设计模式学习——JAVA动态代理原理分析

一、JDK动态代理执行过程

上一篇我们讲了JDK动态代理的简单使用，今天我们就来研究一下它的原理。

首先我们回忆下上一篇的代码：

public class Main {

    public static void main(String[] args)

    {

        IPaymentService paymentService = new WatchPaymentService();

        PaymentIH paymentIH = new PaymentIH(paymentService);

        IPaymentService proxy = (IPaymentService) Proxy.newProxyInstance(

                paymentService.getClass().getClassLoader(),

                new Class[] {IPaymentService.class}, paymentIH);

        proxy.pay();

    }

}

我们通过Proxy.newProxyInstance方法创建了代理对象，我们通过Debug看下这个proxy到底是什么：

我们看到proxy的类是$Proxy0，很显然这是一个自动生成的类，我们使用如下工具类将此动态类保存下来看看：

public class ProxyUtils {



    /**

     * 将动态类的二进制字节码保存到硬盘中，默认的是clazz目录下

     * params: clazz 需要生成动态代理类的类

     * proxyName: 为动态生成的代理类的名称

     */

    public static void generateClassFile(Class clazz, String proxyName) {

        // 根据类信息和提供的代理类名称，生成字节码

        byte[] classFile = ProxyGenerator.generateProxyClass(proxyName, clazz.getInterfaces());

        String paths = clazz.getResource(".").getPath();

        System.out.println(paths);

        FileOutputStream out = null;

        try {

            //保留到硬盘中

            out = new FileOutputStream(paths + proxyName + ".class");

            out.write(classFile);

            out.flush();

        } catch (Exception e) {

            e.printStackTrace();

        } finally {

            try {

                out.close();

            } catch (IOException e) {

                e.printStackTrace();

            }

        }

    }

}

在main方法中执行下面这段代码，我们便可以在target的classes下找到生成的动态类。

ProxyUtils.generateClassFile(paymentService.getClass(), "PaymentServiceProxy");

我们通过IDEA查看这个动态类的内容如下：

public final class PaymentServiceProxy extends Proxy implements IPaymentService {

    private static Method m1;

    private static Method m3;

    private static Method m2;

    private static Method m0;



    public PaymentServiceProxy(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 void pay() throws  {

        try {

            super.h.invoke(this, m3, (Object[])null);

        } 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);

        }

    }



    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);

        }

    }



    static {

        try {

            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));

            m3 = Class.forName("proxy.IPaymentService").getMethod("pay");

            m2 = Class.forName("java.lang.Object").getMethod("toString");

            m0 = Class.forName("java.lang.Object").getMethod("hashCode");

        } catch (NoSuchMethodException var2) {

            throw new NoSuchMethodError(var2.getMessage());

        } catch (ClassNotFoundException var3) {

            throw new NoClassDefFoundError(var3.getMessage());

        }

    }

}

从 PaymentServiceProxy 的代码中我们可以发现：

PaymentServiceProxy 继承了 Proxy 类，并且实现了被代理的所有接口，以及equals、hashCode、toString等方法；
由于 PaymentServiceProxy继承了 Proxy 类，所以每个代理类都会关联一个 InvocationHandler 方法调用处理器；
类和所有方法都被 public final 修饰，所以代理类只可被使用，不可以再被继承；
每个方法都有一个 Method 对象来描述，Method 对象在static静态代码块中创建，以 m + 数字的格式命名；
被代理对象方法的调用是通过super.h.invoke(this, m1, (Object[])null); 完成的，其中的 super.h.invoke 实际上是在创建代理的时候传递给 Proxy.newProxyInstance 的 PaymentIH 对象，即 InvocationHandler的实现类，负责实际的调用处理逻辑；
PaymentIH的invoke方法接收到method、args等参数后，通过反射机制让被代理对象执行对应的方法。

综上，JDK的动态代理执行流程如下：

那么这个类是如何生成的呢？这个类与InvocationHandler又是如何关联起来的？带着这两个问题我们深入研究下Proxy的源码。

二、JDK动态代理源码解读

@CallerSensitive

    public static Object newProxyInstance(ClassLoader loader,

                                          Class<?>[] interfaces,

                                          InvocationHandler h)

        throws IllegalArgumentException{

        // null检查，h为null就抛出NullPointerException

        Objects.requireNonNull(h);

        // 将接口类对象数组clone一份。

        final Class<?>[] intfs = interfaces.clone();



        //执行权限检查

        final SecurityManager sm = System.getSecurityManager();

        if (sm != null) {

            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);

        }



        /*

         * Look up or generate the designated proxy class.

