Spring进阶 - Spring AOP实现原理详解之Cglib代理实现

我们在前文中已经介绍了SpringAOP的切面实现和创建动态代理的过程,那么动态代理是如何工作的呢?本文主要介绍Cglib动态代理的案例和SpringAOP实现的原理。@pdai

引入

我们在前文中已经介绍了SpringAOP的切面实现和创建动态代理的过程,那么动态代理是如何工作的呢?本文主要介绍Cglib动态代理的案例和SpringAOP实现的原理。

要了解动态代理是如何工作的,首先需要了解

  • 什么是代理模式?
  • 什么是动态代理?
  • 什么是Cglib?
  • SpringAOP和Cglib是什么关系?

动态代理要解决什么问题?

什么是代理?

代理模式(Proxy pattern): 为另一个对象提供一个替身或占位符以控制对这个对象的访问

举个简单的例子:

我(client)如果要买(doOperation)房,可以找中介(proxy)买房,中介直接和卖方(target)买房。中介和卖方都实现买卖(doOperation)的操作。中介就是代理(proxy)。

什么是动态代理?

动态代理就是,在程序运行期,创建目标对象的代理对象,并对目标对象中的方法进行功能性增强的一种技术。

在生成代理对象的过程中,目标对象不变,代理对象中的方法是目标对象方法的增强方法。可以理解为运行期间,对象中方法的动态拦截,在拦截方法的前后执行功能操作。

什么是Cglib? SpringAOP和Cglib是什么关系?

Cglib是一个强大的、高性能的代码生成包,它广泛被许多AOP框架使用,为他们提供方法的拦截。

  • 最底层是字节码,字节码相关的知识请参考 JVM基础 - 类字节码详解
  • ASM是操作字节码的工具
  • cglib基于ASM字节码工具操作字节码(即动态生成代理,对方法进行增强)
  • SpringAOP基于cglib进行封装,实现cglib方式的动态代理

Cglib代理的案例

这里我们写一个使用cglib的简单例子。@pdai

pom包依赖

引入cglib的依赖包

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>tech-pdai-spring-demos</artifactId>
        <groupId>tech.pdai</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>007-spring-framework-demo-aop-proxy-cglib</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/cglib/cglib -->
        <dependency>
            <groupId>cglib</groupId>
            <artifactId>cglib</artifactId>
            <version>3.3.0</version>
        </dependency>
    </dependencies>

</project>
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定义实体

User

package tech.pdai.springframework.entity;

/**
 * @author pdai
 */
public class User {

    /**
     * user's name.
     */
    private String name;

    /**
     * user's age.
     */
    private int age;

    /**
     * init.
     *
     * @param name name
     * @param age  age
     */
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}
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被代理的类

即目标类, 对被代理的类中的方法进行增强

package tech.pdai.springframework.service;

import java.util.Collections;
import java.util.List;

import tech.pdai.springframework.entity.User;

/**
 * @author pdai
 */
public class UserServiceImpl {

    /**
     * find user list.
     *
     * @return user list
     */
    public List<User> findUserList() {
        return Collections.singletonList(new User("pdai", 18));
    }

    /**
     * add user
     */
    public void addUser() {
        // do something
    }

}
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cglib代理

cglib代理类,需要实现MethodInterceptor接口,并指定代理目标类target

package tech.pdai.springframework.proxy;

import java.lang.reflect.Method;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

/**
 * This class is for proxy demo.
 *
 * @author pdai
 */
public class UserLogProxy implements MethodInterceptor {

    /**
     * 业务类对象,供代理方法中进行真正的业务方法调用
     */
    private Object target;

    public Object getUserLogProxy(Object target) {
        //给业务对象赋值
        this.target = target;
        //创建加强器,用来创建动态代理类
        Enhancer enhancer = new Enhancer();
        //为加强器指定要代理的业务类(即:为下面生成的代理类指定父类)
        enhancer.setSuperclass(this.target.getClass());
        //设置回调:对于代理类上所有方法的调用,都会调用CallBack,而Callback则需要实现intercept()方法进行拦
        enhancer.setCallback(this);
        // 创建动态代理类对象并返回
        return enhancer.create();
    }

    // 实现回调方法
    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        // log - before method
        System.out.println("[before] execute method: " + method.getName());

