Spring重复扫描导致事务失败的解决方案及深入分析

问题及日志
使用Spring和mybatis，然后配置事务，出现SqlSession was not registered for synchronization because synchronization is not active,事务没有启用成功。

[org.mybatis.spring.SqlSessionUtils] - Creating a new SqlSession
[org.mybatis.spring.SqlSessionUtils] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1a714e6e] was not registered for synchronization because synchronization is not active
[org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource
[org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [jdbc: mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java]* will not be managed by Spring*
[com.haoyifen.mySSMTemplate.dao.UserMapper.selectByPrimaryKey] - ==> Preparing: select ID, USER_ID, USER_NAME, SEX, ADDRESS, PHONE_NUMBER, MAIL_BOX from user where ID = ?
[com.haoyifen.mySSMTemplate.dao.UserMapper.selectByPrimaryKey] - ==> Parameters: 1(Integer)
[com.haoyifen.mySSMTemplate.dao.UserMapper.selectByPrimaryKey] - <== Total: 1
[org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1a714e6e]

IntelliJ IDEA的编辑器显示我的拦截路径是正确的。为UserService类创建事务配置。

问题分析及解决方案
后来深入分析才发现，原来是扫描配置错误。
我把Spring的ApplicationContext和DispatcherServlet(WebApplicationContext)配置使用两个文件分离开的，分别为spring.xml和spring-mvc.xml。在spring.xml中配置了事务。在两个文件中都启用了扫描设置，用于扫描@Service，@Component和@Controller

spring.xml
<!-- 自动扫描 -->
<context:component-scan base-package="com.haoyifen.mySSMTemplate"></context:component-scan>
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spring-mvc.xml
<context:component-scan base-package="com.haoyifen.mySSMTemplate"></context:component-scan>
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以上的配置使得spring-mvc重复扫描了userService类，而spring如果要使事务生效，就需要cglib为userService生成代理子类，在spring.xml中已经生成了代理类，而在spring-mvc.xml中，又重新扫描了一遍，使得原先cglib生成的代理子类失效，从而事务拦截也失效。所以我们应该设置spring-mvc.xml，使其不扫描@Service和@Component，spring.xml不扫描@Controller。配置如下：

spring.xml
<!-- 自动扫描 ,忽略@Controller注解的类-->
<context:component-scan base-package="com.haoyifen.mySSMTemplate">
<context:exclude-filter type="annotation" expression="org.springframework.stereotype.Controller"></context:exclude-filter>
</context:component-scan>
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spring-mvc.xml
<!--自动扫描该包，使spring-mvc只扫描controller包中的类(其中只有@Controller控制器)，不会重复扫描到@Service或者@Component-->
<context:component-scan base-package="com.haoyifen.mySSMTemplate.controller">
</context:component-scan>
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验证
修改后，再查看日志，就发现已经可以正确启用事务了。
[org.springframework.jdbc.datasource.DataSourceTransactionManager] - Acquired Connection [jdbc:mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java] for JDBC transaction
[org.springframework.jdbc.datasource.DataSourceTransactionManager] - Switching JDBC Connection [jdbc:mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java]* to manual commit*
[org.mybatis.spring.SqlSessionUtils] - Creating a new SqlSession
[org.mybatis.spring.SqlSessionUtils] -* Registering transaction synchronization for SqlSession* [org.apache.ibatis.session.defaults.DefaultSqlSession@1031238e]
[org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [jdbc:mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java]* will be managed by Spring*
….
[org.mybatis.spring.SqlSessionUtils] -* Transaction synchronization closing SqlSession* [org.apache.ibatis.session.defaults.DefaultSqlSession@1031238e]
[org.springframework.jdbc.datasource.DataSourceTransactionManager] - Initiating transaction commit
[org.springframework.jdbc.datasource.DataSourceTransactionManager] - Committing JDBC transaction on Connection [jdbc:mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java]
[org.springframework.jdbc.datasource.DataSourceTransactionManager] - Releasing JDBC Connection [jdbc:mysql://localhost:3306/iot, UserName=root@localhost, MySQL Connector Java] after transaction
[org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource

深入日志分析
我们可以查看日志，来分析一下userService的创建过程。在修改xml文件之前：

Application初始化
创建Application时创建了一次userService，并使用cglib代理子类，生成了事务类：
[org.springframework.beans.factory.xml.XmlBeanDefinitionReader] - Loading XML bean definitions from class path resource [spring.xml]
…..
开始创建userService类
[org.springframework.beans.factory.support.DefaultListableBeanFactory] -* Creating shared instance of singleton bean ‘userService’*
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Creating instance of bean ‘userService’
……
缓存userService用于其他bean的注入
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Eagerly caching bean ‘userService’ to allow for resolving potential circular references
……
注入userMapper
[org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor] - Autowiring by type from bean name ‘userService’ to bean named ‘userMapper’
……
生成cglib代理类
[org.springframework.aop.aspectj.autoproxy.AspectJAwareAdvisorAutoProxyCreator] - Creating implicit proxy for bean ‘userService’ with 0 common interceptors and 2 specific interceptors
[org.springframework.aop.framework.CglibAopProxy] - Creating CGLIB proxy: target source is SingletonTargetSource for target object [com.haoyifen.mySSMTemplate.service.UserService@2c06eb2b]
…..
完成创建，此时的userService已经是被cglib代理过的子类的实例
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Finished creating instance of bean ‘userService’

创建WebApplication子容器
接下来创建WebApplication子容器时，又创建了一次userService，并且没有使用cglib进行代理，所以事务失效。
创建WebApplication
[org.springframework.beans.factory.xml.XmlBeanDefinitionReader] - Loading XML bean definitions from class path resource [spring-mvc.xml]
开始创建userService类
[org.springframework.beans.factory.support.DefaultListableBeanFactory] -* Creating shared instance of singleton bean ‘userService’*
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Creating instance of bean ‘userService’
[org.springframework.beans.factory.annotation.InjectionMetadata] - Registered injected element on class [com.haoyifen.mySSMTemplate.service.UserService]: AutowiredFieldElement for private com.haoyifen.mySSMTemplate.dao.UserMapper com.haoyifen.mySSMTemplate.service.UserService.userMapper
缓存userService用于其他bean的注入
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Eagerly caching bean ‘userService’ to allow for resolving potential circular references
[org.springframework.beans.factory.annotation.InjectionMetadata] - Processing injected element of bean ‘userService’: AutowiredFieldElement for private com.haoyifen.mySSMTemplate.dao.UserMapper com.haoyifen.mySSMTemplate.service.UserService.userMapper
[org.springframework.beans.factory.support.DefaultListableBeanFactory] - Returning cached instance of singleton bean ‘userMapper’
注入userMapper
[org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor] - Autowiring by type from bean name ‘userService’ to bean named ‘userMapper’
完成创建，本来由cglib代理的子类实例被替换成原生的userService实例，自然也就没有了事务功能
[org.springframework.beans.factory.support.DefaultListableBeanFactory] -* Finished creating instance of bean ‘userService’*

我们按照上面的方法将spring-mvc.xml文件更改后，在创建WebApplication时，就不会再重新创建userService了。