jdk动态代理实现原理

jdk的动态代理即使用反射来实现，具体由Proxy、InvocationHandler等类来实现的。

具体调用过程比较难理解，但是如果看到生成的代理类就不难理解了。

代码如下：

接口类：Calculator.java

package com.yangjianzhou.javaBasics;

/**
 * Created by yangjianzhou on 16-5-8.
 */
public interface Calculator {

    public int  add(int operator1 , int operator2);

}

实现类：CalculatorImpl.java

package com.yangjianzhou.javaBasics;

/**
 * Created by yangjianzhou on 16-5-8.
 */
public class CalculatorImpl implements Calculator {

    @Override
    public int add(int operator1, int operator2) {
        System.out.println("operator1 + operator2 = " + (operator1 + operator2));
        return operator1 + operator2;
    }
}

代理类：CalculatorProxy.java

package com.yangjianzhou.javaBasics;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

/**
 * Created by yangjianzhou on 16-5-8.
 */
public class CalculatorProxy {

    public void executeProxy(){

        Calculator calculator = new CalculatorImpl();
        LogHandler logHandler = new LogHandler(calculator);
        Calculator proxy = (Calculator) Proxy.newProxyInstance(calculator.getClass().getClassLoader() , calculator.getClass().getInterfaces(),logHandler);
        proxy.add(2,3);
    }
}

class LogHandler implements InvocationHandler{

    Object obj ;

    public LogHandler(Object obj) {
        this.obj = obj;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        this.doBefore();
        Object object = method.invoke(obj , args);
        this.doAfter();
        return object;
    }

    public void doBefore(){
        System.out.println("log before method");
    }

    public void doAfter(){
        System.out.println("log after method");
    }
}

测试代码：TestProxy.java

package com.yangjianzhou.javaBasics;

import sun.misc.ProxyGenerator;

import java.io.FileOutputStream;

/**
 * Created by yangjianzhou on 16-5-8.
 */
public class TestProxy {

    public static void main(String[] args) throws Exception {

        CalculatorProxy calculatorProxy = new CalculatorProxy();
        calculatorProxy.executeProxy();
        byte[] classCode = ProxyGenerator.generateProxyClass("com.yangjianzhou.javaBasics.", new Class[]{Calculator.class});
        FileOutputStream fileOutputStream = new FileOutputStream("/tmp/proxy.class");
        fileOutputStream.write(classCode);
        fileOutputStream.close();
    }
}

反编译proxy.class得到的java代码如下：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.yangjianzhou.javaBasics;

import com.yangjianzhou.javaBasics.Calculator;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class  extends Proxy implements Calculator {
    private static Method m1;
    private static Method m0;
    private static Method m3;
    private static Method m2;

    public (InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final int hashCode() throws  {
        try {
            return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int add(int var1, int var2) throws  {
        try {
            return ((Integer)super.h.invoke(this, m3, new Object[]{Integer.valueOf(var1), Integer.valueOf(var2)})).intValue();
        } catch (RuntimeException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    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.yangjianzhou.javaBasics.Calculator").getMethod("add", new Class[]{Integer.TYPE, Integer.TYPE});
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

因此，调用proxy.add(2,3)，实际上就是调用LogHandler的invoke方法