JDK动态代理案例实现：实现 InvocationHandler 接口重写 invoke 方法，其中包含一个对象变量和提供一个包含对象的构造方法；
public class MyInvocationHandler implements InvocationHandler { Object target;//目标对象 public MyInvocationHandler(Object target){ this.target=target; } /** * @param proxy 代理对象 * @param method 目标对象的目标方法 * @param args 目标方法的参数 */ @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("log"); return method.invoke(target,args); } }
public class MyInvocationHandlerTest { public static void main(String[] args) { //参数： 当前类的classLoader（保证MyInvocationHandlerTest当前类可用） // 接口数组：通过接口反射得到接口里面的方法，对接口里面的所有方法都进行代理 // 实现的InvocationHandler：参数是目标对象 UserDao jdkproxy = (UserDao) Proxy.newProxyInstance(MyInvocationHandlerTest.class.getClassLoader(), new Class[]{UserDao.class},new MyInvocationHandler(new UserService())); jdkproxy.query("query"); //-----结果------ //log //query } }
接下来查看 Proxy.newProxyInstance 源码探究它的实现过程：
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { //检查InvocationHandler是否为空，为空抛出空指针异常 Objects.requireNonNull(h); //克隆拿到接口 final Class<?>[] intfs = interfaces.clone(); //进行安全校验 final SecurityManager sm = System.getSecurityManager(); //检查是否能被代理 if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } /* * Look up or generate the designated proxy class. * 得到代理类 */ Class<?> cl = getProxyClass0(loader, intfs); /* * Invoke its constructor with the designated invocation handler. */ try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } //通过构造方法得到代理类的对象 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; } }); } //new得到代理对象 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); } }
从上面可以看出 Class<?> cl = getProxyClass0(loader, intfs); 这行代码最重要，它可以得到一个代理对象，下面是 Proxy#getProxyClass0 的源码：
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 //如果接口的代理类已经存在缓存中了，直接从缓存中取出来返回，如果不存在则通过ProxyClassFactory创建一个并放入缓存中供下次使用 return proxyClassCache.get(loader, interfaces); }
proxyClassCache.get() 的实现在 WeakCache#get() 中：
public V get(K key, P parameter) { Objects.requireNonNull(parameter); expungeStaleEntries(); //从缓存中取出代理类的key值，因为代理类的生成需要耗时间和性能，所有缓存起来方便下次直接使用 Object cacheKey = CacheKey.valueOf(key, refQueue); // 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 获取到接口信息 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); Supplier<V> supplier = valuesMap.get(subKey); Factory factory = null; //开始进来supplier是null，所以执行后面的代码创建一个Factory然后赋值给supplier while (true) { if (supplier != null) { // supplier might be a Factory or a CacheValue<V> instance //获取到代理类 V value = supplier.get(); if (value != null) { //返回代理类 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);//通过接口信息和classLoader创建一个工厂 } 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;//将工厂赋值给supplier } else { // retry with current supplier supplier = valuesMap.get(subKey); } } } }
下面查看 supplier.get()：WeakCache.Factory#get()
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 { //创建获取到value 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);//根据代理类创建一个缓存对象 // put into reverseMap reverseMap.put(cacheValue, Boolean.TRUE);//将代理放入缓存中 // 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; }
接下来查看 Objects.requireNonNull() 中的 apply方法：Proxy.ProxyClassFactory#apply()
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { //获取到所有实现接口 Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length); //遍历所有接口 for (Class<?> intf : interfaces) { /* * Verify that the class loader resolves the name of this * interface to the same Class object. */ Class<?> interfaceClass = null; try { //对接口再次装载，相当classLoader.loadClass interfaceClass = Class.forName(intf.getName(), false, loader); } catch (ClassNotFoundException e) { } //判断两个接口是不是同一个接口（同一个类加载器加载的）：判断对象是否是同一个 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()); } } //代理对象的包名 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. */ for (Class<?> intf : interfaces) { int flags = intf.getModifiers(); //判断接口的类型是否是public，如果不是public，将代理类放到目标对象同一目录下 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 //默认定义包名 //public static final String PROXY_PACKAGE = "com.sun.proxy"; proxyPkg = ReflectUtil.PROXY_PACKAGE + "."; } /* * Choose a name for the proxy class to generate. */ long num = nextUniqueNumber.getAndIncrement(); //标识：private static final String proxyClassNamePrefix = "$Proxy"; //随机数（防止多线程测试相同的类名）：long num = nextUniqueNumber.getAndIncrement(); //代理类的名字：代理包名+ 标识+ 随机数 String proxyName = proxyPkg + proxyClassNamePrefix + num; /* * Generate the specified proxy class. * 产生指定的代理类信息，是二进制信息，可以使用IO流输出代理类的Java文件（可查看前文有介绍） * ProxyGenerator.generateProxyClass()是一个静态方法，所以可以外部直接调用 */ byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags); try { //产生代理类，返回一个class：将byte字节码转换成class //defineClass0是一个native方法，由JVM实现的：private static native Class<?> defineClass0(); 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()); } }