JDK Proxy动态代理解析

开篇

• 日常阅读开源的代码框架的时候经常会到Proxy动态代理的应用场景，抽空研究一下。
• 借助于Proxy的demo和反编译动态代理代码来分析调用过程。
• 借助于Proxy的动态代理类的源码分析下动态类的生成过程。

Proxy应用场景

package com.sunboy;

import java.io.File;
import java.io.FileOutputStream;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import sun.misc.ProxyGenerator;

public class ViewProxyClass {

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

        Game gamePlayer = new GamePalyer("张三");

        ClassLoader classLoader = gamePlayer.getClass().getClassLoader();
        Class<?>[] interfaces = gamePlayer.getClass().getInterfaces();
        // 生成动态代理的基本操作
        Game proxy = (Game) Proxy.newProxyInstance(classLoader, interfaces, new GameInvocation(gamePlayer));
        proxy.paly();

        // 生成动态代理的源码的调试方法
        byte[] bts = ProxyGenerator.generateProxyClass("$GameProxy", interfaces);
        FileOutputStream fos = new FileOutputStream(new File("D:/tmp/$GameProxy.class"));
        fos.write(bts);
        fos.flush();
        fos.close();
    }

    // 接口的定义
    interface Game {
        public void paly();
    }

    // 接口的实现
    static class GamePalyer implements Game {

        private String name;

        public GamePalyer(String name) {
            this.name = name;
        }

        @Override
        public void paly() {
            System.out.println(this.name + " playing ...");
        }
    }

    // 动态代理的封装对象
    static class GameInvocation implements InvocationHandler {

        private Game target;

        public GameInvocation(Game target) {
            this.target = target;
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            System.out.println(target + "被代理");
            method.invoke(this.target, args);
            return proxy;
        }
    }
}

• Proxy动态代理类基于Interface维度的，被代理类需要实现Interface，如案例中提到的GamePalyer类实现Game接口。
• Proxy的动态代理封装类需要实现InvocationHandler接口，如案例中的GameInvocation对象。
• 动态代理类的生成通过Proxy.newProxyInstance来实现。动态代理类的代码是由Proxy的接口动态定义生成，可以通过ProxyGenerator.generateProxyClass("$GameProxy", interfaces)生成代理类并保存到class文件后反编译后观察。
• InvocationHandler的第一个参数Object是指动态生成的代理类，第二个参数是被代理的方法，第三方参数是动态入参。

Proxy动态代理类

import com.sunboy.ViewProxyClass.Game;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

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

    public $GameProxy(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 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 paly() 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);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    // 加载动态代理类
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.sunboy.ViewProxyClass$Game").getMethod("paly");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}


public class Proxy implements java.io.Serializable {

    private static final long serialVersionUID = -2222568056686623797L;
    protected InvocationHandler h;

     protected Proxy(InvocationHandler h) {
        Objects.requireNonNull(h);
        this.h = h;
    }
}

• 动态代理类GameProxy继承Proxy并implement被代理接口（如案例中的Game）。
• 动态代理类GameProxy的构造函数$GameProxy(InvocationHandler var1)的参数为InvocationHandler 对象，并传给父类Proxy。
• 动态代理类GameProxy通过Class.forName来加载被代理接口，并通过getMethod获取被代理的Method。
• 动态代理类重写了被代理的Method，并通过 super.h.invoke()调用InvocationHandler的invoke方法。
• 动态代理类核心就是重载被代理的方法，并在重载的方法内部调用代理对象Invocation的invoke方法。代理的实现代码在Invocation里面

动态代理类工厂

public class Proxy implements java.io.Serializable {

    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        private static final String proxyClassNamePrefix = "$Proxy";

        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
            // 检查被代理的Interface的合法性
            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class<?> intf : interfaces) {
                Class<?> interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }

                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }

                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }

            // 检查package的合法性
            String proxyPkg = null;     // package to define proxy class in
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

            for (Class<?> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) {
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) {
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }

            if (proxyPkg == null) {
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;
            // 1、生成动态代理的类
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);

            try {
                // 2、加载动态代理的类
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {

                throw new IllegalArgumentException(e.toString());
            }
        }
    }
}

• ProxyGenerator.generateProxyClass()负责生成动态代理类。
• defineClass0负责加载新生成的动态代理类并返回。
• ProxyClassFactory负责动态生成代理类的工厂。