记一次Spring的aop代理Mybatis的DAO所遇到的问题

由来

项目中需要实现某个订单的状态改变后然后推送给第三方的功能，由于更改状态的项目和推送的项目不是同一个项目，所以为了不改变原项目的代码，我们考虑用spring的aop来实现。

项目用的是springmvc + spring + mybatis 的架构，我们知道spring实现了两种代理方式：JDK动态代理和CGLB动态代理。所以spring对接口和类都可以实现代理。所以只需要考虑在DAO接口的相关update状态的方法上加aop就可以了。整理了下共有六个地方对订单的status做了update。所以配置如下：

    <!-- 声明通知类 -->
    <bean id="aspectBean" class="com.info.web.service.BorrowOrderStatusAspect"></bean>
    <aop:config>
        <aop:aspect id="myAspect" ref="aspectBean">
            <aop:pointcut
                expression="execution(public * com.info.risk.dao.IRiskCreditUserDao.updateAssetsSuc(..))
               || execution(public * com.info.risk.dao.IRiskCreditUserDao.updateAssetsFail(..)) 
               || execution(public * com.info.web.dao.IBorrowOrderDao.updateByPrimaryKeySelective(..)) 
               || execution(public * com.info.web.dao.IBorrowOrderDao.updateByPrimaryKeyWithBLOBs(..)) 
               || execution(public * com.info.web.dao.IBorrowOrderDao.updateByPrimaryKey(..))"
                id="servicePointcut" />
            <aop:after-returning method="doAfter"
                pointcut-ref="servicePointcut" />
        </aop:aspect>
    </aop:config>

 可是启动项目的时候发现，启动失败，报错信息如下：

………………………………

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'repaymentService': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.info.web.dao.IBorrowOrderDao com.info.web.service.RepaymentService.borrowOrderDao; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'IBorrowOrderDao': Post-processing of FactoryBean's singleton object failed; nested exception is org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.sun.proxy.$Proxy39]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Cannot subclass final class class com.sun.proxy.$Proxy39
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1210)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:303)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:1120)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1044)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:942)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:533)
... 37 more
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.info.web.dao.IBorrowOrderDao com.info.web.service.RepaymentService.borrowOrderDao; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'IBorrowOrderDao': Post-processing of FactoryBean's singleton object failed; nested exception is org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.sun.proxy.$Proxy39]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Cannot subclass final class class com.sun.proxy.$Proxy39
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:561)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331)
... 48 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'IBorrowOrderDao': Post-processing of FactoryBean's singleton object failed; nested exception is org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.sun.proxy.$Proxy39]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Cannot subclass final class class com.sun.proxy.$Proxy39
at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:116)
at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1517)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:251)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:1120)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1044)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:942)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:533)
... 50 more
Caused by: org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.sun.proxy.$Proxy39]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Cannot subclass final class class com.sun.proxy.$Proxy39
at org.springframework.aop.framework.CglibAopProxy.getProxy(CglibAopProxy.java:212)
at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:109)
at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.createProxy(AbstractAutoProxyCreator.java:447)
at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.wrapIfNecessary(AbstractAutoProxyCreator.java:333)
at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.postProcessAfterInitialization(AbstractAutoProxyCreator.java:293)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:422)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.postProcessObjectFromFactoryBean(AbstractAutowireCapableBeanFactory.java:1719)
at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:113)
... 57 more
Caused by: java.lang.IllegalArgumentException: Cannot subclass final class class com.sun.proxy.$Proxy39
at org.springframework.cglib.proxy.Enhancer.generateClass(Enhancer.java:446)
at org.springframework.cglib.transform.TransformingClassGenerator.generateClass(TransformingClassGenerator.java:33)
at org.springframework.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
at org.springframework.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:216)
at org.springframework.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)
at org.springframework.cglib.proxy.Enhancer.createClass(Enhancer.java:317)
at org.springframework.aop.framework.ObjenesisCglibAopProxy.createProxyClassAndInstance(ObjenesisCglibAopProxy.java:57)
at org.springframework.aop.framework.CglibAopProxy.getProxy(CglibAopProxy.java:202)
... 64 more

