透过字节码分析Java动态代理机制。

一、创建动态代理代码

1、创建接口

public interface Subject {
    void request();
}

　　

2、创建接口实现类

public class RealSubject  implements  Subject
{
    @Override
    public void request() {
        System.out.println("From real subject");
    }
}

　　

3、创建DynamicSubject类

public class DynamicSubject implements InvocationHandler {

    private  Object sub;

    public  DynamicSubject(Object obj){
        this.sub = obj;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before calling:" + method);
        method.invoke(this.sub, args);
        System.out.println("after calling:" + method);
        return null;
    }
}

　　

4、创建测试类

public class Client {
    public static void main(String[] args) {

        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");

        RealSubject rs = new RealSubject();
        InvocationHandler ds = new DynamicSubject(rs);

        Class<?> cls = rs.getClass();
        Subject subject = (Subject) Proxy.newProxyInstance(cls.getClassLoader(), cls.getInterfaces(), ds);
        subject.request();

        System.out.println(subject.getClass());
        System.out.println(subject.getClass().getSuperclass());
    }
}

　　输出结果：

before calling:public abstract void com.example.jvm.bytecode.Subject.request()
From real subject
after calling:public abstract void com.example.jvm.bytecode.Subject.request()
class com.sun.proxy.$Proxy0
class java.lang.reflect.Proxy

　　

查看源码，动态代理类在ProxyGenerator类的generateProxyClass生成。

   public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if(saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if(var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar), new String[0]);
                            Files.createDirectories(var3, new FileAttribute[0]);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class", new String[0]);
                        }

                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

　　可以发现saveGeneratedFiles为true时，可以将代理类保存到磁盘上。

因此在测试类中sun.misc.ProxyGenerator.saveGeneratedFiles属性设置为true

System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");


最终生成的.class 文件位置如下图

打开$Proxy0.class

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

package com.sun.proxy;

import com.example.jvm.bytecode.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements Subject {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    public $Proxy0(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 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 void request() throws  {
        try {
            super.h.invoke(this, m3, (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)).intValue();
        } 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")});
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m3 = Class.forName("com.example.jvm.bytecode.Subject").getMethod("request", new Class[0]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}