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;
    }