摘要

相比于静态代理，动态代理避免了开发人员编写各个繁锁的静态代理类，只需简单地指定一组接口及目标类对象就能动态的获得代理对象。

代理模式

使用代理模式必须要让代理类和目标类实现相同的接口，客户端通过代理类来调用目标方法，代理类会将所有的方法调用分派到目标对象上反射执行，还可以在分派过程中添加"前置通知"和后置处理（如在调用目标方法前校验权限，在调用完目标方法后打印日志等）等功能。
 
使用动态代理的五大步骤
1.通过实现InvocationHandler接口来自定义自己的InvocationHandler;
 
2.通过Proxy.getProxyClass获得动态代理类
 
3.通过反射机制获得代理类的构造方法，方法签名为getConstructor(InvocationHandler.class)
 
4.通过构造函数获得代理对象并将自定义的InvocationHandler实例对象传为参数传入
 
5.通过代理对象调用目标方法
 
动态代理的使用
例1(方式一)
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public class MyProxy {
    public interface IHello{
        void sayHello();
    }
    static class Hello implements IHello{
        public void sayHello() {
            System.out.println("Hello world!!");
        }
    }
    //自定义InvocationHandler
    static  class HWInvocationHandler implements InvocationHandler{
        //目标对象
        private Object target;
        public HWInvocationHandler(Object target){
            this.target = target;
        }
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            System.out.println("------插入前置通知代码-------------");
            //执行相应的目标方法
            Object rs = method.invoke(target,args);
            System.out.println("------插入后置处理代码-------------");
            return rs;
        }
    }
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetExc    eption, InstantiationException {
        //生成$Proxy0的class文件
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        //获取动态代理类
        Class proxyClazz = Proxy.getProxyClass(IHello.class.getClassLoader(),IHello.class);
        //获得代理类的构造函数，并传入参数类型InvocationHandler.class
        Constructor constructor = proxyClazz.getConstructor(InvocationHandler.class);
        //通过构造函数来创建动态代理对象，将自定义的InvocationHandler实例传入
        IHello iHello = (IHello) constructor.newInstance(new HWInvocationHandler(new Hello()));
        //通过代理对象调用目标方法
        iHello.sayHello();
    }
}

输出：
------插入前置通知代码-------------
Hello world!!
------插入后置处理代码-------------
Proxy类中还有个将2~4步骤封装好的简便方法来创建动态代理对象，其方法签名 为：newProxyInstance(ClassLoader loader,Class<?>[] instance, InvocationHandler h)，如下例：
(方式二)
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public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
       //生成$Proxy0的class文件
       System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
       IHello  ihello = (IHello) Proxy.newProxyInstance(IHello.class.getClassLoader(),  //加载接口的类加载器
               new Class[]{IHello.class},      //一组接口
               new HWInvocationHandler(new Hello())); //自定义的InvocationHandler
       ihello.sayHello();
   }
输出结果一样.
 
下面以newProxyInstance方法为切入点来剖析代理类的生成及代理方法的调用
（为了篇幅整洁去掉了次要的代码）
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public static Object newProxyInstance(ClassLoader loader,
                                          Class<!--?-->[] interfaces,
                                          InvocationHandler h)
            throws IllegalArgumentException
    {
        if (h == null) {   //如果h为空直接抛出异常，所以InvocationHandler实例对象是必须的
            throw new NullPointerException();
        }
        //对象的拷贝,暂不知道这里拷贝下的意义是啥？
        final Class<!--?-->[] intfs = interfaces.clone();
        //一些安全的权限检查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        //产生代理类
        Class<!--?--> cl = getProxyClass0(loader, intfs);
 
        //获取代理类的构造函数对象
        //参数constructorParames为常量值：private static final Class<!--?-->[] constructorParams = { InvocationHandler.class };
        final Constructor<!--?--> cons = cl.getConstructor(constructorParames);
        final InvocationHandler ih = h;
        //根据代理类的构造函数对象来创建代理类对象
        return newInstance(cons, ih);
              
    }
这段代码就是对代理类对象的创建，就是对例1中34~38行封装，其中getProxyClass0就是生成代理类的方法
getProxyClass0方法剖析
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private static Class<!--?--> getProxyClass0(ClassLoader loader,
                                           Class<!--?-->... interfaces) {
//接口数不得超过65535个
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
//代理类缓存，如果缓存中有代理类了直接返回，否则将由ProxyClassFactory创建代理类
        return proxyClassCache.get(loader, interfaces);
    }