         */

         // 查找或者是生成一个特定的代理类对象

        Class<?> cl = getProxyClass0(loader, intfs);



        /*

         * Invoke its constructor with the designated invocation handler.

         */

        try {

            if (sm != null) {

                checkNewProxyPermission(Reflection.getCallerClass(), cl);

            }

            // 从代理类对象中查找参数为InvocationHandler的构造器

            final Constructor<?> cons = cl.getConstructor(constructorParams);

            final InvocationHandler ih = h;

            // 判断构造器是否是Public，如果不是则将其设置为可以访问的。

            if (!Modifier.isPublic(cl.getModifiers())) {

                AccessController.doPrivileged(new PrivilegedAction<Void>() {

                    public Void run() {

                        cons.setAccessible(true);

                        return null;

                    }

                });

            }

            // 通过反射，将h作为参数，实例化代理类，返回代理类实例。

            return cons.newInstance(new Object[]{h});

        } catch (IllegalAccessException|InstantiationException e) {

            throw new InternalError(e.toString(), e);

        } catch (InvocationTargetException e) {

            Throwable t = e.getCause();

            if (t instanceof RuntimeException) {

                throw (RuntimeException) t;

            } else {

                throw new InternalError(t.toString(), t);

            }

        } catch (NoSuchMethodException e) {

            throw new InternalError(e.toString(), e);

        }

    }

上面的代码中最重要的就是第21行和第44行，这两行实现了代理类的生成与实例化代理对象。

首先我们看下getProxyClass0(loader, intfs)的实现逻辑：

private static Class<?> getProxyClass0(ClassLoader loader,

                                           Class<?>... interfaces) {

        if (interfaces.length > 65535) {

            throw new IllegalArgumentException("interface limit exceeded");

        }



        // If the proxy class defined by the given loader implementing

        // the given interfaces exists, this will simply return the cached copy;

        // otherwise, it will create the proxy class via the ProxyClassFactory

        // 如果代理类被指定的类加载器定义了，并实现了给定的接口，

        // 那么就返回缓存的代理类对象，否则使用ProxyClassFactory创建代理类。

        return proxyClassCache.get(loader, interfaces);

    }

根据注释分析，proxyClassCache.get方法是获取代理类的入口，那我们接下来首先看看这个proxyClassCache是什么东东：

private static final WeakCache<ClassLoader, Class<?>[], Class<?>>

        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

proxyClassCache是一个WeakCache的对象，我们看看他的定义：（由于代码篇幅较大，这里只展示出私有变量定义与构造函数定义）

final class WeakCache<K, P, V> {



    private final ReferenceQueue<K> refQueue

        = new ReferenceQueue<>();

    // the key type is Object for supporting null key

    private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map

        = new ConcurrentHashMap<>();

    private final ConcurrentMap<Supplier<V>, Boolean> reverseMap

        = new ConcurrentHashMap<>();

    private final BiFunction<K, P, ?> subKeyFactory;

    private final BiFunction<K, P, V> valueFactory;



    /**

     * Construct an instance of {@code WeakCache}

     *

     * @param subKeyFactory a function mapping a pair of

     *                      {@code (key, parameter) -> sub-key}

     * @param valueFactory  a function mapping a pair of

     *                      {@code (key, parameter) -> value}

     * @throws NullPointerException if {@code subKeyFactory} or

     *                              {@code valueFactory} is null.

     */

    public WeakCache(BiFunction<K, P, ?> subKeyFactory,

                     BiFunction<K, P, V> valueFactory) {

        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);

        this.valueFactory = Objects.requireNonNull(valueFactory);

    }

其中map变量是实现缓存的核心变量，它是一个双重的Map结构: (key, subKey) -> value。其中key是传进来的Classloader进行包装后的对象，subKey是由WeakCache构造函数传人的KeyFactory()生成的。value就是产生代理类的对象由WeakCache构造函数传人的ProxyClassFactory()生成，这个可以从proxyClassCache的初始化能看出来。

