无法调用“”,因为“”为空

时间:2020-09-22 00:53:11

标签: java spring nullpointerexception

我终于找到了问题标签。 为什么这个错误是“繁荣” ???? 我不明白要花两天时间.....

tmux new-session python3 --version

控制器

    **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)

控制器很好..我想..

服务

@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;
    }
}

这个位置是错误的,为什么? T.T

我认为此方法是错误的位置,但我不了解。为什么?

dao

@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 ;
    }

}

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;
    }

}

root-context.xml

为什么错误是“繁荣”?我不明白... T.T

->编辑更多错误信息

1 个答案:

答案 0 :(得分:2)

我认为您的问题与@Component类中缺少BoardDao注释有关。 该组件应具有@Component注释,以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,则可以尝试使用@Repository批注。不幸的是,我没有使用过SqlSessionDaoSupport类,所以我不知道最好的注释。