看看ProxyClassFactory是怎样生成代理类的?
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private static final class ProxyClassFactory
        implements BiFunction<classloader, class<?="">[], Class<!--?-->>
    {
        //统一代理类的前缀名都以$Proxy开关
        private static final String proxyClassNamePrefix = "$Proxy";
        //使用唯一的编号给作为代理类名的一部分，如$Proxy0,$Proxy1等
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
        @Override
        public Class<!--?--> apply(ClassLoader loader, Class<!--?-->[] interfaces) {
            Map<class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class<!--?--> intf : interfaces) {
                //验证指定的类加载器(loader)加载接口所得到的Class对象(interfaceClass)是否与intf对象相同
                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");
                }
                //验证该Class对象是不是接口
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                // 验证该接口是否重复了
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }
                  //声明代理类所在包
            String proxyPkg = null;  
            /*验证你传入的接口中是否有非public接口，只要有一个接口是非public的，那么这些接口都必须在同一包中
            这里的接口修饰符直接影响到System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true")所生成
            的代理类的路径，往下看！！*/
            for (Class<!--?--> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    //截取完整包名
                    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) {
                /* 如果都是public接口，那么生成的代理类就在com.sun.proxy包下如果报java.io.FileNotFoundException: com\sun\proxy\$Proxy0.c                  lass (系统找不到指定的路径。)的错误，就先在你项目中创建com.sun.proxy路径*/            
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }
 
             //将当前nextUniqueNumber的值以原子的方式的加1，所以第一次生成代理类的名字为$Proxy0.class
            long num = nextUniqueNumber.getAndIncrement();
            //代理类的完全限定名，如com.sun.proxy.$Proxy0.calss,
            String proxyName = proxyPkg + proxyClassNamePrefix + num;
            //生成代理类字节码文件            
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces);
            try {
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                throw new IllegalArgumentException(e.toString());
            }
        }
    }</class<?></classloader,>
 
而生成代理类字节码文件又主要通过ProxyGenerate的generateProxyClass(proxyName，interfaces)
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public static byte[] generateProxyClass(final String var0, Class[] var1) {
        ProxyGenerator var2 = new ProxyGenerator(var0, var1);
       //生成代理类字节码文件的真正方法
        final byte[] var3 = var2.generateClassFile();
        //保存文件
        if(saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction() {
                public Void run() {
                    try {
                        FileOutputStream var1 = new FileOutputStream(ProxyGenerator.dotToSlash(var0) + ".class");
                        var1.write(var3);
                        var1.close();
                        return null;
                    } catch (IOException var2) {
                        throw new InternalError("I/O exception saving generated file: " + var2);
                    }
                }
            });
        }
        return var3;
    }
 
层层调用后，最终generateClassFile才是真正生成代理类字节码文件的方法，注 意开头的三个addProxyMethod方法是只将Object的hashcode,equals,toString方法添加到代理方法容器中，代理类 除此之外并没有重写其他Object的方法，所以除这三个方法外，代理类调用其他方法的行为与Object调用这些方法的行为一样不通过Invoke
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private byte[] generateClassFile() {
        /addProxyMethod系列方法就是将接口的方法和Object的hashCode,equals,toString方法添加到代理方法容器(proxyMethods),
         其中方法签名作为key,proxyMethod作为value*/
        /*hashCodeMethod方法位于静态代码块中通过Object对象获得，hashCodeMethod=Object.class.getMethod("hashCode",new Class[0]),
         相当于从Object中继承过来了这三个方法equalsMethod,toStringMethod*/    
        this.addProxyMethod(hashCodeMethod, Object.class);   -->
        this.addProxyMethod(equalsMethod, Object.class);
        this.addProxyMethod(toStringMethod, Object.class);
        int var1;
        int var3;
         //获得所有接口中的所有方法，并将方法添加到代理方法中
        for(var1 = 0; var1 < this.interfaces.length; ++var1) {
            Method[] var2 = this.interfaces[var1].getMethods();           
            for(var3 = 0; var3 < var2.length; ++var3) {
                this.addProxyMethod(var2[var3], this.interfaces[var1]);
            }
        }
          
        Iterator var7 = this.proxyMethods.values().iterator();
        List var8;
        while(var7.hasNext()) {
            var8 = (List)var7.next();
            checkReturnTypes(var8);    //验证具有相同方法签名的的方法的返回值类型是否一致，因为不可能有两个方法名相同,参数相同，而返回值却不同的方法
        };
    //接下来就是写代理类文件的步骤了
        Iterator var11
        try {
             //生成代理类的构造函数
            this.methods.add(this.generateConstructor());
            var7 = this.proxyMethods.values().iterator();
            while(var7.hasNext()) {
                var8 = (List)var7.next();
                var11 = var8.iterator();
                while(var11.hasNext()) {
                    ProxyGenerator.ProxyMethod var4 = (ProxyGenerator.ProxyMethod)var11.next();
                    /将代理字段声明为Method，10为ACC_PRIVATE和ACC_STATAIC的与运算，表示该字段的修饰符为private static
                     所以代理类的字段都是private static Method XXX*/
                    this.fields.add(new ProxyGenerator.FieldInfo(var4.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    //生成代理类的代理方法
                    this.methods.add(var4.generateMethod());
                }
            }
            //为代理类生成静态代码块，对一些字段进行初始化
            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var6) {
            throw new InternalError("unexpected I/O Exception");
        }
       
