserMgr.class,h);

Proxy动态生成一个$Proxy类，该类实现了UserMgr接口，于是能够转化为UserMgr对象，该类里面有一个Handler属性，这个Handler对象就是我们传进去的h，这个h里面的target 就是我们自己创建好的mgr。。当我们调用u.addUser()方法的时候，就是调用h.invoke()方法，在这个方法里面通过反射调用mgr .addUser()方法。

JDK的动态代理实例：

UserMgr类 和UserMgrImpl()不变。。Hander类里面实现的接口不再是我们自己写的InvocationHandler，变为java.lang.reflect.InvocationHandler。测试类里面的Proxy改为

import java.lang.reflect.Proxy;

如下：

package com.bjsxt.proxy.test;

import java.lang.reflect.Method;

import java.lang.reflect.InvocationHandler;

public class TransactionHandler implements InvocationHandler {

private Object target;

public TransactionHandler(Object target) {
super();
this.target = target;
}

@Override
public void invoke(Object o, Method m) {
System.out.println("Transaction Start");
try {
m.invoke(target);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("Transaction Commit");
}

}

测试类

package com.bjsxt.proxy.test;

import com.bjsxt.proxy.InvocationHandler;

import java.lang.reflect.Proxy;

import com.bjsxt.proxy.TimeHandler;

public class Client {
public static void main(String[] args) throws Exception {
UserMgr mgr = new UserMgrImpl();
InvocationHandler h = new TransactionHandler(mgr);
//TimeHandler h2 = new TimeHandler(h);
UserMgr u = (UserMgr)Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),   
UserMgr.class,h);
u.addUser();
}

}

备注：

Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader()（classLoader）,
target.getClass().getInterfaces()(所有的需要实现的接口), InvocationHandler对象);