threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with

异常描述：空值异常，java.lang.NullPointerException 这是因为我自己在拦截器拦截的时候进行的登录判断时通过session获取到一个空的user并使用这个实体类的getUserName方法获取用户名出现了错误,这是因为我还没有登录，并没有session保存所以出现了异常。这是程序本身设计错误，当然有时候其他的可能也会出现空值异常。

严重: Servlet.service() for servlet [dispatcherServlet] in context with path [/ssm-demo2] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at com.liangfufeng.interceptor.LoginInterceptor.preHandle(LoginInterceptor.java:40)
    at org.springframework.web.servlet.HandlerExecutionChain.applyPreHandle(HandlerExecutionChain.java:134)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:958)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)

出现错误没修改之前的代码如下：

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        //创建session
        HttpSession session=request.getSession();
        //无需登录可以访问的地址
        String [] allowUrls=new String[]{"toLogin","/login"};
        //获取当前请求地址
        String url=request.getRequestURL().toString();
        //获取session中的用户
        User user=(User) session.getAttribute("user");
        System.out.println(user);
        //判断是否已经登录
        for (String strUrl : allowUrls) {  
            if(url.contains(strUrl))  
            {  
                return true;  
            } 
        }  
        if(user.getUserName()==null){
            request.getRequestDispatcher("/login").forward(request, response);
            return false;
        }
        return true;

    }

错误出现在：

user.getUserName()==null

修改思路：
直接使用user作判断而不是使用的的userName属性

user==null

修改后的代码如下:

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        //创建session
        HttpSession session=request.getSession();
        //无需登录可以访问的地址
        String [] allowUrls=new String[]{"toLogin","/login"};
        //获取当前请求地址
        String url=request.getRequestURL().toString();
        //获取session中的用户
        User user=(User) session.getAttribute("user");
        System.out.println(user);
        //判断是否已经登录
        for (String strUrl : allowUrls) {  
            if(url.contains(strUrl))  
            {  
                return true;  
            } 
        }  
        if(user==null){
            request.getRequestDispatcher("/login").forward(request, response);
            return false;
        }
        return true;

    }