java - servlet Controller 中出现空指针异常-6ren

java - servlet Controller 中出现空指针异常

转载作者：行者123 更新时间：2023-12-02 12:50:00

我是 JSP-Servlet 的新手，我正在为学生跟踪系统创建简单的 servlet Controller “StudentControllerServlet”。代码如下:

package com.luv2code.web.jdbc;

import java.io.IOException;
import java.sql.SQLException;
import java.util.List;

import javax.annotation.Resource;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.sql.DataSource;

/**
 * Servlet implementation class StudentControllerServlet
 */
@WebServlet("/StudentControllerServlet")
public class StudentControllerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private StudentDbUtil studentDbUtil;

    @Resource(name="jdbc/web_student_tracker")
    private DataSource dataSource;

    @Override
    public void init() throws ServletException {
        // TODO Auto-generated method stub
        super.init();
        try{
        studentDbUtil = new StudentDbUtil(dataSource);
        }
        catch(Exception ex){
            throw new ServletException(ex);
        }
    }



    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String theCommand = request.getParameter("command");

        switch (theCommand) {
        case "LIST":
            listStudents(request,response);
            break;

        case "ADD":
            try {
                addStudents(request,response);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            break;

        default:
            listStudents(request,response);
            break;
        }

    }



    private void addStudents(HttpServletRequest request, HttpServletResponse response) throws Exception {
        // TODO Auto-generated method stub
        String firstName = request.getParameter("firstName");
        String lastName = request.getParameter("lastName");
        String email = request.getParameter("email");

        Student theStudent = new Student(firstName, lastName, email);

        StudentDbUtil.addStudent(theStudent);

        listStudents(request, response);
    }



    private void listStudents(HttpServletRequest request, HttpServletResponse response) {
        try {
            List<Student> students = studentDbUtil.getStudents();
            request.setAttribute("STUDENT_LIST", students);

            RequestDispatcher dispatcher = request.getRequestDispatcher("/list-students.jsp");
            dispatcher.forward(request, response);

        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

现在，我在 switch 部分遇到空指针异常。在 switch 部分，一些方法在名为“StudentDbUtil”的单独 Java 文件上调用。 StudentDBUtil 的代码如下:

package com.luv2code.web.jdbc;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

import javax.sql.DataSource;

import com.mysql.jdbc.PreparedStatement;

public class StudentDbUtil {
    private static DataSource dataSource;

    public StudentDbUtil(DataSource theDataSource){
        dataSource = theDataSource;

    }

    public List<Student> getStudents() throws Exception{
        List<Student> students = new ArrayList<>();

        Connection myConn = null;
        Statement myStmt = null;
        ResultSet myRs = null;

        try{
            myConn = dataSource.getConnection();
            myStmt = myConn.createStatement();
            String query = "select * from Student order by last_name";
            myRs = myStmt.executeQuery(query);
            while(myRs.next()){
                int id = myRs.getInt("id");
                String firstName = myRs.getString("first_name");
                String lastName = myRs.getString("last_name");
                String emailID = myRs.getString("email");
                Student tempStudent = new Student(id,firstName,lastName,emailID);
                students.add(tempStudent);
            }

            return students;

        }
        finally{
            close(myConn,myStmt,myRs);
        }
    }

    private static void close(Connection myConn, Statement myStmt, ResultSet myRs) {
        // TODO Auto-generated method stub
        try{
            if(myConn!=null){
                myConn.close();
            }
            if(myStmt!=null){
                myStmt.close();
            }
            if(myRs!=null){
                myRs.close();
            }
        }
        catch(Exception ex){
            ex.printStackTrace();

        }
    }

    public static void addStudent(Student theStudent) throws SQLException  {
        // TODO Auto-generated method stub
        Connection myConn = null;
        java.sql.PreparedStatement myPrepState = null;

        try{
            myConn = dataSource.getConnection();