分析原因

从报错信息可以了解说是代理了final修饰的类。可是哪里来的final类？ 原来，DAO层使用的是mybatis，可以只写接口不用写实现类。而我们项目中就是没有写实现类。但是spring也可以对接口进行代理，继续分析。

Mapper开发规则

1. 在mapper.xml中将namespace设置为mapper.java的全限定名 
2. 将mapper.java接口的方法名和mapper.xml中statement的id保持一致。 
3. 将mapper.java接口的方法输入参数类型和mapper.xml中statement的parameterType保持一致 
4. 将mapper.java接口的方法输出 结果类型和mapper.xml中statement的resultType保持一致。

注意遵循上边四点规范！这样抛弃Dao实现类的写法： 具有更好的可扩展性，提高了灵活度。

先来说明下mybatis为何可以只写接口而不写实现类，通过mybatis源码分析可知：

mybatis通过JDK的动态代理方式，在启动加载配置文件时，根据配置mapper的xml去生成Dao的实现。session.getMapper()使用了代理，当调用一次此方法，都会产生一个代理class的instance,看看这个代理class的实现.

public class MapperProxy implements InvocationHandler { 
... 
public static <T> T newMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) { 
    ClassLoader classLoader = mapperInterface.getClassLoader(); 
    Class<?>[] interfaces = new Class[]{mapperInterface}; 
    MapperProxy proxy = new MapperProxy(sqlSession); 
    return (T) Proxy.newProxyInstance(classLoader, interfaces, proxy); 
  } 

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
    if (!OBJECT_METHODS.contains(method.getName())) { 
      final Class<?> declaringInterface = findDeclaringInterface(proxy, method); 
      final MapperMethod mapperMethod = new MapperMethod(declaringInterface, method, sqlSession); 
      final Object result = mapperMethod.execute(args); 
      if (result == null && method.getReturnType().isPrimitive()) { 
        throw new BindingException("Mapper method '" + method.getName() + "' (" + method.getDeclaringClass() + ") attempted to return null from a method with a primitive return type (" +    method.getReturnType() + ")."); 
      } 
      return result; 
    } 
    return null; 
  } 

这里是用到了JDK的代理Proxy。 newMapperProxy()可以取得实现interfaces 的class的代理类的实例。

当执行interfaces中的方法的时候，会自动执行invoke()方法，其中public Object invoke(Object proxy, Method method, Object[] args)中 method参数就代表你要执行的方法.

MapperMethod类会使用method方法的methodName 和declaringInterface去取 sqlMapxml 取得对应的sql，也就是拿declaringInterface的类全名加上 sql-id..

因此，dao类被多次代理，第二次aop进行代理的时候拿到的是第一次代理后的对象，这个对象是个final形式的，因此报错。

解决方法：最后我在外层封装了一个service接口和接口的实现类，将dao注入到该service中，最后对该service实现aop，问题就解决了。

总结

动态代理解决问题的检查点： 

1. 需要AOP拦截的类是否是final的，final类不可使用CGLIB来代理。
2. 是否在给BEAN配AOP的时候强制使用CGLIB，如果是则可指定proxyTargetClass属性以让spring强制代理目标类。
3. 类是否被多次代理了，如果类被多次代理过，则第二次进行代理的时候拿到的是第一次代理后的对象，这个对象是个final形式的，所以会出现这个错误。

基于第三点要注意，类是否被多次代理不紧紧取决于类是否被配置了多次AOP，如果类实现了某个接口，则还要看类实现的接口是否被aop拦截过。如果类实现了接口且接口也被AOP拦截了，则很可能出现上面的错误（是否出错取决于AOP代理执行的顺序）。

spring配置aop需要注意：

1、proxy-target-class属性值决定是基于接口的还是基于类的代理被创建，启动对@Aspectj的支持 true为cglib（基于类），false为jdk代理（基于接口），不写的话默认为false。为true的话，会导致拦截不了mybatis的mapper

<aop:aspectj-autoproxy proxy-target-class="false" />

2、在类没有实现任何接口，并且没有默认构造函数的情况下，通过构造函数注入时，目前的Spring是无法实现AOP切面拦截的。 参考通过CGLIB实现AOP的浅析（顺便简单对比了一下JDK的动态代理）