下面就直接给出例子:
//利用JDK的InvocationHandler和Proxy
private class MyInvocationHandler implements InvocationHandler {
private Object o;
MyInvocationHandler(Object o) {
this.o = o;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
...//插入自己的操作
method.invoke(mvpView, args);//调用 代理类的方法
...//插入自己的操作
}
}
InvocationHandler handler = new MyInvocationHandler();
MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
MyInterface.class.getClassLoader(),
new Class[] { MyInterface.class },
handler);
实际应用中都是直接调用 JDK 的 Proxy.newProxyInstance ,不过我们可以了解一下它的底层原理。
1. 底层模拟动态代理
package dynamicProxy;
import java.lang.reflect.Method;
public interface InvocationHandler {
void invoke(Object proxy, Method method);
}
-------------------------------------------------------------------------------------------
package dynamicProxy;
import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
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;
/**
* 功能：
*/
public class Proxy {
public static Object newProxyInstance(Class interfaces,InvocationHandler invocationHandler) throws Exception {
String methodStr="";
String rt = "\r\n";
Method[] methods=interfaces.getMethods();
for (Method m: methods){
methodStr+= " @Override" + rt +
" public "+m.getReturnType().getName()+" "+m.getName()+"("+" "+") {"+rt+
" try{"+rt+
" Method md = "+interfaces.getName()+".class.getMethod(\""+m.getName()+"\");"+rt+
/* " "+invocationHandler.invoke(this,"md");+rt+*/
" handler.invoke(this,md);" +rt+
" }catch (NoSuchMethodException e) {\n" +
" e.printStackTrace();\n" +
" }"+rt+
" }";
}
String src = "package dynamicProxy;" + rt +
"import java.lang.reflect.Method;" + rt +
"public class $Proxy1 implements " + interfaces.getName() + "{" + rt +
" dynamicProxy.InvocationHandler handler;" +rt+
" public $Proxy1(dynamicProxy.InvocationHandler handler) {" + rt +
" super();" + rt +
" this.handler = handler;" + rt +
" }" + rt +
methodStr + rt +
"}";
String fileName = System.getProperty("user.dir")
+ "/src/dynamicProxy/$Proxy1.java";
//在/src/dynamicProxy下动态创建TankTimeProxy.java文件
File f = new File(fileName);
FileWriter fileWriter = new FileWriter(f);
fileWriter.write(src);
fileWriter.flush();
fileWriter.close();
//编译动态生成的java文件
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileManager =
compiler.getStandardFileManager(null, null, null);
Iterable units = fileManager.getJavaFileObjects(fileName);
JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager,
null, null, null, units);
task.call(); //进行编译
fileManager.close();
//把编译完成的.class文件加载进内存中并创建实例
URL[] urls = new URL[]{new URL("file:/" + System.getProperty("user.dir"))};
URLClassLoader urlClassLoader = new URLClassLoader(urls);
Class c = urlClassLoader.loadClass("dynamicProxy.$Proxy1"); //报错是因为配置的原因
System.out.println(c);
Constructor constructor=c.getConstructor(InvocationHandler.class);
return constructor.newInstance(invocationHandler);
}
}
例子如下:
public interface Moveable {
void move();
}
/**
* 功能：看作是第三方不可修改的类
*/
public class Tank implements Moveable {
@Override
public void move() {
System.out.println("正在移动");
try {
new Thread().sleep(new Random().nextInt(1000));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
在不修改源码的情况下,对move方法进行时间计算(在move前后进行自己的操作)
/**
* 功能：处理类
*/
public class TimeHandler implements InvocationHandler{
Object object;
public TimeHandler(Object object){
super();
this.object=object;
}
@Override
public void invoke(Object proxy, Method method) {
long start=System.currentTimeMillis();
System.out.println("startTime: "+start);
try {
// !!! 注意,这里m.invoke执行的对象,应该是外界传入的object,如tank
method.invoke(object,new Object[]{});
} catch (Exception e) {
e.printStackTrace();
}
long end=System.currentTimeMillis();
System.out.println("time: "+(end-start));
}
}
//测试类
public class Text {
public static void main(String[] args) throws Exception {
Tank tank=new Tank();
InvocationHandler handler=new TimeHandler(tank);
Moveable m= (Moveable) Proxy.newProxyInstance(Moveable.class,handler);
m.move();
}
}
//log
class dynamicProxy.$Proxy1
startTime: 1520388195900
正在移动
time: 215
这样做,最明显的好处就是 Proxy类不用修改,会动态生成与代理类具有相同方法的$Proxy1类,通过实现InvocationHandler的方法,就能对代理类(此类是Tank),增加操作.
动态生成的代码如下:
package dynamicProxy;
import java.lang.reflect.Method;
public class $Proxy1 implements interfaces.Moveable{
dynamicProxy.InvocationHandler handler;
public $Proxy1(dynamicProxy.InvocationHandler handler) {
super();
this.handler = handler;
}
@Override
public void move( ) {
try{
Method md = interfaces.Moveable.class.getMethod("move");
handler.invoke(this,md);
}catch (NoSuchMethodException e) {
e.printStackTrace();
}
}
}
2. 分析底层模拟动态代理
Moveable m= (Moveable) Proxy.newProxyInstance(Moveable.class,handler);
-->
public static Object newProxyInstance(Class interfaces,InvocationHandler invocationHandler) throws Exception {
...
Class c = urlClassLoader.loadClass("dynamicProxy.$Proxy1"); //报错是因为配置的原因
System.out.println(c);
Constructor constructor=c.getConstructor(InvocationHandler.class);
return constructor.newInstance(invocationHandler);
}
其中c是生成的代理类 $Proxy1,所以 Proxy.newProxyInstance(Moveable.class,handler); 最终就是返回 $Proxy1 对象
package dynamicProxy;
import java.lang.reflect.Method;
public class $Proxy1 implements interfaces.Moveable{
dynamicProxy.InvocationHandler handler;
public $Proxy1(dynamicProxy.InvocationHandler handler) {
super();
this.handler = handler;
}
@Override
public void move( ) {
try{
Method md = interfaces.Moveable.class.getMethod("move");
handler.invoke(this,md);
}catch (NoSuchMethodException e) {
e.printStackTrace();
}
}
}
-->
所以 m.move(),实际就是调用
$Proxy1.move(){
...
Method md = interfaces.Moveable.class.getMethod("move");
handler.invoke(this,md); //就是调用handler的invoke方法
}
-->
public class TimeHandler implements InvocationHandler{
Object object;
public TimeHandler(Object object){
super();
this.object=object;
}
@Override
public void invoke(Object proxy, Method method) {
...//your action
// 注意!!!,第一个参数是调用外界 需要代理的类
method.invoke(object,new Object[]{});
...//your action
}
}
-->
而
Tank tank=new Tank();
InvocationHandler handler=new TimeHandler(tank);
所以,handler里面的 method.invoke(object,new Object[]{}); 相当于调用了 tank.move();
由此可见,动态代理能实现 在每个方法前后 判空,权限检查,打印LOG等等操作.
参考：
详细介绍: 10分钟看懂动态代理设计模式
简洁介绍: Java 动态代理
代码来源: 马士兵的设计模式--java动态代理(视频)