这篇文章将为大家详细讲解有关JDK中动态代理的示例分析，小编觉得挺实用的，因此分享给大家做个参考，希望大家阅读完这篇文章后可以有所收获。
1.创建一个实现接口InvocationHandler的类，它必须实现invoke方法
2.创建被代理的类以及接口
3.通过Proxy的静态方法
通过Proxy的静态方法
ProxyObject proxyObject = new ProxyObject(); InvocationHandler invocationHandler = new DynamicProxy(proxyObject); ClassLoader classLoader = proxyObject.getClass().getClassLoader(); ProxyObjectInterface proxy = (IRoom) Proxy.newProxyInstance(classLoader,new Class[] {ProxyObjectInterface.class},invocationHandler); proxy.execute(); public class DynamicProxy implements InvocationHandler { private Object object; public DynamicProxy(Object object){ this.object = object; } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { Object result = method.invoke(object,args); return result; } }创建一个代理 newProxyInstance
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { //检验h不为空，h为空抛异常 Objects.requireNonNull(h); //接口的类对象拷贝一份 final Class<?>[] intfs = interfaces.clone(); //进行一些安全性检查 final SecurityManager sm = System.getSecurityManager(); if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } /* * Look up or generate the designated proxy class. * 查询（在缓存中已经有）或生成指定的代理类的class对象。 */ Class<?> cl = getProxyClass0(loader, intfs); /* * Invoke its constructor with the designated invocation handler. */ try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } //得到代理类对象的构造函数，这个构造函数的参数由constructorParams指定 //参数constructorParames为常量值： private static final Class<?>[] constructorParams = { InvocationHandler.class }; final Constructor<?> cons = cl.getConstructor(constructorParams); final InvocationHandler ih = h; if (!Modifier.isPublic(cl.getModifiers())) { AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { cons.setAccessible(true); return null; } }); } //这里生成代理对象，传入的参数new Object[]{h}后面讲 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); } }先对h进行判空处理。
这段代码核心就是通过getProxyClass0(loader, intfs)得到代理类的Class对象，然后通过Class对象得到构造方法，进而创建代理对象。下一步看getProxyClass0这个方法。从1可知，先接口得到接口类，当接口的数量超过65535，则报异常。
//此方法也是Proxy类下的方法 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 //意思是：如果代理类被指定的类加载器loader定义了，并实现了给定的接口interfaces， //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); }proxyClassCache 是一个弱引用的缓存
这里看到proxyClassCache，有Cache便知道是缓存的意思，正好呼应了前面Look up or generate the designated proxy class。查询（在缓存中已经有）或生成指定的代理类的class对象这段注释。
在进入get方法之前，我们看下 proxyClassCache是什么？高能预警，前方代码看起来可能有乱，但我们只需要关注重点即可。
private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory()); //K代表key的类型，P代表参数的类型，V代表value的类型。 // WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache 说明proxyClassCache存的值是Class<?>对象，正是我们需要的代理类对象。 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; 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，他是一个双重的Map结构: (key, sub-key) -> value。其中key是传进来的Classloader进行包装后的对象，sub-key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，是由WeakCache构造函数传人的ProxyClassFactory()生成的。如下，回顾一下:
proxyClassCache是个WeakCache类的对象，调用proxyClassCache.get(loader, interfaces)； 可以得到缓存的代理类或创建代理类（没有缓存的情况）。
说明WeakCache中有get这个方法。先看下WeakCache类的定义（这里先只给出变量的定义和构造函数），继续看它的get（）；
//K和P就是WeakCache定义中的泛型，key是类加载器，parameter是接口类数组 public V get(K key, P parameter) { //检查parameter不为空 Objects.requireNonNull(parameter); //清除无效的缓存 expungeStaleEntries(); // cacheKey就是(key, sub-key) -> value里的一级key， Object cacheKey = CacheKey.valueOf(key, refQueue); // lazily install the 2nd level valuesMap for the particular cacheKey //根据一级key得到 ConcurrentMap<Object, Supplier<V>>对象。如果之前不存在，则新建一个ConcurrentMap<Object, Supplier<V>>和cacheKey（一级key）一起放到map中。 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 //这部分就是调用生成sub-key的代码，上面我们已经看过怎么生成的了 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); //通过sub-key得到supplier Supplier<V> supplier = valuesMap.get(subKey); //supplier实际上就是这个factory Factory factory = null; while (true) { //如果缓存里有supplier ，那就直接通过get方法，得到代理类对象，返回，就结束了，一会儿分析get方法。 if (supplier != null) { // supplier might be a Factory or a CacheValue<V> instance 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 //下面的所有代码目的就是：如果缓存中没有supplier，则创建一个Factory对象，把factory对象在多线程的环境下安全的赋给supplier。 //因为是在while（true）中，赋值成功后又回到上面去调get方法，返回才结束。 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); } } } }所以接下来我们看Factory类中的get方法。接下来看supplier的get()
public synchronized V get() { // serialize access // re-check Supplier<V> supplier = valuesMap.get(subKey); //重新检查得到的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 { //代理类就是在这个位置调用valueFactory生成的 //valueFactory就是我们传入的 new ProxyClassFactory() //一会我们分析ProxyClassFactory()的apply方法 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); // put into reverseMap // reverseMap是用来实现缓存的有效性 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"); } // successfully replaced us with new CacheValue -> return the value // wrapped by it return value; } }拨云见日，来到ProxyClassFactory的apply方法，代理类就是在这里生成的。
首先看proxyClassCache的定义WeakCache<ClassLoader, Class<?>[], Class<?>>，泛型里面第一个表示加载器K，第二个表示接口类P，第三个则是生成的代理类V。而V的生成则是通过ProxyClassFactory生成的。调用其apply（）；