产生subKey的KeyFactory代码如下：

private static final class KeyFactory

        implements BiFunction<ClassLoader, Class<?>[], Object>

{

        @Override

        public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {

            switch (interfaces.length) {

                case 1: return new Key1(interfaces[0]); // the most frequent

                case 2: return new Key2(interfaces[0], interfaces[1]);

                case 0: return key0;

                default: return new KeyX(interfaces);

            }

        }

    }

这部分代码没有必要深究，我们只需要知道它是根据传进去的interface生成subKey就行了，我们接着来看WeakCache.get方法：

public V get(K key, P parameter) {

        // 校验parameter不为空

        Objects.requireNonNull(parameter);

        // 清除无效缓存

        expungeStaleEntries();

        // cacheKey就是缓存的一级键

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // 根据一级键得到ConcurrentMap<Object, Supplier<V>>，如果不存在则创建之

        // lazily install the 2nd level valuesMap for the particular cacheKey

        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);

        if (valuesMap == null) {

            ConcurrentMap<Object, Supplier<V>> oldValuesMap

                = map.putIfAbsent(cacheKey,

                                  valuesMap = new ConcurrentHashMap<>());

            if (oldValuesMap != null) {

                valuesMap = oldValuesMap;

            }

        }



        // create subKey and retrieve the possible Supplier<V> stored by that

        // subKey from valuesMap

        // 根据classloader和interfaces获取二级键，即KeyFactory的apply方法

        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));

        // 根据下面的代码可以得知这个supplier就是factory，这里同样也是先看缓存中有没有，没有就重新创建，

        // 这也是此处代码使用一个while循环的原因

        Supplier<V> supplier = valuesMap.get(subKey);

        Factory factory = null;



        while (true) {

            if (supplier != null) {

                // supplier might be a Factory or a CacheValue<V> instance

                // 这里调用的就是Factory.get,得到代理类并返回

                V value = supplier.get();

                if (value != null) {

                    return value;

                }

            }

            // else no supplier in cache

            // or a supplier that returned null (could be a cleared CacheValue

            // or a Factory that wasn't successful in installing the CacheValue)



            // lazily construct a Factory

            if (factory == null) {

                factory = new Factory(key, parameter, subKey, valuesMap);

            }



            if (supplier == null) {

                supplier = valuesMap.putIfAbsent(subKey, factory);

                if (supplier == null) {

                    // successfully installed Factory

                    supplier = factory;

                }

                // else retry with winning supplier

            } else {

                if (valuesMap.replace(subKey, supplier, factory)) {

                    // successfully replaced

                    // cleared CacheEntry / unsuccessful Factory

                    // with our Factory

                    supplier = factory;

                } else {

                    // retry with current supplier

                    supplier = valuesMap.get(subKey);

                }

            }

        }

    }

上面的代码稍微比较绕，总之就是先尝试从key获取（subKey -> value）如果不存在则创建（subKey -> value），然后再根据subKey获取value，同样的不存在则创建新的value。最终通过Factory.get方法获取代理类，接下来我们来看看Factory的代码：

private final class Factory implements Supplier<V> {



        private final K key;

        private final P parameter;

        private final Object subKey;

        private final ConcurrentMap<Object, Supplier<V>> valuesMap;



        Factory(K key, P parameter, Object subKey,

                ConcurrentMap<Object, Supplier<V>> valuesMap) {

            this.key = key;

            this.parameter = parameter;

            this.subKey = subKey;

            this.valuesMap = valuesMap;

        }



        @Override

        public synchronized V get() { // serialize access

            // re-check

            Supplier<V> supplier = valuesMap.get(subKey);

            // 检测得到的supplier是不是当前对象

            if (supplier != this) {

                // something changed while we were waiting:

                // might be that we were replaced by a CacheValue

                // or were removed because of failure ->

                // return null to signal WeakCache.get() to retry

                // the loop

                return null;

            }

            // else still us (supplier == this)



            // create new value

            V value = null;

            try {

                // 调用ProxyClassFactory创建代理类

                value = Objects.requireNonNull(valueFactory.apply(key, parameter));

            } finally {

                if (value == null) { // remove us on failure

                    valuesMap.remove(subKey, this);

                }

            }

            // the only path to reach here is with non-null value

            assert value != null;



            // wrap value with CacheValue (WeakReference)

            // 把value包装成弱引用

            CacheValue<V> cacheValue = new CacheValue<>(value);



