java - Autowiring 类为空

Question

总结

我正在构建一个 spring-mvc web 应用程序并尝试使用 Mybatis 和 DAO 进行数据访问。

我成功地配置了 Mybatis，并且能够从 Mysql 服务器获取预期的数据。

但是当我尝试在 Mybatis 中使用 DAO 时出现了问题。

错误

正如您在下面看到的，这是一个 NullPointerException。

INFO : com.*****.web.controller.ExampleController - testDaoSelect action has been executed. No parameter has been taken.
INFO : com.*****.web.service.exampleService - Executed or not??
09, March, 2015 5:07:28 PM org.apache.catalina.core.StandardWrapperValve invoke
Fatal: Servlet.service() for servlet [appServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at com.*****.web.service.exampleService.sampleList(exampleService.java:29)
    at com.*****.web.controller.ExampleController.testDaoSelect(ExampleController.java:188)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:213)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:126)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:96)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:617)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:578)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:789)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:644)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:516)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1086)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:659)
    at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:223)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1558)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1515)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

---

重现错误

从 AJAX 调用开始。

    $('#DaoSelectTest').click(function(e) {
    $.ajax({
        type: "POST",
        url: "/testDaoSelect.fst",
        data: {},
        success: function (result) {
            console.log(result);

            var html = "Querying was successful. Check out console logs. <br/>";
            html += "Done with the test. <a href='/daoSample.fst'>Click here</a> ";
            html += "And proceed to the next.";

            $('#sampleTable').html(html);
        },
        error: function (result) {
            // error...
        }
    });
});

接下来，执行 Action ..

@Controller
public class ExampleController {

    // Mybatis direct
    @Autowired
    private SqlSession sqlSession;

    // Dao  
    @Autowired
    private exampleService exService;
.
.
.
.
.


@RequestMapping("/testDaoSelect")
        @ResponseBody
        public List<HashMap<String, Object>> testDaoSelect(HttpServletRequest request, HttpServletResponse response) {
            logger.info("testDaoSelect action has been executed. No parameter has been taken.");

            List<HashMap<String, Object>> result = exService.sampleList();

            logger.info("result size is... " + result.size());

            return result;
        }

调用textDaoSelect就可以了，接下来是Service类。

@Service
@Transactional
public class exampleService {

    private Log logger = LogFactory.getLog(this.getClass());

    @Autowired
    private exampleDao exDao;

    @Transactional(readOnly = true)
    public List<HashMap<String, Object>> sampleList() {
        logger.info("Executed or not??");

        return exDao.sampleList();
    }
}

砰~!这里

此处exDao为null，此处出现NullPointException。接线不正确。为什么？？我该怎么办？？

我的 exampleDao

public interface exampleDao {
    public List<HashMap<String, Object>> sampleList();
}

**我的例子DaoImplement

public class exampleDaoImplement extends SqlSessionDaoSupport implements exampleDao {

    private static final Logger logger = LoggerFactory.getLogger(exampleDaoImplement.class);    

    public List<HashMap<String, Object>> sampleList() {
        // TODO Auto-generated method stub

        logger.info("I've reached impl class....");
        return getSqlSession().selectList("Example.selectTest");

    }
}

我的根上下文

(忘记 ****** 无论如何......)

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

    <!-- Root Context: defines shared resources visible to all other web components -->

    <bean id="dataSource" class="org.apache.ibatis.datasource.pooled.PooledDataSource">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://*******:3306/*****_web"/>
        <property name="username" value="root"/>
        <property name="password" value="*******"/>
    </bean>

    <bean id ="sqlSessionFactory" class= "org.mybatis.spring.SqlSessionFactoryBean" >
        <property name ="dataSource" ref= "dataSource"></property >
        <property name="configLocation" value="classpath:mybatis/mybatis-config.xml" />
    </bean >

    <bean id ="transactionManager"
        class= "org.springframework.jdbc.datasource.DataSourceTransactionManager" >
        <property name ="dataSource" ref= "dataSource"></property >
    </bean >

    <bean id ="sqlSession"
        class= "org.mybatis.spring.SqlSessionTemplate" >
        <constructor-arg ref= "sqlSessionFactory"></constructor-arg >
    </bean >

    <bean id ="exService" class= "com.*****.web.service.exampleService" ></bean >

</beans>

我的 servlet 上下文

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

    <!-- DispatcherServlet Context: defines this servlet's request-processing infrastructure -->

    <!-- Enables the Spring MVC @Controller programming model -->
    <annotation-driven />

    <!-- Handles HTTP GET requests for /resources/** by efficiently serving up static resources in the ${webappRoot}/resources directory -->
    <resources mapping="/resources/**" location="/resources/" />

    <!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory -->
    <beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <beans:property name="prefix" value="/WEB-INF/views/" />
        <beans:property name="suffix" value=".jsp" />
    </beans:bean>

    <context:component-scan base-package="com.******.web.controller" />

</beans:beans>

我已经用谷歌搜索了半天，得到了一些提示，但没有确切的解决方案。好像是怎么回事？？

---

已添加

在谷歌搜索时，我得到了这个提示并进行了相应的配置。

<bean id ="exDao" class= "com.*****.web.dao.implement.exampleDaoImplement" >
    <property name="sqlSessionFactory" ref="sqlSessionFactory"></property>
</bean >

(没有定义属性，在启动服务器时抛出错误，说属性sqlSessionFactory或SqlSessionTemplate是必需的。)

但它不起作用......即使使用这种配置，仍然存在空点错误。

Answer 1

第一个想法:在 servlet-context.xml 中，您编写了:

<context:component-scan base-package="com.******.web.controller" />

这意味着 Spring 将在该文件夹中仅查找您的注释类(如 @Service 和其他)。你有没有把你的 DAO 放在“com.******.web.controller”文件夹之外？这可以解释为什么它仍然是 null .

一个简单的测试是使用“com.******”来确保每个文件夹都被扫描。

第二个想法:如果你想让 Spring 实例化你的 DAO 类，你必须添加 @Repository在 DAO 接口(interface)的具体实现中注释。

第三个想法:您不应该对同一个类使用 XML 配置和注释。在您的示例中，您不需要使用 <bean id ="exService" class= "com.*****.web.service.exampleService"></bean >如果你使用 @Service在 exampleService 类和你的 <component-scan> 中覆盖服务所在的目录。

最后说明:您在评论中使用 @Service("exService")解决了你的问题。这是因为，如果您不指定组件的名称，Spring 会查找具有给定类型(而不是其名称)的任何现有 bean ).所以，它注入(inject)了你在 XML 中声明的 bean，它有一个 null。道。

为避免此类问题，您应该始终在使用它的变量名称之后显式命名您的 bean，此处为 @Service("exService")切换“按名称绑定(bind)”行为，这使您的代码按预期工作:

// This works because the name of the variable
// matches the @Service("exService") annotation
@Autowired
private ExampleService exService;

感谢@RohitJain 的最后一个说明。

---

与您的问题无关，Java 类和接口(interface)应始终用大写字母书写(例如“ExampleDao”、“ExampleDaoImplement”和“ExampleService”)。