Question

我正在尝试开发一个连接到数据库的Web应用程序。我试图连接到数据库，但它继续给我一个空指针异常。我已经尝试了很多东西，我似乎无法找到错误。如果有人可以帮助我，那就太棒了！

请注意，我可以通过mysql workbench连接到数据库，其中包含下面的详细信息。 JDBC连接器被添加到构建路径中，因此是servlet。

HTML

<form action="./servlets/Login" method="GET">
                    <div>
                        <label for="name">Username:</label>
                        <input type="text" name="name"/><br/>
                        <label for="pass">Password:</label>
                        <input type ="password" name="pass"/><br/><br/>
                        <label for="submit"></label>
                        <input type="submit" class="btn btn-primary btn-xl page-scroll" name="submit" value="Login">
                    </div>
                    </form>

连接MySQL服务器的Java

公共类VerifyUser {

private static Connection connect = null;
  private static String host="127.0.0.1:3307";
  private static String database="activities";
  private static String username="root";
  private static String password="Hitesh123";
  //connecting to the database
  public static Connection getConnection(){
        if(connect ==null){
            try{
             Class.forName("com.mysql.jdbc.Driver");
              String conn_string="jdbc:mysql://"+host+"/"+database;
              Connection connect = DriverManager.getConnection(conn_string,username,password);
              return connect;
            }catch(Exception ex){
                 System.out.println("Cannot connect to databse"); 
                return null;    
                //ex.printStackTrace();
            }
        }else{
            return connect;
        }
    }
  public Admin checkAdmin(String user,String password){
      String sql="SELECT * from Admin WHERE Username=? AND PasswordHash=?";  
      Admin a=null;
        try( Connection connect = getConnection(); 
             PreparedStatement pstmt = connect.prepareStatement(sql);
            ){  
             pstmt.setString(1,user);
             pstmt.setString(2,MD5HashGenerator.getMD5Hash(password));
             try (ResultSet rs = pstmt.executeQuery();){
               while(rs.next()){
                  //int id=rs.getInt("AdminID");
                   int id=1;
                  String uname=rs.getString("Username");        
                  String pass=rs.getString("PasswordHash");     
                  a=new Admin(id,uname,pass);
                  break;

              }
             }
        }catch(SQLException ex){
            System.out.println("does not work");
            ex.printStackTrace();   
        }
        return a;

  }

错误消息

Cannot connect to databseMar 19, 2018 7:43:29 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [Login] in context with path [/activities_connect] threw exception
java.lang.NullPointerException
    at main.bean.VerifyUser.checkAdmin(VerifyUser.java:80)
    at main.Login.doGet(Login.java:36)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    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:198)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    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)

编辑：再次检查构建路径并且没有更改...向部署程序集添加了jar并仍然没有更改..

Answer 1

方式，你尝试获取连接显示典型的菜鸟错误，特别是在Java中，我们检查了异常。核心或库异常通常会揭示它被抛出的原因。而不是打印一些通用和不明确的消息并返回null（我猜测会导致NullPointerException），记录实际的异常消息并关注可能出错的内容。

Answer 2

这意味着，您Connection connect = getConnection()返回了null。检查数据库属性是否正确。

servlet的数据库连接问题 - Java webapp

2 个答案: