只有掌握了这三种代理模式,才能进军Spring AOP哦!

代理模式定义

首先咱们来看看代理模式: java

image.png
所谓代理模式,是指客户端(Client)并不直接调用实际的对象(下图右下角的RealSubject),而是经过调用代理(ProxySubject),来间接的调用实际的对象。

代理模式的使用场合,通常是因为客户端不想直接访问实际对象,或者访问实际的对象存在技术上的障碍,于是经过代理对象做为桥梁,来完成间接访问。spring

业务场景

首先有个UserService接口,接口里有一个添加用户的方法ide

public interface UserService {
    void addUser();
}
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这是它的实现类函数

public class UserServiceImpl implements UserService {
    @Override
    public void addUser() {
        System.out.println("添加一个用户");
    }
}
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如今须要在添加用户的时候记录一下日志。固然,你能够直接在addUser里面直接写添加日志的代码,工具

public void addUser() {
        System.out.println("添加一个用户");
	System.out.println("拿个小本本记一下");
    }
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可是Java推崇单一职责原则,若是这样写就违背了这个原则,咱们须要将添加日志的代码解耦出来,让addUser()方法专一写本身的业务逻辑。测试

静态代理

根据类图,建立一个静态代理类this

public class UserStaticProxy implements UserService{
    private UserService userService;
    public UserStaticProxy(UserService userService) {
        this.userService = userService;
    }

    @Override
    public void addUser() {
        userService.addUser();
        System.out.println("拿个小本本记录一下");
    }
}
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咱们创建一个测试类来测试静态代理:编码

public class Test {

    public static void main(String[] args) {
        UserStaticProxy userStaticProxy = new UserStaticProxy(new UserServiceImpl());
        userStaticProxy.addUser();
    }
}
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运行结果: spa

image.png
如此,一个静态代理类就建立好了,咱们能够专一在Service写业务逻辑,添加日志等非业务逻辑交给这个静态代理类来完成。

静态代理的缺点

缺点一:接口增长方法,代理类须要同步维护

随着业务扩大,UserService类里不知有addUser方法,还有updateUser、deleteUser、batchUpdateUser、batchDeleteUser等方法,这些方法都须要记录日志。3d

UserServiceImpl类以下:

public class UserServiceImpl implements UserService {
    @Override
    public void addUser() {
        System.out.println("添加一个用户");
    }

    @Override
    public void updateUser() {
        System.out.println("更新一个用户");
    }

    @Override
    public void deleteUser() {
        System.out.println("删除一个用户");
    }

    @Override
    public void batchUpdateUser() {
        System.out.println("批量更新用户");
    }

    @Override
    public void batchDeleteUser() {
        System.out.println("批量删除用户");
    }
}
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那么对应的静态代理类以下:

public class UserStaticProxy implements UserService{
    private UserService userService;
    public UserStaticProxy(UserService userService) {
        this.userService = userService;
    }

    @Override
    public void addUser() {
        userService.addUser();
        System.out.println("拿个小本本记录一下");
    }

    @Override
    public void updateUser() {
        userService.updateUser();
        System.out.println("拿个小本本记录一下");
    }

    @Override
    public void deleteUser() {
        userService.deleteUser();
        System.out.println("拿个小本本记录一下");
    }

    @Override
    public void batchUpdateUser() {
        userService.batchUpdateUser();
        System.out.println("拿个小本本记录一下");
    }

    @Override
    public void batchDeleteUser() {
        userService.batchDeleteUser();
        System.out.println("拿个小本本记录一下");
    }
}
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从上面咱们能够看到,代理类里有不少重复的日志代码。由于代理类和目标对象实现同一个接口,一旦接口增长方法,代理类也得同步增长方法而且得同步增长重复的额外功能代码,增大了代码量

缺点二:接口越多,致使代理类繁多

若是须要增长业务类,如StudentService,TeacherService等等,这些类里的方法也都须要实现增长日志的方法,那么就须要同步建立对应的代理类。此外静态代理类不是自动生成的,须要在编译以前就编写好的,若是业务愈来愈庞大,那么建立的代理类愈来愈多,这样又增大了代码量

如何解决这些缺点呢?这时候就须要动态代理方法了

JDK动态代理

其实动态代理和静态代理的本质是同样的,最终程序运行时都须要生成一个代理对象实例,经过它来完成相关加强以及业务逻辑,只不过静态代理须要硬编码的方式指定,而动态代理支持运行时动态生成这种实现方式。

JDK自己帮咱们实现了动态代理,只须要使用newProxyInstance方法:

public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h) 复制代码

注意该方法是在Proxy类中是静态方法,且接收的三个参数依次为:

  • ClassLoader loader,:指定当前目标对象使用类加载器
  • Class<?>[] interfaces,:代理类须要实现的接口列表
  • InvocationHandler h:调用处理程序,将目标对象的方法分派到该调用处理程序

