设计模式 - 代理模式
基本介绍:
- 代理模式:为一个对象提供一个替身,以控制对这个对象的访问。即通过代理 对象访问目标对象.这样做的好处是:可以在目标对象实现的基础上,增强额外的 功能操作,即扩展目标对象的功能。
- 被代理的对象可以是远程对象、创建开销大的对象或需要安全控制的对象
- 代理模式有不同的形式, 主要有三种 静态代理、动态代理 (JDK代理、接口代 理)和 Cglib代理 (可以在内存动态的创建对象,而不需要实现接口, 他是属于 动态代理的范畴)
静态代理
基本介绍:
静态代理在使用时,需要定义接口或者父类,被代理对象(即目标对象)与代理对象一 起实现相同的接口或者是继承相同父类
接口:
/*** 接口*/
public interface ITeacherDao {/*** 授课方式*/public void teach();
}
接口实现:
public class TeacherDao implements ITeacherDao{@Overridepublic void teach() {System.out.println("开始授课教学....");}
}
静态代理:
public class TeacherDaoProxy implements ITeacherDao{private ITeacherDao proxy;public TeacherDaoProxy(ITeacherDao proxy) {this.proxy = proxy;}@Overridepublic void teach() {System.out.println("增强教学中....");proxy.teach();System.out.println("增强教学结束..");}public static void main(String[] args) {// 创建目标对象(被代理对象)TeacherDao teacherDao = new TeacherDao();// 创建代理对象,同时将被代理对象传递给代理对象TeacherDaoProxy proxy = new TeacherDaoProxy(teacherDao);proxy.teach();}
}
优点:
在不修改目标对象的功能前提下,能通过代理对象对目标功能扩展
缺点:
- 因为代理对象需要与目标对象实现一样的接口,所以会有很多代理类
- 一旦接口增加方法,目标对象与代理对象都要维护
JDK 动态代理
基本介绍:
- 代理对象,不需要实现接口,但是目标对象要实现接口,否则不能用动态代理
- 代理对象的生成,是利用JDK的API,动态的在内存中构建代理对象
- 动态代理也叫做:JDK代理、接口代理
JDK 生成代理对象的 API:
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;public class ProxyFactory {// 维护一个目标对象, Objectprivate Object target;// 构造器, 对 target 进行初始化public ProxyFactory(Object target) {this.target = target;}// 生成代理对象public Object getProxyInstance(){//1. ClassLoader loader : 指定当前目标对象使用的类加载器, 获取加载器的方法固定//2. Class<?>[] interfaces: 目标对象实现的接口类型,使用泛型方法确认类型//3. InvocationHandler h : 事情处理,执行目标对象的方法时,会触发事情处理器方法, 会把当前执行的目标对象方法作为参数传入return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("JDK 代理开始...");Object proxyObj = method.invoke(target, args);System.out.println("JDK 代理结束...");return proxyObj;}});}public static void main(String[] args) {TeacherDao techerDao = new TeacherDao();ProxyFactory proxyFactory = new ProxyFactory(techerDao);ITeacherDao proxy = (ITeacherDao) proxyFactory.getProxyInstance();proxy.teach();}
}
源码分析:
newProxyInstance方法:
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException{// 如果 h 为空直接抛出空指针异常Objects.requireNonNull(h);// 拷贝类实现的所有接口final Class<?>[] intfs = interfaces.clone();// 获取当前系统安全接口final SecurityManager sm = System.getSecurityManager();if (sm != null) {// Reflection.getCallerClass() 返回调用该方法的方法的调用类// 访问权限以及类加载器权限等检查checkProxyAccess(Reflection.getCallerClass(), loader, intfs);}// 查找或者生成指定代理类Class<?> cl = getProxyClass0(loader, intfs);try {if (sm != null) {checkNewProxyPermission(Reflection.getCallerClass(), cl);}// 获取代理类的构造方法对象// constructorParams 是类常量,作为代理类构造函数的参数类型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;}});}// 根据代理类的构造函数对象来创建需要返回的代理对象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);}}
getProxyClass0:
private static Class<?> getProxyClass0(ClassLoader loader,Class<?>... interfaces) {if (interfaces.length > 65535) {throw new IllegalArgumentException("interface limit exceeded");}// 如果缓存有代理类直接返回,否则将由类工厂 ProxyClassFactory 创建代理类return proxyClassCache.get(loader, interfaces);}
get 方法:
public V get(K key, P parameter) {// 检查指定类型的对象引用不为空Objects.requireNonNull(parameter);// 清空已经被 GC 回收的弱引用expungeStaleEntries();// 将 classLoader 包装成 CacheKey,作为一级缓存的 KEYObject cacheKey = CacheKey.valueOf(key, refQueue);// 获取二级缓存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;}}// 生成某个 KEY,根据接口生成 KEYObject subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));// 通过 subKey 获取二级缓存值Supplier<V> supplier = valuesMap.get(subKey);Factory factory = null;// 轮询到真为止while (true) {if (supplier != null) {V value = supplier.get();if (value != null) {return value;}}if (factory == null) {// 将信息存入Factoryfactory = new Factory(key, parameter, subKey, valuesMap);}if (supplier == null) {// subKey 没有对应值,就会将 factory 作为 subKey 的值放入supplier = valuesMap.putIfAbsent(subKey, factory);if (supplier == null) {supplier = factory;}} else {// 如果期间其他线程修改了值,那么将原来的值替换if (valuesMap.replace(subKey, supplier, factory)) {supplier = factory;} else {supplier = valuesMap.get(subKey);}}}}