        // call method
        Object result = proxy.invokeSuper(obj, args);

        // log - after method
        System.out.println("[after] execute method: " + method.getName() + ", return value: " + result);
        return null;
    }
}
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使用代理

启动类中指定代理目标并执行。

package tech.pdai.springframework;

import tech.pdai.springframework.proxy.UserLogProxy;
import tech.pdai.springframework.service.UserServiceImpl;

/**
 * Cglib proxy demo.
 *
 * @author pdai
 */
public class ProxyDemo {

    /**
     * main interface.
     *
     * @param args args
     */
    public static void main(String[] args) {
        // proxy
        UserServiceImpl userService = (UserServiceImpl) new UserLogProxy().getUserLogProxy(new UserServiceImpl());

        // call methods
        userService.findUserList();
        userService.addUser();
    }
}
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简单测试

我们启动上述类main 函数,执行的结果如下:

[before] execute method: findUserList
[after] execute method: findUserList, return value: [User{name='pdai', age=18}]
[before] execute method: addUser
[after] execute method: addUser, return value: null
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Cglib代理的流程

我们把上述Demo的主要流程画出来,你便能很快理解

更多细节:

  • 在上图中,我们可以通过在Enhancer中配置更多的参数来控制代理的行为,比如如果只希望增强这个类中的一个方法(而不是所有方法),那就增加callbackFilter来对目标类中方法进行过滤;Enhancer可以有更多的参数类配置其行为,不过我们在学习上述主要的流程就够了。
  • final方法为什么不能被代理?很显然final方法没法被子类覆盖,当然不能代理了。
  • Mockito为什么不能mock静态方法?因为mockito也是基于cglib动态代理来实现的,static方法也不能被子类覆盖,所以显然不能mock。但PowerMock可以mock静态方法,因为它直接在bytecode上工作,更多可以看Mockito单元测试。(pdai: 通了没?是不是so easy...)

SpringAOP中Cglib代理的实现

SpringAOP封装了cglib,通过其进行动态代理的创建。

我们看下CglibAopProxy的getProxy方法

@Override
public Object getProxy() {
  return getProxy(null);
}

@Override
public Object getProxy(@Nullable ClassLoader classLoader) {
  if (logger.isTraceEnabled()) {
    logger.trace("Creating CGLIB proxy: " + this.advised.getTargetSource());
  }

  try {
    Class<?> rootClass = this.advised.getTargetClass();
    Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");

    // 上面流程图中的目标类
    Class<?> proxySuperClass = rootClass;
    if (rootClass.getName().contains(ClassUtils.CGLIB_CLASS_SEPARATOR)) {
      proxySuperClass = rootClass.getSuperclass();
      Class<?>[] additionalInterfaces = rootClass.getInterfaces();
      for (Class<?> additionalInterface : additionalInterfaces) {
        this.advised.addInterface(additionalInterface);
      }
    }

    // Validate the class, writing log messages as necessary.
    validateClassIfNecessary(proxySuperClass, classLoader);

    // 重点看这里,就是上图的enhancer,设置各种参数来构建
    Enhancer enhancer = createEnhancer();
    if (classLoader != null) {
      enhancer.setClassLoader(classLoader);
      if (classLoader instanceof SmartClassLoader &&
          ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
        enhancer.setUseCache(false);
      }
    }
    enhancer.setSuperclass(proxySuperClass);
    enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
    enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(classLoader));

    // 设置callback回调接口,即方法的增强点
    Callback[] callbacks = getCallbacks(rootClass);
    Class<?>[] types = new Class<?>[callbacks.length];
    for (int x = 0; x < types.length; x++) {
      types[x] = callbacks[x].getClass();
    }
    // 上节说到的filter
    enhancer.setCallbackFilter(new ProxyCallbackFilter(
        this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
    enhancer.setCallbackTypes(types);

    // 重点:创建proxy和其实例
    return createProxyClassAndInstance(enhancer, callbacks);
  }
  catch (CodeGenerationException | IllegalArgumentException ex) {
    throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() +
        ": Common causes of this problem include using a final class or a non-visible class",
        ex);
  }
  catch (Throwable ex) {
    // TargetSource.getTarget() failed
    throw new AopConfigException("Unexpected AOP exception", ex);
  }
}
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获取callback的方法如下,提几个理解的要点吧,具体读者在学习的时候建议把我的例子跑一下,然后打一个断点进行理解。