            String sql = "insert into student " +
                         "(first_Name, last_Name, email) " +
                         "values(?,?,?)";

            myPrepState = myConn.prepareStatement(sql);

            myPrepState.setString(1, theStudent.getFirstName());
            myPrepState.setString(2, theStudent.getLastName());
            myPrepState.setString(3, theStudent.getEmail());

            myPrepState.execute();



        }
        finally {
            close(myConn, myPrepState, null);
        }
    }
}

在我添加“addStudent”方法后，错误开始出现。谁能帮我看看这个空指针异常是从哪里来的？我自己尝试过，但无法弄清楚。

这是部分堆栈跟踪:

Jun 19, 2017 4:13:52 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:my_1st_JSP' did not find a matching property.
Jun 19, 2017 4:13:52 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:tagDemo' did not find a matching property.
Jun 19, 2017 4:13:52 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:servletDemo' did not find a matching property.
Jun 19, 2017 4:13:52 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:web_student_tracker' did not find a matching property.
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Server version:        Apache Tomcat/8.0.39
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Server built:          Nov 9 2016 08:48:39 UTC
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Server number:         8.0.39.0
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: OS Name:               Windows 7
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: OS Version:            6.1
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Architecture:          x86
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Java Home:             C:\Program Files\Java\jdk1.8.0_101\jre
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: JVM Version:           1.8.0_101-b13
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: JVM Vendor:            Oracle Corporation
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: CATALINA_BASE:         E:\study material\udemy jsp-servlet chad\jsp servlet workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: CATALINA_HOME:         C:\Program Files\Apache Software Foundation\Tomcat 8.0
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Command line argument: -Dcatalina.base=E:\study material\udemy jsp-servlet chad\jsp servlet workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Command line argument: -Dcatalina.home=C:\Program Files\Apache Software Foundation\Tomcat 8.0
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Command line argument: -Dwtp.deploy=E:\study material\udemy jsp-servlet chad\jsp servlet workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Command line argument: -Djava.endorsed.dirs=C:\Program Files\Apache Software Foundation\Tomcat 8.0\endorsed
Jun 19, 2017 4:13:52 PM org.apache.catalina.startup.VersionLoggerListener log
INFO: Command line argument: -Dfile.encoding=Cp1252
Jun 19, 2017 4:13:52 PM org.apache.catalina.core.AprLifecycleListener lifecycleEvent
INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: C:\Program Files\Java\jdk1.8.0_101\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;C:/Program Files/Java/jdk1.8.0_101/bin/../jre/bin/client;C:/Program Files/Java/jdk1.8.0_101/bin/../jre/bin;C:/Program Files/Java/jdk1.8.0_101/bin/../jre/lib/i386;C:\Program Files\Java\jdk1.8.0_101\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;c:\Program Files\Microsoft SQL Server\100\Tools\Binn\;c:\Program Files\Microsoft SQL Server\100\DTS\Binn\;c:\Program Files\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\;C:\Program Files\Skype\Phone\;C:\Program Files\MySQL\MySQL Utilities 1.6\;E:\softwares\study software\eclipse-jee-neon-RC3-win32\eclipse;;.
Jun 19, 2017 4:13:52 PM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["http-nio-8080"]
Jun 19, 2017 4:13:53 PM org.apache.tomcat.util.net.NioSelectorPool getSharedSelector
INFO: Using a shared selector for servlet write/read
Jun 19, 2017 4:13:53 PM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["ajp-nio-8009"]
Jun 19, 2017 4:13:53 PM org.apache.tomcat.util.net.NioSelectorPool getSharedSelector
INFO: Using a shared selector for servlet write/read
Jun 19, 2017 4:13:53 PM org.apache.catalina.startup.Catalina load
INFO: Initialization processed in 3158 ms
Jun 19, 2017 4:13:53 PM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina
Jun 19, 2017 4:13:53 PM org.apache.catalina.core.StandardEngine startInternal
INFO: Starting Servlet Engine: Apache Tomcat/8.0.39
Jun 19, 2017 4:13:55 PM org.apache.jasper.servlet.TldScanner scanJars
INFO: At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
Jun 19, 2017 4:13:56 PM org.apache.tomcat.dbcp.dbcp2.BasicDataSourceFactory getObjectInstance
WARNING: Name = web_student_tracker Property maxActive is not used in DBCP2, use maxTotal instead. maxTotal default value is 8. You have set value of "20" for "maxActive" property, which is being ignored.
Jun 19, 2017 4:13:56 PM org.apache.tomcat.dbcp.dbcp2.BasicDataSourceFactory getObjectInstance
WARNING: Name = web_student_tracker Property maxWait is not used in DBCP2 , use maxWaitMillis instead. maxWaitMillis default value is -1. You have set value of "10000" for "maxWait" property, which is being ignored.
Jun 19, 2017 4:13:56 PM org.apache.jasper.servlet.TldScanner scanJars
INFO: At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
Jun 19, 2017 4:13:56 PM org.apache.jasper.servlet.TldScanner scanJars
INFO: At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
Jun 19, 2017 4:13:57 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-nio-8080"]
Jun 19, 2017 4:13:57 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["ajp-nio-8009"]
Jun 19, 2017 4:13:57 PM org.apache.catalina.startup.Catalina start
INFO: Server startup in 3838 ms
Jun 19, 2017 4:15:21 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.luv2code.web.jdbc.StudentControllerServlet] in context with path [/web_student_tracker] threw exception
java.lang.NullPointerException
    at com.luv2code.web.jdbc.StudentControllerServlet.doGet(StudentControllerServlet.java:48)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    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:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$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)