代码示例:

public class DynamicProxy implements InvocationHandler {

    private Object target; // 目标对象

    public Object bind(Object target) {
        this.target = target;
        return Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = method.invoke(target, args);
        System.out.println("拿个小本本记录一下");
        return result;
    }

}
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上文的invoke方法,负责加强目标对象的方法,接口类的全部方法都会走这个invoke方法。另外bind方法简单封装了JDK的代理方法newProxyInstance,负责返回接口类。

测试类:

public static void main(String[] args) {
        DynamicProxy dynamicProxy = new DynamicProxy();
        UserService userService = (UserService)dynamicProxy.bind(new UserServiceImpl());
        userService.addUser();
        userService.updateUser();
    }
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运行结果以下:

image.png

如图UserService接口里的全部方法都已经加上了日志逻辑了,此外,咱们看一下UserDynamicProxy这个类里的target属性是Object类型的。因此,这个动态代理的方法一样能够给其余Service复用。能够这样调用:

DynamicProxy dynamicProxy = new DynamicProxy();
TeacherService teacherService = (TeacherService)dynamicProxy.bind(new TeacherServiceImpl());
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综上,动态代理解决了静态代理的缺点

用arthas查看JDK动态代理生成的类

动态代理是运行时候动态生成代理类的,这个类放在内存中,咱们要怎么才能看到这个类呢?

artias是阿里开源的一个牛逼闪闪的Java诊断工具,不懂的能够看看这篇文章http://www.dblearn.cn/article/5,用它就能够线上反编译代码。

这里咱们添加一个断点:

public static void main(String[] args) throws IOException {
        DynamicProxy dynamicProxy = new DynamicProxy();
        UserService userService = (UserService)dynamicProxy.bind(new UserServiceImpl());
        userService.addUser();
        userService.updateUser();
        System.in.read();
    }
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运行 arthas

image.png

jad命令反编译,java生成的代理类都在com.sun.proxy目录下。所以反编译命令以下

jad com.sun.proxy.$Proxy0

package com.sun.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import proxy.UserService;

public final class $Proxy0 extends Proxy implements UserService {
    private static Method m1;
    private static Method m6;
    private static Method m2;
    private static Method m7;
    private static Method m0;
    private static Method m3;
    private static Method m4;
    private static Method m5;

    public final void addUser() {
        try {
            this.h.invoke(this, m3, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void updateUser() {
        try {
            this.h.invoke(this, m4, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void deleteUser() {
        try {
            this.h.invoke(this, m5, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void batchUpdateUser() {
        try {
            this.h.invoke(this, m6, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void batchDeleteUser() {
        try {
            this.h.invoke(this, m7, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public $Proxy0(InvocationHandler invocationHandler) {
        super(invocationHandler);
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m6 = Class.forName("proxy.UserService").getMethod("batchUpdateUser", new Class[0]);
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m7 = Class.forName("proxy.UserService").getMethod("batchDeleteUser", new Class[0]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
            m3 = Class.forName("proxy.UserService").getMethod("addUser", new Class[0]);
            m4 = Class.forName("proxy.UserService").getMethod("updateUser", new Class[0]);
            m5 = Class.forName("proxy.UserService").getMethod("deleteUser", new Class[0]);
            return;
        }
        catch (NoSuchMethodException noSuchMethodException) {
            throw new NoSuchMethodError(noSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException classNotFoundException) {
            throw new NoClassDefFoundError(classNotFoundException.getMessage());
        }
    }

    public final boolean equals(Object object) {
        try {
            return (Boolean)this.h.invoke(this, m1, new Object[]{object});
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final String toString() {
        try {
            return (String)this.h.invoke(this, m2, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final int hashCode() {
        try {
            return (Integer)this.h.invoke(this, m0, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }
}

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由上面的代码能够看到咱们的代理类已经生成好了,没当咱们调用方法如 addUser(),实际分派到h变量的invoke方法上执行:

this.h.invoke(this, m3, null);

h变量是什么呢?其实就是咱们实现了InvocationHandler的DynamicProxy类。

cglib动态代理

经过观察上面的静态代理和JDK动态代理模式,发现要求目标对象实现一个接口,可是有时候目标对象只是一个单独的对象,并无实现任何的接口。这时候要怎么处理呢?下面引出大名鼎鼎的CGlib动态代理

cglib代理,也叫做子类代理,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的扩展。

要用cglib须要引入它的jar包,由于spring已经集成了它,所以引入spring包便可

编写代理类:

public class CGLibProxy implements MethodInterceptor {
    private Object target; // 目标对象
    public Object bind(Object target) {
        this.target = target;
        Enhancer enhancer = new Enhancer();
        //设置父类
        enhancer.setSuperclass(this.target.getClass());
        //设置回调函数
        enhancer.setCallback(this);
        //建立子类(代理对象)
        return enhancer.create();
    }
    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
        Object result = methodProxy.invokeSuper(obj, args);
        System.out.println("拿个小本本记录一下");
        return result;
    }
}
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其中,Enhancer须要设置目标对象为父类(由于生成的代理类须要继承目标对象)

