Mybatis + Spring 时出错 java.lang.NoClassDefFoundError

一、首先项目中使用的是Mybatis3.2.2 + Spring3.2.2，同时使用了mybatis-spring-1.2.0.jar来做Mybatis与Spring的整合。

二、使用了Mybatis推荐的mapper加载方式：

<!-- mybatis配置 -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="writeDataSource" />
        <property name="mapperLocations" value="classpath*:com/baofoo/admin/dao/**/impl/*Mapper.xml" />
    </bean>
                                                                                                                                                                                         
    <!-- mybatis自动扫描包下的mapper -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.baofoo.admin.dao" />
        <!-- <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" /> -->
    </bean>

值得需要注意的是，在配置MapperScannerConfigurer时，这里并没有指定sqlSessionFactoryName以及sqlTemplateName。在没有指定的情况下，spring就会指定默认的查找规则进行查询，如分别查找到默认的sqlSessionFactory实现和sqlSessionTemplate实现，并注入到MapperFactoryBean中。

这种配置方式，在一个数据源时，没有问题，但是在如果存在多个数据源时，上面的配置就存在问题了。在多个数据源时，如果配置不正确，或者配置的步骤不正确，将直接产生莫名奇妙的问题

最初从tomcat启动一切正常，然后后面突然启动服务时报出如下异常：

java.lang.NoClassDefFoundError: Could not initialize class org.springframework.beans.factory.BeanCreationException
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:532)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:461)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:626)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:479)
    at org.springframework.web.servlet.FrameworkServlet.configureAndRefreshWebApplicationContext(FrameworkServlet.java:651)
    at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:599)
    at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:665)
    at org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext(FrameworkServlet.java:518)
    at org.springframework.web.servlet.FrameworkServlet.initServletBean(FrameworkServlet.java:459)
    at org.springframework.web.servlet.HttpServletBean.init(HttpServletBean.java:136)
    at javax.servlet.GenericServlet.init(GenericServlet.java:160)
    at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1274)
    at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1186)
    at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:1081)
    at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:5033)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5320)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:633)
    at org.apache.catalina.startup.HostConfig.deployDescriptor(HostConfig.java:657)
    at org.apache.catalina.startup.HostConfig$DeployDescriptor.run(HostConfig.java:1637)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:619)

我的解决方案是在“MapperScannerConfigurer”配置为其指定“sqlSessionFactoryBeanName”，于是修改配置如下：

<!-- mybatis自动扫描包下的mapper -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.baofoo.admin.dao" />
        <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" /> <!-- 指定sqlSessionFactoryName -->
    </bean>

注：以上问题为本人在实战项目中解决问题的理解