        if(this.methods.size() > '\uffff') {  //代理方法超过65535将抛出异常
            throw new IllegalArgumentException("method limit exceeded");
        } else if(this.fields.size() > '\uffff') {   //代理类的字段超过65535将抛出异常
            throw new IllegalArgumentException("field limit exceeded");
        } else {
         //这里开始就是一些代理类文件的过程，此过程略过
            this.cp.getClass(dotToSlash(this.className));
            this.cp.getClass("java/lang/reflect/Proxy");
            for(var1 = 0; var1 < this.interfaces.length; ++var1) {
                this.cp.getClass(dotToSlash(this.interfaces[var1].getName()));
            }
            this.cp.setReadOnly();
            ByteArrayOutputStream var9 = new ByteArrayOutputStream();
            DataOutputStream var10 = new DataOutputStream(var9);
            try {
                var10.writeInt(-889275714);
                var10.writeShort(0);
                var10.writeShort(49);
                this.cp.write(var10);
                var10.writeShort(49);
                var10.writeShort(this.cp.getClass(dotToSlash(this.className)));
                var10.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
                var10.writeShort(this.interfaces.length);
                for(var3 = 0; var3 < this.interfaces.length; ++var3) {
                    var10.writeShort(this.cp.getClass(dotToSlash(this.interfaces[var3].getName())));
                }
                var10.writeShort(this.fields.size());
                var11 = this.fields.iterator();
                while(var11.hasNext()) {
                    ProxyGenerator.FieldInfo var12 = (ProxyGenerator.FieldInfo)var11.next();
                    var12.write(var10);
                }
                var10.writeShort(this.methods.size());
                var11 = this.methods.iterator();
                while(var11.hasNext()) {
                    ProxyGenerator.MethodInfo var13 = (ProxyGenerator.MethodInfo)var11.next();
                    var13.write(var10);
                }
                var10.writeShort(0);
                return var9.toByteArray();
            } catch (IOException var5) {
                throw new InternalError("unexpected I/O Exception");
            }
        }
    }
 
addProxyMethod方法剖析
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private void addProxyMethod(Method var1, Class var2) {
        String var3 = var1.getName();  //方法名
        Class[] var4 = var1.getParameterTypes();   //方法参数类型数组
        Class var5 = var1.getReturnType();    //返回值类型
        Class[] var6 = var1.getExceptionTypes();   //异常类型
        String var7 = var3 + getParameterDescriptors(var4);   //方法签名
        Object var8 = (List)this.proxyMethods.get(var7);   //根据方法签名却获得proxyMethods的Value
        if(var8 != null) {    //处理多个代理接口中重复的方法的情况
            Iterator var9 = ((List)var8).iterator();
            while(var9.hasNext()) {
                ProxyGenerator.ProxyMethod var10 = (ProxyGenerator.ProxyMethod)var9.next();
                if(var5 == var10.returnType) {
                    /*归约异常类型以至于让重写的方法抛出合适的异常类型，我认为这里可能是多个接口中有相同的方法，而这些相同的方法抛出的异常类                      型又不同，所以对这些相同方法抛出的异常进行了归约*/
                    ArrayList var11 = new ArrayList();
                    collectCompatibleTypes(var6, var10.exceptionTypes, var11);
                    collectCompatibleTypes(var10.exceptionTypes, var6, var11);
                    var10.exceptionTypes = new Class[var11.size()];
                    //将ArrayList转换为Class对象数组
                    var10.exceptionTypes = (Class[])var11.toArray(var10.exceptionTypes);
                    return;
                }
            }
        } else {
            var8 = new ArrayList(3);
            this.proxyMethods.put(var7, var8);
        }    
        ((List)var8).add(new ProxyGenerator.ProxyMethod(var3, var4, var5, var6, var2, null));
       /*24~27行的意思就是如果var8为空，就创建一个数组，并以方法签名为key,proxymethod对象数组为value添加到proxyMethods*/
    }
InvocationHandler的作用
在动态代理中InvocationHandler是核心，每个代理实例都具有一个关联的调用处理程序(InvocationHandler)。对代理实例调用方法时，将对方法调用进行编码并将其指派到它的调用处理程序(InvocationHandler)的 invoke 方法。所以对代理方法的调用都是通InvocationHadler的invoke来实现中，而invoke方法根据传入的代理对象，方法和参数来决定调用代理的哪个方法
invoke方法签名：invoke（Object Proxy，Method method，Object[] args）
 
$Proxy0.class
来看看例1(MyProxy)的代理类是怎样的？
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public final class $Proxy0 extends Proxy implements IHello {   //继承了Proxy类和实现IHello接口
    //变量，都是private static Method  XXX
    private static Method m3;  
    private static Method m1;
    private static Method m0;
    private static Method m2;
    //代理类的构造函数，其参数正是是InvocationHandler实例，Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }
    //接口代理方法
    public final void sayHello() throws  {
        try {
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    //以下Object中的三个方法
    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 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);
        }
    }
    //对变量进行一些初始化工作
    static {
        try {
            m3 = Class.forName("com.mobin.proxy.IHello").getMethod("sayHello", new Class[0]);
            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]);
            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());
        }
    }
}
 
以上就是对代理类如何生成，代理类方法如何被调用的分析！在很多框架都使用了动态代理如Spring,HDFS的RPC调用等等，分析过程中收获很多，如果想深入的了解JDK动态代理机制一定要深入到源码去剖析！！