apply 方法:
@Overridepublic Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);for (Class<?> intf : interfaces) {// 验证指定的类加载器(Loader) 加载接口所得到的 Class 对象(interfaceClass) 是否与 intf 对象相同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");}// 验证 Class 对象是不是接口if (!interfaceClass.isInterface()) {throw new IllegalArgumentException(interfaceClass.getName() + " is not an interface");}// 验证接口是否重复if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {throw new IllegalArgumentException("repeated interface: " + interfaceClass.getName());}}// 声明代理类所在包String proxyPkg = null; int accessFlags = Modifier.PUBLIC | Modifier.FINAL;// 验证所有非公共的接口在同一包内for (Class<?> intf : interfaces) {int flags = intf.getModifiers();if (!Modifier.isPublic(flags)) {accessFlags = Modifier.FINAL;String name = intf.getName();int n = name.lastIndexOf('.');// 截取完整包名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");}}}// 根据规则生成文件名if (proxyPkg == null) {// 如果都是public 接口,那么生成的代理类就在com.sun.proxy 包下// 如果报文件找不到,则在项目中创建 com.sun.proxy 路径proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";}long num = nextUniqueNumber.getAndIncrement();// 代理类的完全限定类名 如 com.sun.proxy.$Proxy0.classString proxyName = proxyPkg + proxyClassNamePrefix + num;// 生成代理类的字节码数组byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);// 生成 class try {return defineClass0(loader, proxyName,proxyClassFile, 0, proxyClassFile.length);} catch (ClassFormatError e) {throw new IllegalArgumentException(e.toString());}}}
动态代理为什么要有接口:
通过父类 Proxy 的构造方法,保存了创建代理对象过程中传进来的 InvocationHandler 的实例,使用 protected 修饰保证了它可以在子类中被访问和使用。但是同时,因为 Java 是单继承的,因此在代理类 $Proxy0 继承了 Proxy 后,其只能通过实现目标接口的方式来实现方法的扩展,达到我们增强目标方法逻辑的目的
流程图:
Cglib 代理
基本介绍:
- 静态代理和JDK代理模式都要求目标对象是实现一个接口,但是有时候目标对象只 是一个单独的对象,并没有实现任何的接口,这个时候可使用目标对象子类来实现 代理-这就是Cglib代理
- Cglib代理也叫作子类代理,它是在内存中构建一个子类对象从而实现对目标对象功 能扩展, 有些书也将Cglib代理归属到动态代理
- Cglib是一个强大的高性能的代码生成包,它可以在运行期扩展java类与实现java接 口.它广泛的被许多AOP的框架使用,例如Spring AOP,实现方法拦截
- 在AOP编程中如何选择代理模式: 1. 目标对象需要实现接口,用JDK代理 2. 目标对象不需要实现接口,用Cglib代理
- Cglib包的底层是通过使用字节码处理框架ASM来转换字节码并生成新的类
代理实现:
import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;import java.lang.reflect.Method;public class ProxyFactory implements MethodInterceptor {// 维护一个目标对象private Object target;// 构造器,传一个被代理的对象public ProxyFactory(Object target){this.target = target;}// 返回一个代理对象: 是 target 对象的代理对象public Object getProxyInstance(){// 创建一个工具类,增强器Enhancer enhancer = new Enhancer();// 设置父类enhancer.setSuperclass(target.getClass());// 设置回调函数enhancer.setCallback(this);// 创建子类对象,即代理对象return enhancer.create();}@Overridepublic Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {System.out.println("Cglib 代理模式...开始");Object obj = method.invoke(target, objects);System.out.println("Cglib 代理模式...结束");return obj;}public static void main(String[] args) {TeacherDao teacherDao = new TeacherDao();ProxyFactory proxyFactory = new ProxyFactory(teacherDao);TeacherDao proxy = (TeacherDao) proxyFactory.getProxyInstance();proxy.teach();}
}
源码分析:
createHelper
private Object createHelper() {this.preValidate();// 生成了一个 EnhancerKey 代理对象,该代理对象里面保存了用户所设置的代理信息// 缓存到 LoadingCache<AbstractClassGenerator, Object, Object>Object key = KEY_FACTORY.newInstance(this.superclass != null ? this.superclass.getName() : null, ReflectUtils.getNames(this.interfaces), this.filter == ALL_ZERO ? null : new WeakCacheKey(this.filter), this.callbackTypes, this.useFactory, this.interceptDuringConstruction, this.serialVersionUID);this.currentKey = key;// 去创建代理对象,避免重复 createObject result = super.create(key);return result;}
