service里無法注入mapper，mapper空指針

被困擾了一天，終於解決了，記錄一下

下面是mapper的代碼

@Mapper
public interface ProductDao {
    @Select("select * from product")
    List<Product> findAll() throws Exception;
}

然后是service的代碼

@Service
@Transactional
public class ProductServiceImpl implements ProductService {

    @Autowired
    private ProductDao productDao;

    @Override
    public List<Product> findAll() throws Exception {
        return productDao.findAll();
    }
}

然后是applicationcontext.xml的配置

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/cache http://www.springframework.org/schema/cache/spring-cache.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd">

    <!--配置注解掃描，掃描service和dao-->
    <context:component-scan base-package="com.rao.travel.service"/>
    <context:component-scan base-package="com.rao.travel.dao"/>

    <!--引入數據庫配置文件-->
    <context:property-placeholder location="classpath:db.properties"/>

    <!--配置連接池-->
    <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <property name="driverClass" value="${jdbc.driver}"/>
        <property name="jdbcUrl" value="${jdbc.url}"/>
        <property name="user" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
    </bean>

    <!--配置session工廠-->
    <bean id="sessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource"/>
    </bean>

    <!--掃描dao接口-->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.rao.travel.dao"/>
    </bean>

    <!--配置事務管理器-->
    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <property name="dataSource" ref="dataSource"/>
    </bean>

    <!--開啟事務注解-->
    <tx:annotation-driven transaction-manager="transactionManager"/>
</beans>

然后是springmvc的代碼

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">

    <!--掃描所有包下的注解-->
    <context:component-scan base-package="com.rao.travel"/>

    <!--配置視圖解析器-->
    <bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/pages/"/>
        <property name="suffix" value=".jsp"/>
    </bean>

    <!-- 設置靜態資源不過濾 -->
    <mvc:resources location="/css/" mapping="/css/**" />
    <mvc:resources location="/img/" mapping="/img/**" />
    <mvc:resources location="/js/" mapping="/js/**" />
    <mvc:resources location="/plugins/" mapping="/plugins/**" />

    <!-- 開啟對SpringMVC注解的支持 -->
    <mvc:annotation-driven/>

    <!--aop的動態代理使用cglib-->
    <aop:aspectj-autoproxy proxy-target-class="true"/>
</beans>

問題來了：

spring和springmvc使用的是兩個容器，spring容器是springmvc容器的父容器，子容器可以訪問父容器，反過來不行。

初始化時先加載父容器，然后加載子容器，所以如果父容器已經有的類，子容器在注入是會覆蓋父容器已有的bean。

上面我在spring里面注入了service和mapper，然后在springmvc里面又注入了一遍，這樣可能會導致空指針異常。

所以我們在配置時，controller在springmvc中注入，service和mapper在spring中注入。

問題解決。