动态代理的实现

代理

所谓代理就是给一个不具有某个功能的类增加某个目标类的功能，如下，被代理类LogHandler 增加一个Calculator 的add方法

//接口
package gn.k48.proxy;

public interface Calculator {
    int add(int a, int b);
}

//实现类
package gn.k48.proxy;

public class CalculatorImpl implements Calculator {
    @Override
    public int add(int a, int b) {
        System.out.println(a + b);
        return a + b;
    }
}

//被代理目标类
package gn.k48.proxy;

import sun.misc.ProxyGenerator;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class LogHandler implements InvocationHandler {

    Object object;

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

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

        return o;
    }

    public void doBefore() {
        System.out.println("do this before");
    }

    public void doAfter() {
        System.out.println("do this after");
    }

    public static void main(String[] args) throws IOException {
        Calculator calculator = new CalculatorImpl();
        LogHandler logHandler = new LogHandler(calculator);
        Calculator proxy = (Calculator)
                Proxy.newProxyInstance(
                        calculator.getClass().getClassLoader(),
                        calculator.getClass().getInterfaces(),
                        logHandler);
        byte[] bs = ProxyGenerator.generateProxyClass("Calculator", new Class[]{proxy.getClass()});
        FileOutputStream fos = new FileOutputStream("../Calculator.class");
        fos.write(bs);
        proxy.add(1, 2);
    }
}

打印出通过Proxy.newProxyInstance生成的动态类class文件

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

import com.sun.proxy..Proxy0;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class Calculator extends Proxy implements Proxy0 {
    private static Method m1;
    private static Method m6;
    private static Method m2;
    private static Method m7;
    private static Method m11;
    private static Method m13;
    private static Method m0;
    private static Method m8;
    private static Method m12;
    private static Method m3; //add
    private static Method m5;
    private static Method m10;
    private static Method m4;
    private static Method m9;

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

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

    public final InvocationHandler getInvocationHandler(Object var1) throws IllegalArgumentException {
        try {
            return (InvocationHandler)super.h.invoke(this, m6, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    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);
        }
    }

    public final Class getProxyClass(ClassLoader var1, Class[] var2) throws IllegalArgumentException {
        try {
            return (Class)super.h.invoke(this, m7, new Object[]{var1, var2});
        } catch (RuntimeException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final Class getClass() throws  {
        try {
            return (Class)super.h.invoke(this, m11, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void notifyAll() throws  {
        try {
            super.h.invoke(this, m13, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

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

    public final void wait() throws InterruptedException {
        try {
            super.h.invoke(this, m8, (Object[])null);
        } catch (RuntimeException | InterruptedException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void notify() throws  {
        try {
            super.h.invoke(this, m12, (Object[])null);
        } 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[]{var1, var2});
        } catch (RuntimeException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final Object newProxyInstance(ClassLoader var1, Class[] var2, InvocationHandler var3) throws IllegalArgumentException {
        try {
            return (Object)super.h.invoke(this, m5, new Object[]{var1, var2, var3});
        } catch (RuntimeException | Error var5) {
            throw var5;
        } catch (Throwable var6) {
            throw new UndeclaredThrowableException(var6);
        }
    }

    public final void wait(long var1) throws InterruptedException {
        try {
            super.h.invoke(this, m10, new Object[]{var1});
        } catch (RuntimeException | InterruptedException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final boolean isProxyClass(Class var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m4, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final void wait(long var1, int var3) throws InterruptedException {
        try {
            super.h.invoke(this, m9, new Object[]{var1, var3});
        } catch (RuntimeException | InterruptedException | Error var5) {
            throw var5;
        } catch (Throwable var6) {
            throw new UndeclaredThrowableException(var6);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m6 = Class.forName("com.sun.proxy.$Proxy0").getMethod("getInvocationHandler", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m7 = Class.forName("com.sun.proxy.$Proxy0").getMethod("getProxyClass", Class.forName("java.lang.ClassLoader"), Class.forName("[Ljava.lang.Class;"));
            m11 = Class.forName("com.sun.proxy.$Proxy0").getMethod("getClass");
            m13 = Class.forName("com.sun.proxy.$Proxy0").getMethod("notifyAll");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m8 = Class.forName("com.sun.proxy.$Proxy0").getMethod("wait");
            m12 = Class.forName("com.sun.proxy.$Proxy0").getMethod("notify");
            m3 = Class.forName("com.sun.proxy.$Proxy0").getMethod("add", Integer.TYPE, Integer.TYPE);
            m5 = Class.forName("com.sun.proxy.$Proxy0").getMethod("newProxyInstance", Class.forName("java.lang.ClassLoader"), Class.forName("[Ljava.lang.Class;"), Class.forName("java.lang.reflect.InvocationHandler"));
            m10 = Class.forName("com.sun.proxy.$Proxy0").getMethod("wait", Long.TYPE);
            m4 = Class.forName("com.sun.proxy.$Proxy0").getMethod("isProxyClass", Class.forName("java.lang.Class"));
            m9 = Class.forName("com.sun.proxy.$Proxy0").getMethod("wait", Long.TYPE, Integer.TYPE);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看到生成的字节码是一个Proxy的子类，将在add方法中调用了Proxy的m3方法