java动态代理

jdk动态代理

目标接口和实现类 public interface UserService { void save(); } public class UserServiceImpl implements UserService { @Override public void save() { System.out.println("save user"); } } 代理类 public class ProxyInvocation implements InvocationHandler { private Object target; public UserService getProxy() { return (UserService) Proxy.newProxyInstance(Thread.currentThread() .getContextClassLoader(), target.getClass().getInterfaces(), this); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("before invoke"); Object object = method.invoke(target, args); System.out.println("after invoke"); return object; } public ProxyInvocation() { } public ProxyInvocation(Object target) { this.target = target; } } 测试 @org.junit.Test public void test(){ UserService userService = new UserServiceImpl(); ProxyInvocation proxyInvocation = new ProxyInvocation(userService); proxyInvocation.getProxy().save(); }

测试结果

before invoke save user after invoke

过程分析

进入newProxyInstance方法进入Class<?> cl = getProxyClass0(loader, intfs)进入proxyClassCache.get(loader, interfaces)看到这里Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter))查看apply代码,就可以看到反射生成class对象了. @CallerSensitive public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h) throws IllegalArgumentException{ Objects.requireNonNull(h); final Class<?>[] intfs = interfaces.clone(); final SecurityManager sm = System.getSecurityManager(); if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } //获取代理对象字节码 Class<?> cl = getProxyClass0(loader, intfs); try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } //获取代理对象的构造器 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; } }); } //通过构造器生成代理对象实例 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); } } 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 //优先从缓存获取class return proxyClassCache.get(loader, interfaces); } public V get(K key, P parameter) { Objects.requireNonNull(parameter); expungeStaleEntries(); 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 //如果不存在则新建 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(); 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); } } } } private static final class ProxyClassFactory implements BiFunction<ClassLoader, Class<?>[], Class<?>> { // prefix for all proxy class names private static final String proxyClassNamePrefix = "$Proxy"; // next number to use for generation of unique proxy class names private static final AtomicLong nextUniqueNumber = new AtomicLong(); @Override 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 { //终于看到熟悉的反射创建class对象 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 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); 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()); } } }

cglib代理

目标接口和实现类 public interface UserService { void save(); } public class UserServiceImpl implements UserService { @Override public void save() { System.out.println("save user"); } } 代理类 public class ProxyCglib implements MethodInterceptor{ @Override public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable { System.out.println("before"); methodProxy.invokeSuper(o, objects); System.out.println("after"); return null; } } 测试 @org.junit.Test public void test(){ ProxyCglib proxyCglib = new ProxyCglib(); Enhancer enhancer = new Enhancer(); enhancer.setSuperclass(UserServiceImpl.class); enhancer.setCallback(proxyCglib); UserService userService= (UserService) enhancer.create(); userService.save(); }

这里只是简单地展示了cglib的用法,实际上cglib能做的远远不止如此,待看springaop源码的时候再来深究.

测试结果 before save user after

两者的不同点

JDK动态代理只能对实现了接口的类生成代理，而不能针对类 CGLIB是针对类实现代理，主要是对指定的类生成一个子类，覆盖其中的方法 因为是继承，所以该类或方法最好不要声明成final 在使用spring来管理事物的时候,如果采用jdk动态代理的话注解可以加在接口或者实现类上面,采用cglib动态代理的话只能加在实现类上面.

最后是关于动态代理一些博客推荐

Spring AOP 实现原理与 CGLIB 应用深入浅出 cglib，打造无入侵的类代理JDK动态代理实现原理CGlib使用笔记Java动态代理机制详解（JDK 和CGLIB，Javassist，ASM）

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