Java 动态代理Proxy应用和底层源码分析

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前言

Java Proxy主要用于创建动态代理实例，这些实例实现了指定的一组接口，并在调用方法时将调用转发给指定的调用处理器（InvocationHandler）。这种机制常用于实现AOP（面向切面编程）框架、RPC（远程过程调用）框架等，以及任何需要动态改变对象行为的场景。

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一、Proxy代码示例

在这个示例中，我们创建了一个实现了 MyInterface 接口的匿名内部类对象 myObject。然后，我们创建了一个 MyInvocationHandler 对象，并将 myObject 作为目标对象传递给它。接下来，我们使用 Proxy.newProxyInstance 方法创建了一个代理对象 proxy，并将 MyInterface 接口和 handler 作为参数传递给它。最后，我们调用代理对象的 doSomething 方法，这个方法调用会转发给 handler 的 invoke 方法，并在其中添加自定义逻辑。

interface MyInterface {
    void doSomething();
}

class MyInvocationHandler implements InvocationHandler {
    private Object target;

    public MyInvocationHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Before method call");
        Object result = method.invoke(target, args);
        System.out.println("After method call");
        return result;
    }
}

public class DynamicProxyExample {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {
            @Override
            public void doSomething() {
                System.out.println("Doing something");
            }
        };

        MyInvocationHandler handler = new MyInvocationHandler(myObject);
        MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
                MyInterface.class.getClassLoader(),
                new Class<?>[]{MyInterface.class},
                handler
        );

        proxy.doSomething(); // This will call the proxy's doSomething method, which forwards to the handler's invoke method.
    }
}
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二、底层源码分析

• 从newProxyInstance这个入口来看看Proxy的重点方法。

public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h) throws IllegalArgumentException {
        //将接口clone，之后对此clone类进行操作
        final Class<?>[] intfs = interfaces.clone();
        //进行权限检查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.
         * 根据传入的类加载器和接口类数组，生成相应的代理类
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            //获取构造
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doexposingd(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //返回代理对象
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } 
    }
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• 再来看查找/生成的代理类getProxyClass0

    /**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // 如果由实现给定接口的给定加载器定义的代理类存在，将返回缓存的副本;
        // 否则，它将通过ProxyClassFactory创建代理类
        return proxyClassCache.get(loader, interfaces);
    }
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/**
     * a cache of proxy classes
     * 动态代理类的弱缓存容器
     * KeyFactory:根据接口的数量，映射一个最佳的key生成函数,其中表示接口的类对象被弱引用；
     * 也就是key对象被弱引用继承自WeakReference（key0、key1、key2、keyX），  保存接口密钥(hash值)
     * ProxyClassFactory：生成动态类的工厂
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
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• WeakCache.get(loader, interfaces)

  	// key 表示缓存键，parameter 表示参数。它的返回值类型为 V，表示缓存值。  
		public V get(K key, P parameter) {
        // 判断parameter不能为空
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);
        // 该方法将 key 和 refQueue 作为参数调用 CacheKey.valueOf() 方法，生成一个缓存键 cacheKey，并从缓存中获取与该缓存键相关的 valuesMap。
        // 如果 valuesMap 不存在，则创建一个新的 ConcurrentHashMap 对象，并将其加入到缓存中。valuesMap 是一个 ConcurrentMap 对象，它用于存储缓存键下的子键和值的提供者，其中子键由 subKeyFactory 参数生成。
        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;
            }
        }

        // 利用Key的工厂类根据key和parameter参数生成键
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        // 根据该键获取到Supplier对象，表示结果的提供者。Supplier接口包含了一个get方法
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier可能是一个工厂类实例，或者是一个CacheValue<V>实例
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            if (factory == null) {
                // 核心的创建代理类的原理在Factory类里实现
                // Factory类实际上也是Supplier接口的实现类，实现了get方法
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // 将上面的Factory类对象赋值给suppler，在下一次循环时，就可以调用Factory类的get方法来生成代理类
                    supplier = factory;
                }
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