测试类:

public static void main(String[] args) throws IOException {
        CGLibProxy cgLibProxy = new CGLibProxy();
        UserServiceImpl userService = (UserServiceImpl)cgLibProxy.bind(new UserServiceImpl());
        userService.addUser();
        userService.updateUser();
        System.in.read();
    }
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运行结果:

image.png

咱们看到已经成功代理了。可是结果有乱码出现,此处设置一个// TODO,我猜想是Spring对CGlib再封装致使的,也请知道的大大回答一下。

用arthas查看cglib动态代理生成的类

步骤和JDK代理类雷同,只不过cglib的代理类生成在和测试类同一个包下,因为代码太多,只上部分代码

package com.example.demo.proxy;

import com.example.demo.proxy.UserServiceImpl;
import java.lang.reflect.Method;
import org.springframework.cglib.core.ReflectUtils;
import org.springframework.cglib.core.Signature;
import org.springframework.cglib.proxy.Callback;
import org.springframework.cglib.proxy.Factory;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;

public class UserServiceImpl$$EnhancerByCGLIB$$3ca8cfc3 extends UserServiceImpl implements Factory {
    private boolean CGLIB$BOUND;
    public static Object CGLIB$FACTORY_DATA;
    private static final ThreadLocal CGLIB$THREAD_CALLBACKS;
    private static final Callback[] CGLIB$STATIC_CALLBACKS;
    private MethodInterceptor CGLIB$CALLBACK_0;
    private static Object CGLIB$CALLBACK_FILTER;
    private static final Method CGLIB$deleteUser$0$Method;
    private static final MethodProxy CGLIB$deleteUser$0$Proxy;
    private static final Object[] CGLIB$emptyArgs;
    private static final Method CGLIB$addUser$1$Method;
    private static final MethodProxy CGLIB$addUser$1$Proxy;
    private static final Method CGLIB$updateUser$2$Method;
    private static final MethodProxy CGLIB$updateUser$2$Proxy;
    private static final Method CGLIB$batchUpdateUser$3$Method;
    private static final MethodProxy CGLIB$batchUpdateUser$3$Proxy;
    private static final Method CGLIB$batchDeleteUser$4$Method;
    private static final MethodProxy CGLIB$batchDeleteUser$4$Proxy;
    private static final Method CGLIB$equals$5$Method;
    private static final MethodProxy CGLIB$equals$5$Proxy;
    private static final Method CGLIB$toString$6$Method;
    private static final MethodProxy CGLIB$toString$6$Proxy;
    private static final Method CGLIB$hashCode$7$Method;
    private static final MethodProxy CGLIB$hashCode$7$Proxy;
    private static final Method CGLIB$clone$8$Method;
    private static final MethodProxy CGLIB$clone$8$Proxy;

    public final void deleteUser() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            UserServiceImpl$$EnhancerByCGLIB$$3ca8cfc3.CGLIB$BIND_CALLBACKS(this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor != null) {
            Object object = methodInterceptor.intercept(this, CGLIB$deleteUser$0$Method, CGLIB$emptyArgs, CGLIB$deleteUser$0$Proxy);
            return;
        }
        super.deleteUser();
    }

    public final void addUser() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            UserServiceImpl$$EnhancerByCGLIB$$3ca8cfc3.CGLIB$BIND_CALLBACKS(this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor != null) {
            Object object = methodInterceptor.intercept(this, CGLIB$addUser$1$Method, CGLIB$emptyArgs, CGLIB$addUser$1$Proxy);
            return;
        }
        super.addUser();
    }

    public final void updateUser() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            UserServiceImpl$$EnhancerByCGLIB$$3ca8cfc3.CGLIB$BIND_CALLBACKS(this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor != null) {
            Object object = methodInterceptor.intercept(this, CGLIB$updateUser$2$Method, CGLIB$emptyArgs, CGLIB$updateUser$2$Proxy);
            return;
        }
        super.updateUser();
    }

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其中

public class UserServiceImpl$$EnhancerByCGLIB$$3ca8cfc3 extends UserServiceImpl

能够看到生成的代理类继承了目标对象,所以有两个注意点:

  1. 目标对象不能处理被final关键字修饰,由于被final修饰的对象是不可继承的。
  2. 目标对象的方法若是为final/static,那么就不会被拦截,即不会执行目标对象额外的业务方法.
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