Jun 19, 2017 4:16:37 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.luv2code.web.jdbc.StudentControllerServlet] in context with path [/web_student_tracker] threw exception
java.lang.NullPointerException
    at com.luv2code.web.jdbc.StudentControllerServlet.doGet(StudentControllerServlet.java:48)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    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:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$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)

Jun 19, 2017 4:32:01 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/web_student_tracker] threw exception [<h3>Validation error messages from TagLibraryValidator for c in /list-students.jsp</h3><p>32: Empty var attribute in "c:url" tag.</p>] with root cause
org.apache.jasper.JasperException: <h3>Validation error messages from TagLibraryValidator for c in /list-students.jsp</h3><p>32: Empty var attribute in "c:url" tag.</p>
    at org.apache.jasper.compiler.DefaultErrorHandler.jspError(DefaultErrorHandler.java:55)
    at org.apache.jasper.compiler.ErrorDispatcher.dispatch(ErrorDispatcher.java:277)
    at org.apache.jasper.compiler.ErrorDispatcher.jspError(ErrorDispatcher.java:75)
    at org.apache.jasper.compiler.Validator.validateXmlView(Validator.java:1913)
    at org.apache.jasper.compiler.Validator.validateExDirectives(Validator.java:1859)
    at org.apache.jasper.compiler.Compiler.generateJava(Compiler.java:217)
    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:356)
    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:336)
    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:323)
    at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:585)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:363)
    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.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$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)

Jun 19, 2017 4:34:10 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.luv2code.web.jdbc.StudentControllerServlet] in context with path [/web_student_tracker] threw exception
java.lang.NullPointerException
    at com.luv2code.web.jdbc.StudentControllerServlet.doGet(StudentControllerServlet.java:48)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    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:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$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)

Jun 19, 2017 4:38:26 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.luv2code.web.jdbc.StudentControllerServlet] in context with path [/web_student_tracker] threw exception
java.lang.NullPointerException
    at com.luv2code.web.jdbc.StudentControllerServlet.doGet(StudentControllerServlet.java:48)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    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:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$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)

最佳答案

在类 StudentControllerServlet 行的第 48 行失败的是:

switch (theCommand) {

theCommand 为 null，您没有在请求中发送命令参数

request.getParameter("command");

在这种情况下添加默认命令或日志错误

使用相关命令调用您的 servlet /StudentControllerServlet?command=LIST

关于java - servlet Controller 中出现空指针异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44629455/

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java - servlet Controller 中出现空指针异常