create
protected Object create(Object key) {try {ClassLoader loader = this.getClassLoader();Map<ClassLoader, AbstractClassGenerator.ClassLoaderData> cache = CACHE;AbstractClassGenerator.ClassLoaderData data = (AbstractClassGenerator.ClassLoaderData)cache.get(loader);if (data == null) {Class var5 = AbstractClassGenerator.class;synchronized(AbstractClassGenerator.class) {cache = CACHE;data = (AbstractClassGenerator.ClassLoaderData)cache.get(loader);if (data == null) {Map<ClassLoader, AbstractClassGenerator.ClassLoaderData> newCache = new WeakHashMap(cache);// 构造方法中会生成一个代理类的 lambda 表达式data = new AbstractClassGenerator.ClassLoaderData(loader);newCache.put(loader, data);CACHE = newCache;}}}this.key = key;// 利用 ClassLoaderData 拿到代理类, ClassLoaderData 中有一个 generatedClasses 用来缓存生成好的代理类Object obj = data.get(this, this.getUseCache());// 调用代理类的构造发那个发生成一个代理对象return obj instanceof Class ? this.firstInstance((Class)obj) : this.nextInstance(obj);} catch (Error | RuntimeException var9) {throw var9;} catch (Exception var10) {throw new CodeGenerationException(var10);}}
createUsingReflection
private Object createUsingReflection(Class type) {// 设置 callbacks 到代理类中setThreadCallbacks(type, this.callbacks);Object var2;try {if (this.argumentTypes == null) {var2 = ReflectUtils.newInstance(type);return var2;}var2 = ReflectUtils.newInstance(type, this.argumentTypes, this.arguments);} finally {setThreadCallbacks(type, (Callback[])null);}return var2;}
setCallbacksHelper
// 设置 ThreadLocal CGLIB$SET_THREAD_CALLBACKS 属性private static void setThreadCallbacks(Class type, Callback[] callbacks) {setCallbacksHelper(type, callbacks, "CGLIB$SET_THREAD_CALLBACKS");}private static void setCallbacksHelper(Class type, Callback[] callbacks, String methodName) {try {Method setter = getCallbacksSetter(type, methodName);setter.invoke((Object)null, callbacks);} catch (NoSuchMethodException var4) {throw new IllegalArgumentException(type + " is not an enhanced class");} catch (IllegalAccessException var5) {throw new CodeGenerationException(var5);} catch (InvocationTargetException var6) {throw new CodeGenerationException(var6);}}
generate
protected Class generate(ClassLoaderData data) {this.validate();// 生成类的前缀名字if (this.superclass != null) {this.setNamePrefix(this.superclass.getName());} else if (this.interfaces != null) {this.setNamePrefix(this.interfaces[ReflectUtils.findPackageProtected(this.interfaces)].getName());}return super.generate(data);}protected Class generate(AbstractClassGenerator.ClassLoaderData data) {Object save = CURRENT.get();CURRENT.set(this);Class var8;try {ClassLoader classLoader = data.getClassLoader();if (classLoader == null) {throw new IllegalStateException("ClassLoader is null while trying to define class " + this.getClassName() + ". It seems that the loader has been expired from a weak reference somehow. Please file an issue at cglib's issue tracker.");}String className;synchronized(classLoader) {className = this.generateClassName(data.getUniqueNamePredicate());data.reserveName(className);this.setClassName(className);}Class gen;if (this.attemptLoad) {try {gen = classLoader.loadClass(this.getClassName());Class var23 = gen;return var23;} catch (ClassNotFoundException var19) {}}byte[] b = this.strategy.generate(this);className = ClassNameReader.getClassName(new ClassReader(b));ProtectionDomain protectionDomain = this.getProtectionDomain();synchronized(classLoader) {gen = ReflectUtils.defineClass(className, b, classLoader, protectionDomain, this.contextClass);}var8 = gen;} catch (Error | RuntimeException var20) {throw var20;} catch (Exception var21) {throw new CodeGenerationException(var21);} finally {CURRENT.set(save);}return var8;}