Requestdispatcher空指针异常

Question

我有以下Servlet代码：

@WebServlet("/Login")
public class Login extends HttpServlet {
private static final long serialVersionUID = 1L;
public String lang = "";   
/**
 * @see HttpServlet#HttpServlet()
 */
public Login() {
    super();
    // TODO Auto-generated constructor stub
}

/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    String lcLog = request.getParameter("LogType").toUpperCase();
    switch(lcLog)
    {
        case "LOGIN":
            try {
                boolean llErfolg = doLogin(request.getParameter("user").toUpperCase(),request.getParameter("pw"));
                if(llErfolg==true)
                {
                    System.out.println("Login");
                    HttpSession session = request.getSession();
                    session.setAttribute("username", request.getParameter("user").toUpperCase());
                    session.setAttribute("Language", lang);
                    request.getRequestDispatcher("/WEB-INF/jsp/view/Kacheln_Test.jsp").forward(request, response);
                }
                else
                {
                    System.out.println("Login Failed");
                    request.setAttribute("LogInFailed", true);
                    request.getRequestDispatcher("/WEB-INF/jsp/view/Startseite.jsp").forward(request, response);
                }
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            break;
        case "LOGOUT":
            System.out.println("Logout");
            HttpSession session = request.getSession();
            session.invalidate();
            request.getRequestDispatcher("/WEB-INF/jsp/view/Startseite.jsp").forward(request, response);
            break;
        default:
            System.out.println("Warum?");
            break;
    }
}

直到现在，一切都运行得很好，但最近每当我遇到"LogOut“这样的情况时，我都会在调度程序中得到一个空指针异常，我完全不知道为什么，因为几天前它还起作用了。

我有下面的Kacheln_Test.jsp文件，也许是问题的根源，因为当我点击"Kunden“时，同样的错误也会发生

<c:url var="post_url2" value="/Login" />
<form id="LogOut" method="POST" action="${post_url2}" name="LogOut">
    <input type="hidden" id="LogType" name="LogType" value = "" />
    <fmt:message key="Kacheln.Label.Angemeldetals"/>&emsp; <%=session.getAttribute("username") %>,
    <a href="javascript:{}" onclick="ausloggen_setzen('LogOut');"><fmt:message key="Kacheln.Label.LogOut"/> </a>
</form>

这里也有同样的错误

<div class="image-container">
    <div class="frame">
       <input class="live-slide"  type ="image" src="Images/Kunden.png" onClick="return setValue('Mail')" />
    </div>
    <div class="image-overlay" onclick="return setValue('Kunden')">
        <fmt:message key="Kacheln.Button.Kunden"/>
    </div>
</div>

它们是被调用的函数

<script>
    function ausloggen_setzen(lcTest)
    {
        document.getElementById('LogType').value = lcTest;
        document.getElementById('LogOut').submit();
        return true;
    }
    function setValue(lcArt)
    {
        document.getElementById("OF").value = lcArt;
        document.getElementById("OFform").submit();
        return true;
    }
</script>

希望有人能帮上忙，我想这只是一件小事，但我不知道原因

错误堆栈跟踪：

Jul 12, 2017 8:46:22 AM org.apache.catalina.core.StandardWrapperValve    
invoke
SCHWERWIEGEND: Servlet.service() for servlet [Login] in context with path
 [/Tuev_Test] threw exception [org.apache.jasper.JasperException:   
java.lang.NullPointerException] with root cause
java.lang.NullPointerException
at     org.apache.jsp.WEB_002dINF.jsp.view.Menu_jsp._jspService(Menu_jsp.java:251)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:716)
at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:584)
at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:523)
at org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:885)
at org.apache.jsp.WEB_002dINF.jsp.view.Startseite_jsp._jspService(Startseite_jsp.java:136)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:716)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:466)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:391)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:318)
at Login.doPost(Login.java:73)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:616)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1533)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1489)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

大家好，

凯文

Answer 1

我已经发现了我的两个问题，通过仔细地一步一步地检查代码，我发现在"Kunden“和"Startseite”的包含jsp中都有一个request.getAttribute()。在我的两种情况下，都有一个nullpointerexception，我忘记在调用jsp之前设置属性。

但是因为一开始只有一个Nullpointerexception，所以我变得不确定，所以我完全忽略了显而易见的原因

感谢你的回答，并为我的盲目和愚蠢道歉：

大家好，

凯文