            // try replacing us with CacheValue (this should always succeed)

            if (valuesMap.replace(subKey, this, cacheValue)) {

                // put also in reverseMap

                reverseMap.put(cacheValue, Boolean.TRUE);

            } else {

                throw new AssertionError("Should not reach here");

            }



            // successfully replaced us with new CacheValue -> return the value

            // wrapped by it

            return value;

        }

    }

终于我们来到了ProxyClassFactory类了，这个类就是用来创建代理类的工厂类：

private static final class ProxyClassFactory

        implements BiFunction<ClassLoader, Class<?>[], Class<?>>

    {

        // prefix for all proxy class names

        // 代理类的类名前缀

        private static final String proxyClassNamePrefix = "$Proxy";



        // next number to use for generation of unique proxy class names

        // 代理类类名编号，即$Proxy0,$Proxy1,$Proxy2......

        private static final AtomicLong nextUniqueNumber = new AtomicLong();



        @Override

        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {



            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);

            // 校验接口是否能被当前classloader加载以及其是否是接口类

            for (Class<?> intf : interfaces) {

                /*

                 * Verify that the class loader resolves the name of this

                 * interface to the same Class object.

                 */

                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");

                }

                /*

                 * Verify that the Class object actually represents an

                 * interface.

                 */

                if (!interfaceClass.isInterface()) {

                    throw new IllegalArgumentException(

                        interfaceClass.getName() + " is not an interface");

                }

                /*

                 * Verify that this interface is not a duplicate.

                 */

                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {

                    throw new IllegalArgumentException(

                        "repeated interface: " + interfaceClass.getName());

                }

            }



            // 生成代理类包名

            String proxyPkg = null;     // package to define proxy class in

            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;



            /*

             * Record the package of a non-public proxy interface so that the

             * proxy class will be defined in the same package.  Verify that

             * all non-public proxy interfaces are in the same package.

             */

            //验证所有非公共的接口在同一个包内；公共的就无需处理

            //生成包名和类名的逻辑，包名默认是com.sun.proxy，类名默认是$Proxy 加上一个自增的整数值

            //如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名

            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) {

                // if no non-public proxy interfaces, use com.sun.proxy package

                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";

            }



            /*

             * Choose a name for the proxy class to generate.

             */

            long num = nextUniqueNumber.getAndIncrement();

            // 代理类的完全限定名，如com.sun.proxy.$Proxy0.calss

            String proxyName = proxyPkg + proxyClassNamePrefix + num;



            /*

             * Generate the specified proxy class.

             */

            // 代理类字节码生成

            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(

                proxyName, interfaces, accessFlags);

            try {

                //把代理类加载到JVM中，至此动态代理过程基本结束了

                return defineClass0(loader, proxyName,

                                    proxyClassFile, 0, proxyClassFile.length);

            } catch (ClassFormatError e) {

                /*

                 * A ClassFormatError here means that (barring bugs in the

                 * proxy class generation code) there was some other

                 * invalid aspect of the arguments supplied to the proxy

                 * class creation (such as virtual machine limitations

                 * exceeded).

                 */

                throw new IllegalArgumentException(e.toString());

            }

        }

    }

在ProxyGenerator.generateProxyClass()方法中完成代理类的字节码的组装，最终就生成了本文一开始保存出来的代理类的内容，感兴趣的读者可以自行研究，值得注意的是在创建代理类的构造函数时，此处设定需要传入InvocationHandler对象，所以这样就能确保我们代理类能够通过我们实现的InvocationHandler接口去调用被代理类的方法。

private ProxyGenerator.MethodInfo generateConstructor() throws IOException {

        ProxyGenerator.MethodInfo var1 = new ProxyGenerator.MethodInfo("<init>", "(Ljava/lang/reflect/InvocationHandler;)V", 1);

        DataOutputStream var2 = new DataOutputStream(var1.code);

        this.code_aload(0, var2);

        this.code_aload(1, var2);

        var2.writeByte(183);

        var2.writeShort(this.cp.getMethodRef("java/lang/reflect/Proxy", "<init>", "(Ljava/lang/reflect/InvocationHandler;)V"));

        var2.writeByte(177);

        var1.maxStack = 10;

        var1.maxLocals = 2;

        var1.declaredExceptions = new short[0];

        return var1;

    }

巴特西

设计模式学习——JAVA动态代理原理分析

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