//这里的BiFunction<T, U, R>是个函数式接口，可以理解为用T，U两种类型做参数，得到R类型的返回值 private static final class ProxyClassFactory implements BiFunction<ClassLoader, Class<?>[], Class<?>> { // prefix for all proxy class names //所有代理类名字的前缀 private static final String proxyClassNamePrefix = "$Proxy"; // next number to use for generation of unique proxy class names //用于生成代理类名字的计数器 private static final AtomicLong nextUniqueNumber = new AtomicLong(); @Override public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length); //验证代理接口，可不看 for (Class<?> intf : interfaces) { /* * Verify that the class loader resolves the name of this * interface to the same Class object. */ Class<?> interfaceClass = null; try { interfaceClass = Class.forName(intf.getName(), false, loader); } catch (ClassNotFoundException e) { } if (interfaceClass != intf) { throw new IllegalArgumentException( intf + " is not visible from class loader"); } /* * Verify that the Class object actually represents an * interface. */ if (!interfaceClass.isInterface()) { throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface"); } /* * Verify that this interface is not a duplicate. */ if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) { throw new IllegalArgumentException( "repeated interface: " + interfaceClass.getName()); } } //生成的代理类的包名 String proxyPkg = null; // package to define proxy class in //代理类访问控制符: public ,final int accessFlags = Modifier.PUBLIC | Modifier.FINAL; /* * Record the package of a non-public proxy interface so that the * proxy class will be defined in the same package. Verify that * all non-public proxy interfaces are in the same package. */ //验证所有非公共的接口在同一个包内；公共的就无需处理 //生成包名和类名的逻辑，包名默认是com.sun.proxy， // 类名默认是$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. */ //核心部分，生成代理类的字节码 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()); } } }然后调用getMethod（）,将equals(),hashcode(),toString()等方法添加进去。然后遍历所有接口的方法，添加到代理类中。最后将这些方法进行排序。
private static List<Method> getMethods(Class<?>[] interfaces) { List<Method> result = new ArrayList<Method>(); try { result.add(Object.class.getMethod("equals", Object.class)); result.add(Object.class.getMethod("hashCode", EmptyArray.CLASS)); result.add(Object.class.getMethod("toString", EmptyArray.CLASS)); } catch (NoSuchMethodException e) { throw new AssertionError(); } getMethodsRecursive(interfaces, result); return result; } private static void getMethodsRecursive(Class<?>[] interfaces, List<Method> methods) { for (Class<?> i : interfaces) { getMethodsRecursive(i.getInterfaces(), methods); Collections.addAll(methods, i.getDeclaredMethods()); } }最后输出相关proxy class
package com.zhb.jdk.proxy; import java.io.FileOutputStream; import java.io.IOException; import java.lang.reflect.Proxy; import com.zhb.jdk.dynamicProxy.HelloworldImpl; import sun.misc.ProxyGenerator; /** * @author ZHB * @date 2018年8月31日下午11:35:07 * @todo TODO */ public class DynamicProxyTest { public static void main(String[] args) { IUserService target = new UserServiceImpl(); MyInvocationHandler handler = new MyInvocationHandler(target); //第一个参数是指定代理类的类加载器（我们传入当前测试类的类加载器） //第二个参数是代理类需要实现的接口（我们传入被代理类实现的接口，这样生成的代理类和被代理类就实现了相同的接口） //第三个参数是invocation handler，用来处理方法的调用。这里传入我们自己实现的handler IUserService proxyObject = (IUserService) Proxy.newProxyInstance(DynamicProxyTest.class.getClassLoader(), target.getClass().getInterfaces(), handler); proxyObject.add("陈粒"); String path = "D:/$Proxy0.class"; byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", HelloworldImpl.class.getInterfaces()); FileOutputStream out = null; try { out = new FileOutputStream(path); out.write(classFile); out.flush(); } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } } } } // Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov. // Jad home page: http://kpdus.tripod.com/jad.html // Decompiler options: packimports(3) fieldsfirst ansi space import com.zhb.jdk.proxy.IUserService; import java.lang.reflect.*; public final class $Proxy0 extends Proxy implements IUserService { private static Method m1; private static Method m2; private static Method m3; private static Method m0; //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 invocationhandler) { super(invocationhandler); } // Object类中的三个方法，equals，toString， hashCode public final boolean equals(Object obj) { try { return ((Boolean)super.h.invoke(this, m1, new Object[] { obj })).booleanValue(); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final String toString() { try { return (String)super.h.invoke(this, m2, null); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } //接口代理方法 public final void add(String s) { try { // invocation handler的 invoke方法在这里被调用 super.h.invoke(this, m3, new Object[] { s }); return; } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final int hashCode() { try { // 在这里调用了invoke方法。 return ((Integer)super.h.invoke(this, m0, null)).intValue(); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } // 静态代码块对变量进行一些初始化工作 static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") }); m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]); m3 = Class.forName("com.zhb.jdk.proxy.IUserService").getMethod("add", new Class[] { Class.forName("java.lang.String") }); m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]); } catch (NoSuchMethodException nosuchmethodexception) { throw new NoSuchMethodError(nosuchmethodexception.getMessage()); } catch (ClassNotFoundException classnotfoundexception) { throw new NoClassDefFoundError(classnotfoundexception.getMessage()); } } }关于“JDK中动态代理的示例分析”这篇文章就分享到这里了，希望以上内容可以对大家有一定的帮助，使各位可以学到更多知识，如果觉得文章不错，请把它分享出去让更多的人看到。
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