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代码核心逻辑：
factory = new Factory(key, parameter, subKey, valuesMap);：// 创建Factory对象，Factory类是Supplier接口的实现类
supplier = factory; // 当缓存中不存在代理类时，将supplier=factory对象
V value = supplier.get(); // 调用Factory类的get方法来创建代理类

• Factory.get()

        public synchronized V get() {               
            V value = null;
            try {
                //valueFactory实际上是一个ProxyClassFactory实例，调用的是ProxyClassFactory.apply方法
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { 
                    valuesMap.remove(subKey, this);
                }
            }
            // wrap value with CacheValue (WeakReference)
            CacheValue<V> cacheValue = new CacheValue<>(value);
            // put into reverseMap
            reverseMap.put(cacheValue, Boolean.TRUE);
            // try replacing us with CacheValue (this should always succeed)
            if (!valuesMap.replace(subKey, this, cacheValue)) {
                throw new AssertionError("Should not reach here");
            }
            return value;
        }
    }
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• ProxyClassFactory.apply
  ProxyClassFactory是Proxy的静态内部类，apply 用于生成代理类的Class对象。

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

            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            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) {
                }
                /*
                 * Verify that the Class object actually represents an interface.
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
            }

            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.
             */
            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();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);

    }
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• ProxyGenerator.generateProxyClass
  generateProxyClass 生成代理类字节码

    private byte[] generateClassFile() {
        //生成hashCode方法
        this.addProxyMethod(hashCodeMethod, Object.class);
        //生成equals方法
        this.addProxyMethod(equalsMethod, Object.class);
        //生成toString方法
        this.addProxyMethod(toStringMethod, Object.class);
        Class[] var1 = this.interfaces;
        int var2 = var1.length;

        int var3;
        Class var4;
        for(var3 = 0; var3 < var2; ++var3) {
            var4 = var1[var3];
            Method[] var5 = var4.getMethods();
            int var6 = var5.length;

            for(int var7 = 0; var7 < var6; ++var7) {
                Method var8 = var5[var7];
                this.addProxyMethod(var8, var4);
            }
        }

        Iterator var11 = this.proxyMethods.values().iterator();

        List var12;
        while(var11.hasNext()) {
            var12 = (List)var11.next();
            checkReturnTypes(var12);
        }

        Iterator var15;
        try {
            this.methods.add(this.generateConstructor());
            var11 = this.proxyMethods.values().iterator();

            while(var11.hasNext()) {
                var12 = (List)var11.next();
                var15 = var12.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
                    this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    this.methods.add(var16.generateMethod());
                }
            }

            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var10) {
            throw new InternalError("unexpected I/O Exception", var10);
        }

        if (this.methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        } else if (this.fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        } else {
            this.cp.getClass(dotToSlash(this.className));
            this.cp.getClass("java/lang/reflect/Proxy");
            var1 = this.interfaces;
            var2 = var1.length;

            for(var3 = 0; var3 < var2; ++var3) {
                var4 = var1[var3];
                this.cp.getClass(dotToSlash(var4.getName()));
            }

            this.cp.setReadOnly();
            ByteArrayOutputStream var13 = new ByteArrayOutputStream();
            DataOutputStream var14 = new DataOutputStream(var13);

            try {
                var14.writeInt(-889275714);
                var14.writeShort(0);
                var14.writeShort(49);
                this.cp.write(var14);
                var14.writeShort(this.accessFlags);
                var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
                var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
                var14.writeShort(this.interfaces.length);
                Class[] var17 = this.interfaces;
                int var18 = var17.length;

                for(int var19 = 0; var19 < var18; ++var19) {
                    Class var22 = var17[var19];
                    var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
                }

                var14.writeShort(this.fields.size());
                var15 = this.fields.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
                    var20.write(var14);
                }

                var14.writeShort(this.methods.size());
                var15 = this.methods.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
                    var21.write(var14);
                }

                var14.writeShort(0);
                return var13.toByteArray();
            } catch (IOException var9) {
                throw new InternalError("unexpected I/O Exception", var9);
            }
        }
    }
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