java无法调用“”，因为“”为null

4 月，4 周 Questions & Answers 31946

我终于找到了问题标签。为什么这个错误是“轰”？？？？？我不明白。我花了两天

    **java.lang.NullPointerException: Cannot invoke 
    "com.board.dao.BoardDao.listboard(com.board.dto.BoardDto)" because "this.bDao" is null**
    at com.board.service.BoardService.listboard(BoardService.java:19)
    at com.board.controller.BoardController.boardlist(BoardController.java:25)
    at com.board.controller.BoardController.boardlist(BoardController.java:26)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.base/java.lang.reflect.Method.invoke(Unknown Source)
    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.doGet(FrameworkServlet.java:778)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:626)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:543)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:616)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:818)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1626)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.base/java.lang.Thread.run(Unknown Source)

控制器

@Controller
public class BoardController {
    
    @Autowired
    BoardService service;
    
    
    @RequestMapping("/board")
    public ModelAndView boardlist (ModelAndView mv ,BoardDto dto) {
        System.out.println("boardlist ");
        
        System.out.println(dto+"dto");
        List<BoardDto> dtolist = service.listboard(dto);
        mv.addObject("boardlist", dtolist);
        mv.setViewName("boardlist");
        return mv;
    }
}

管制员很好。。我想

服务

@Service
public class BoardService {
    
    @Autowired
    BoardDao bDao;

    public List<BoardDto> listboard(BoardDto dto) {
        System.out.println("boardservice");
        List<BoardDto> result = bDao.listboard(dto);
        return result ;
    }

}

这个位置是错误的，为什么？？T.T

我认为这种方法是错误的，但我不知道。为什么

刀

public class BoardDao extends SqlSessionDaoSupport{
    
    @Autowired
    SqlSessionTemplate session;

    public List<BoardDto> listboard(BoardDto dto) {
        System.out.println("dao.");
        List<BoardDto> result = session.selectList("boarddate.listboard", dto);
        return result;
    }

}

<?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"
    xsi:schemaLocation="http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd">
    
    <!-- Root Context: defines shared resources visible to all other web components -->


    <bean id="dataSource" 
        class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/>
        <property name="url" value="jdbc:oracle:thin:@localhost:1521:orcl"/>
        <property name="username" value="board"/>
        <property name="password" value="1234"/>
    </bean> 


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



    <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg name="sqlSessionFactory" ref="sqlSessionFactory"/>
    </bean>
    <!-- DI  -->
    <bean class="com.board.service.BoardService"></bean>
    <bean class="com.board.dao.BoardDao">
        <property name="sqlSessionTemplate" ref="sqlSession"/>
    </bean>
    
</beans>

根上下文。xml

为什么错误是“爆炸”？我不明白。。。。T.T

——>；编辑更多信息以避免出错

共 (1) 个答案

# 1 楼答案
我认为您的问题与BoardDao类中缺少@Component注释有关。组件应该有一个@Component注释来实例化要注入到服务层中的单例
```
@Component
public class BoardDao extends SqlSessionDaoSupport{

    @Autowired
    SqlSessionTemplate session;

    public List<BoardDto> listboard(BoardDto dto) {
        System.out.println("dao.");
        List<BoardDto> result = session.selectList("boarddate.listboard", dto);
        return result;
    }

}
```
如果问题仍然存在，您可以尝试使用@Repository注释。遗憾的是，我没有使用类SqlSessionDaoSupport，因此我不知道该类的最佳注释

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有 Java 编程相关的问题?

java无法调用“”，因为“”为null

共 (1) 个答案

# 1 楼答案