  • rootClass: 即目标代理类
  • advised: 包含上文中我们获取到的advisor增强器的集合
  • exposeProxy: 在xml配置文件中配置的,背景就是如果在事务A中使用了代理,事务A调用了目标类的的方法a,在方法a中又调用目标类的方法b,方法a,b同时都是要被增强的方法,如果不配置exposeProxy属性,方法b的增强将会失效,如果配置exposeProxy,方法b在方法a的执行中也会被增强了
  • DynamicAdvisedInterceptor: 拦截器将advised(包含上文中我们获取到的advisor增强器)构建配置的AOP的callback(第一个callback)
  • targetInterceptor: xml配置的optimize属性使用的(第二个callback)
  • 最后连同其它5个默认的Interceptor 返回作为cglib的拦截器链,之后通过CallbackFilter的accpet方法返回的索引从这个集合中返回对应的拦截增强器执行增强操作。
private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
  // Parameters used for optimization choices...
  boolean exposeProxy = this.advised.isExposeProxy();
  boolean isFrozen = this.advised.isFrozen();
  boolean isStatic = this.advised.getTargetSource().isStatic();

  // Choose an "aop" interceptor (used for AOP calls).
  Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);

  // Choose a "straight to target" interceptor. (used for calls that are
  // unadvised but can return this). May be required to expose the proxy.
  Callback targetInterceptor;
  if (exposeProxy) {
    targetInterceptor = (isStatic ?
        new StaticUnadvisedExposedInterceptor(this.advised.getTargetSource().getTarget()) :
        new DynamicUnadvisedExposedInterceptor(this.advised.getTargetSource()));
  }
  else {
    targetInterceptor = (isStatic ?
        new StaticUnadvisedInterceptor(this.advised.getTargetSource().getTarget()) :
        new DynamicUnadvisedInterceptor(this.advised.getTargetSource()));
  }

  // Choose a "direct to target" dispatcher (used for
  // unadvised calls to static targets that cannot return this).
  Callback targetDispatcher = (isStatic ?
      new StaticDispatcher(this.advised.getTargetSource().getTarget()) : new SerializableNoOp());

  Callback[] mainCallbacks = new Callback[] {
      aopInterceptor,  // 
      targetInterceptor,  // invoke target without considering advice, if optimized
      new SerializableNoOp(),  // no override for methods mapped to this
      targetDispatcher, this.advisedDispatcher,
      new EqualsInterceptor(this.advised),
      new HashCodeInterceptor(this.advised)
  };

  Callback[] callbacks;

  // If the target is a static one and the advice chain is frozen,
  // then we can make some optimizations by sending the AOP calls
  // direct to the target using the fixed chain for that method.
  if (isStatic && isFrozen) {
    Method[] methods = rootClass.getMethods();
    Callback[] fixedCallbacks = new Callback[methods.length];
    this.fixedInterceptorMap = CollectionUtils.newHashMap(methods.length);

    // TODO: small memory optimization here (can skip creation for methods with no advice)
    for (int x = 0; x < methods.length; x++) {
      Method method = methods[x];
      List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, rootClass);
      fixedCallbacks[x] = new FixedChainStaticTargetInterceptor(
          chain, this.advised.getTargetSource().getTarget(), this.advised.getTargetClass());
      this.fixedInterceptorMap.put(method, x);
    }

    // Now copy both the callbacks from mainCallbacks
    // and fixedCallbacks into the callbacks array.
    callbacks = new Callback[mainCallbacks.length + fixedCallbacks.length];
    System.arraycopy(mainCallbacks, 0, callbacks, 0, mainCallbacks.length);
    System.arraycopy(fixedCallbacks, 0, callbacks, mainCallbacks.length, fixedCallbacks.length);
    this.fixedInterceptorOffset = mainCallbacks.length;
  }
  else {
    callbacks = mainCallbacks;
  }
  return callbacks;
}
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可以结合调试,方便理解

示例源码

https://github.com/realpdai/tech-pdai-spring-demos

联系我

添加@pdai微信

PS:添加时请备注Java全栈,谢谢!