Proxy源码

1，成员变量

?代理类的构造函数参数。默认每个代理类都具有一个invocationHandler的构造方法。（本文代码主要基于jdk 1.7）

/** parameter types of a proxy class constructor */
    private static final Class<?>[] constructorParams =
        { InvocationHandler.class };

?缓存代理对象。

private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

代理对象的InvacationHandler，每个代理对象都有一个与之对应的Invocationhandler对象。

 /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;

2，构造方法

?私有构造方法，禁止外部直接通过new关键字生成代理对象。

 /**
     * Prohibits instantiation.
     */
    private Proxy() {
    }

protected构造函数。

 /**
     * Constructs a new {@code Proxy} instance from a subclass
     * (typically, a dynamic proxy class) with the specified value
     * for its invocation handler.
     *
     * @param   h the invocation handler for this proxy instance
     */
    protected Proxy(InvocationHandler h) {
        doNewInstanceCheck();
        this.h = h;
    }

3，newProxyInstance方法

?newProxyInstance方法是我们外部生成代理对象时候主要调用的方法。

@CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        if (h == null) {//检查InvocationHandler，如果为空，直接抛出异常
            throw new NullPointerException();
        }

        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {

//检查对应的classLoader是否为空，以及接口是否可见

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

        /*
         * Look up or generate the designated proxy class.
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
                // create proxy instance with doPrivilege as the proxy class may
                // implement non-public interfaces that requires a special permission
                return AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    public Object run() {
                        return newInstance(cons, ih);
                    }
                });
            } else {
                return newInstance(cons, ih);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString());
        }
    }

我们可以看到，最主要的方法在getProxyClass0方法，我们查看其对应的方法。

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
        return proxyClassCache.get(loader, interfaces);
    }

我们从代码注释中可以看到，此出主要出缓存中获取代理对象。缓存表中主要存放以类加载器为key的Map对象。我们查看缓存的proxyClassCache的get方法。

 public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);//校验接口不为空

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);//生成缓存对象

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

//内部缓存map中获取对应的对象

        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {//为空
            ConcurrentMap<Object, Supplier<V>> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());//map中存入对应cacheKey和Map
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }

        // create subKey and retrieve the possible Supplier<V> stored by that
        // subKey from valuesMap
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();//Supplier实现了一个get接口。主要由Factory实现其具体接口。
                if (value != null) {
                    return value;//存在，则直接返回
}
            }
            // 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);//创建Factory
            }

            if (supplier == null) {//Supplier
                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);
                }
            }
        }
    }

此处主要用到了ConcurrentHashMap的相关操作。

我们主要的操作都是通过自定义的Factory的get方法来获取我们对应的缓存对象。

  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);
            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 {
                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)
            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;
        }
    }

在此处，通过valueFactory的apply方法来生成一个新的代理对象，然后放入缓存。valueFactory为BiFunction接口实例，主要实现有KeyFactory和ProxyClassFactory。ProxyClassFactory是生成代理对象的关键。

 /**
     * A factory function that generates, defines and returns the proxy class given
     * the ClassLoader and array of interfaces.
     */
    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";//代理名称前缀

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();//代理名称的下一个名字

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

//循环检查接口
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) {
                }
                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());
                }
            }

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

            /*
             * 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)) {
                    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);//生成最终的代理对象
            try {
                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());
            }
        }
    }

最终的代理对象由ProxyGenerator.generateProxyClass实现。改类的源码SUN并未提供，只能尝试一些反编译技术查看，在此省略。

以上即为JDK实现动态代理的基本原理。我们抛开各种技术细节总结，可以整体的将实现过程概括为下面：

?1）检查内部缓存代码是否存在对应ClassLoader对应的代理对象。存在则直接返回，否则生成。

?2）生成代理对象时，获取对应的Factory视力，如果存在Factory实例，则直接调用其get方法，否则生成Factory实例。

?3）调用Factory的get方法，再调用ProxyClassFactory的apply方法，apply方法调用ProxyGenerator.generateProxyClass方法生成最终的代理对象。在Fatory内部，生成代理对象后，缓存代理对象。

实现自己的Proxy类

看了如上JDK Proxy类的实现，整体结构我们可以有个较为清晰的认识。但是对具体生成代理对象，以为SUN隐藏了内部实现，我们可能较为模糊，下面我们可以自己使用反射来完成一个自己的Proxy类。

个人认为，Proxy内的内部，只是将我们的接口，以及InvocationHandler方法进行组装而已。

?此部分代码参考博客：http://www.ibm.com/developerworks/cn/java/j-lo-proxy1/

?

package com.jing.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

/**
 * 自定义代理，实现对实例进行代理
 * 
 * @author jinglongjun
 *
 * @param <T>
 */
public final class MyProxy {

	private InvocationHandler handler;// 处理器

	/**
	 * 禁止外部访问
	 */
	private MyProxy() {

	}

	public MyProxy(InvocationHandler handler) {
		
		this.handler = handler;
	}

	/**
	 * 代理执行方法
	 * 
	 * @param methodName
	 *            代理方法名称
	 * @param obj
	 *            目标对象
	 * @param args
	 *            方法执行参数
	 * @param parameterTypes
	 *            方法参数类型数组
	 * @return 方法执行返回值
	 * @throws Exception
	 */
	public Object invoke(String methodName, Object obj,
			Class<?>[] parameterTypes, Object... args) throws Exception {

		if (obj == null)
			throw new NullPointerException("目标对象不能为空！");

		// 1，获取代理对象方法
		Method method = obj.getClass().getMethod(methodName, parameterTypes);// 获取方法

		// 2.获取InvocationHandler的invke方法，并执行。此处传入了目标对象的Method对象
		Object result = null;
		try {
			//执行Handler的Invoke方法
			result = handler.invoke(this, method, args);
		} catch (Throwable e) {

			e.printStackTrace();
		}

		return result;
	}
}

调用实例

package com.jing.proxy;

public class TestMyProxy {

	public static void main(String[] args) throws Exception {
		
		HelloSubject helloSubject = new HelloSubject();
		SubHandler subHandler = new SubHandler(helloSubject);
		MyProxy proxy = new MyProxy(subHandler);
		Object result = proxy.invoke("doSomething", helloSubject, null, new Object[]{});
		System.out.println(result);
	}
}

如上，即为整体JDK动态代理的实现，部分代码我写的比较模糊，比如缓存的具体实现步骤，此处可以在后续有时间再深入地学习，毕竟不能一口吃成胖子，还是要循环渐进。

我们通过对源码的分析很容易帮助我们更好的理解动态代理模式。

了解了动态代理的实现原理，后续我们可以分析开源框架中对动态代理模式的应用。

具体后续会根据Spring AOP的代码实现来查看动态代理对框架的作用。

?

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