java.lang.NullPointerException错误

是这样的,java.lang.NullPointerException
at org.apache.jsp.users$jsp._jspService(users$jsp.java:97)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:202)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:382)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:474)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:190)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.valves.CertificatesValve.invoke(CertificatesValve.java:246)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2343)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:170)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:170)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:468)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.connector.http.HttpProcessor.process(HttpProcessor.java:1012)
at org.apache.catalina.connector.http.HttpProcessor.run(HttpProcessor.java:1107)
at java.lang.Thread.run(Thread.java:536)

解决方案 »

  1.   

    估计是数据库连接有问题.conn==null
      

  2.   

    你必须先调用Opendb4()方法，然后再调用executeQuery(String sql)方法！
    而且每次当一个连接用完以后你都必须调用Opendb4()方法.
    你的这个错误一定是连接为空了！
      

  3.   

    jsp页面是这样的:<%@ page contentType="text/html;charset=gb2312" %> 
    <%@ page import="java.sql.*"%><jsp:useBean id="Opendb" class="odbcbean.Opendb4" scope="page" />
    <HTML>
    <BODY>
    <%
    String sql="select *from users";
    ResultSet rs=Opendb.executeQuery(sql);while (rs.next()){
    out.println(rs.getString("username")+"<br>");
    }
    %>
    </BODY>
    </HTML>
    也是报:java.lang.NullPointerException
    at org.apache.jsp.test$jsp._jspService(test$jsp.java:96)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
    at org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:202)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:382)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:474)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)        .......
            ....... at java.lang.Thread.run(Thread.java:536)
      

  4.   

    Connection conn=null;
    .......
    Connection conn=DriverManager.getConnection(sConnStr);你用了好多connection哦。
      

  5.   

    换成
    <%@ page contentType="text/html;charset=gb2312" %> 
    <%@ page import="java.sql.*"%><jsp:useBean id="Opendb" class="odbcbean.Opendb4" scope="page" />
    <HTML>
    <BODY>
    <%
    String sql="select *from users";
    try {
    ResultSet rs=Opendb.executeQuery(sql);while (rs.next()){
    out.println(rs.getString("username")+"<br>");
    }
    } catch (Exception e) {
        e.printStackTrace();
    }
    %>
    </BODY>
    </HTML>看看打出来什么错误？？
      

  6.   


    try{
       Connection conn=DriverManager.getConnection(sConnStr); //可能会抛出NullPointerException。
       Statement stmt=conn.createStatement();
       rs=stmt.executeQuery(sql);
      }
      catch(SQLException ex){
       System.err.print(ex.getMessage());
      }//加上看看
     catch(java.lang.ClassNotFoundException e){
       System.err.print(e.getMessage());
      }
      

  7.   

    to scbb(星际Baby):} catch (Exception e) {
        e.printStackTrace();
    }调用jsp,什么都没有.
      

  8.   

    to farmer0512(风总是朝我吹)://加上看看
     catch(java.lang.ClassNotFoundException e){
       System.err.print(e.getMessage());
      }
    编译时:
    Opendb4.java:32: exception java.lang.ClassNotFoundException is never thrown in body of corresponding try statement
     catch(java.lang.ClassNotFoundException e){
     ^
    1 error
      

  9.   

    不好意思，应该是java.lang.NullPointerException e