出现错误500，请问问题出在哪儿，在线等

Question

HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: java.lang.NullPointerException
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
com.hfut.oa.dao.DB.getRs(DB.java:66)
com.hfut.oa.action.LogonAction.executeLogonNo(LogonAction.java:74)
com.hfut.oa.action.LogonAction.execute(LogonAction.java:31)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.29

Answer 1

java.lang.NullPointerException
com.hfut.oa.dao.DB.getRs(DB.java:66)空指针异常，看看代码啥样先。

Answer 2

public class DB {
    private ResultSet rs;
    private Statement stm;
    private Connection con;
    private String url=Content.URL;
    private String classname=Content.CLASSNAME;
    private int num_per=0;
    private int num_rs=0;
    private int num_currentpage=1;
    private int num_pages=1;
    public DB(){
     try{
     Class.forName(classname);
     }
     catch(ClassNotFoundException e){
     e.printStackTrace();
     }
    }
    
    public Connection getCon(){       
       try{
           con=DriverManager.getConnection(url,"sa","123");
       }catch(Exception e){e.printStackTrace(System.err);}
       return con;
    }
    public Statement getStm(){
       try{
          con=getCon();
          stm=con.createStatement();
       }catch(Exception e){e.printStackTrace(System.err);}
        return stm;
      }
      public Statement getStmed(){
       try{
          con=getCon();
          stm=con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
       }catch(Exception e){e.printStackTrace(System.err);}
       return stm;
    }
      
    public ResultSet getRs(String sql){
System.out.println(sql);
    if(sql==null)sql="";
try{
stm=getStmed();
rs=stm.executeQuery(sql);
}
catch(SQLException e){e.printStackTrace();}
return rs;
}
   
    public int update(String sql){
     System.out.println(sql);
int num=0;
     if(sql==null)sql="";
     try{
         stm=getStmed();
         num=stm.executeUpdate(sql);
     }
     catch(SQLException e){e.printStackTrace();num=0;}
     return num;
    }

Answer 3

public int inseart(String sql){
     int num=0;
     if(sql==null)sql="";
     System.out.println(sql);
     try{
         stm=getStmed();
         num=stm.executeUpdate(sql);
     }
     catch(Exception e){e.printStackTrace();num=0;}
     return num;
    }
     
    public int del(String sql){
     int num=0;
     if(sql==null)sql="";
     System.out.println(sql);
     try{
     stm=getStmed();
     num=stm.executeUpdate(sql);
     }
     catch(SQLException e){e.printStackTrace();}
     return num;
    }

    public ArrayList getalltextlist(String sql,int pos){
     ArrayList list=new ArrayList();
     if(sql==null)sql="";
     stm=getStmed();
     try{
     rs=stm.executeQuery(sql);
     Page page=new Page(rs,pos);       //通过实例化一个Page类对象设置每页记录数(num_per)、总记录数(num_rs)、总页数(num_pages)、当前页数(current_page)。
     setPage(page);                    //调用DB类的setPage(Page page)方法，将上面实例化的Page对象的属性值复给DB类中的属性。
     int i=1;
     rs=page.getRs();                  //调用Page类的getRs()方法，获得当前页的记录指针。
     while(rs.next()&&i<=num_per){    //变量i控制显示的记录数。
        TextForm textform=new TextForm();
        textform.setTextid(rs.getInt(Content.TEXTID));
        textform.setSubject(rs.getString(Content.TEXTSUBJECT));
        textform.setSender(rs.getString(Content.TEXTSENDER));
        textform.setGeter(rs.getString(Content.TEXTGETER));
        textform.setMark(rs.getString(Content.TEXTMARK));
        textform.setContent(rs.getString(Content.TEXTCONTENT));
        textform.setSendtime(rs.getString(Content.TEXTSENDTIME));
        list.add(textform);
            i++;
     }
     }
     catch(Exception e){e.printStackTrace();}
        return list;
    }   
    
   
    public ArrayList getallmeetlist(String sql,int pos){
     System.out.println(sql);
     ArrayList list=new ArrayList();
     if(sql==null)sql="";
     stm=getStmed();
     try{
     rs=stm.executeQuery(sql);
     Page page=new Page(rs,pos);
     setPage(page);
     int i=1;
     rs=page.getRs();
     while(rs.next()&&i<=num_per){
        MeetForm meetform=new MeetForm();
        meetform.setMeetid(rs.getInt(Content.MEETID));
        meetform.setSubject(rs.getString(Content.MEETSUBJECT));
        meetform.setSpeaker(rs.getString(Content.MEETSPEAKER));
        meetform.setListener(rs.getString(Content.MEETLIST));
        meetform.setTime(rs.getString(Content.MEETTIME));
        meetform.setAddress(rs.getString(Content.MEETADDRESS));
        meetform.setContent(rs.getString(Content.MEETCONTENT));
        list.add(meetform);
        i++;
         }
     }
     catch(Exception e){e.printStackTrace();}
        return list;
    }
    
    public ArrayList getallpcardlist(String sql,int pos){
     System.out.println(sql);
     ArrayList list=new ArrayList();
     if(sql==null)sql="";
     stm=getStmed();
     try{
     rs=stm.executeQuery(sql);
     Page page=new Page(rs,pos);
     setPage(page);
     int i=1;
     rs=page.getRs();
     while(rs.next()&&i<=num_per){
        PcardForm pcardform=new PcardForm();
        pcardform.setPcardid(rs.getInt(Content.PCARDID));
        pcardform.setSubject(rs.getString(Content.PCARDSUBJECT));
        pcardform.setAuthor(rs.getString(Content.PCARDAUTHOR));
        pcardform.setTime(rs.getString(Content.PCARDTIME));
        pcardform.setContent(rs.getString(Content.PCARDCONTENT));
        list.add(pcardform);
        i++;
         }
     }
     catch(Exception e){e.printStackTrace();}
        return list;
    }
    
    public ArrayList getallpersonlist(String sql,int pos){
     System.out.println(sql);
     ArrayList list=new ArrayList();
     if(sql==null)sql="";
     stm=getStmed();
     try{
     rs=stm.executeQuery(sql);
     Page page=new Page(rs,pos);
     setPage(page);
     int i=1;
     rs=page.getRs();
     while(rs.next()&&i<=num_per){
        UserForm userform=new UserForm();
        userform.setUserid(rs.getInt(Content.USERID));
        userform.setUsername(rs.getString(Content.USERNAME));
        userform.setUsertruename(rs.getString(Content.USERTRUENAME));
        userform.setUserable(rs.getInt(Content.USERABLE));
        userform.setUsergood(rs.getString(Content.USERGOOD));
        userform.setUserbranch(rs.getString(Content.USERBRANCH));
        userform.setUserjob(rs.getString(Content.USERJOB));
        userform.setUsersex(rs.getString(Content.USERSEX));
        userform.setUseremail(rs.getString(Content.USEREMAIL));
        userform.setUsertel(rs.getString(Content.USERTEL));
        userform.setUseraddress(rs.getString(Content.USERADDRESS));
        userform.setUseraccesstimes(rs.getInt(Content.USERACCESSTIMES));
        userform.setUserfoundtime(rs.getString(Content.USERFOUNDTIME));
        list.add(userform);
        i++;
     }
     }
     catch(Exception e){e.printStackTrace();}
        return list;
    }   

Answer 4

请将这一行标注一下com.hfut.oa.dao.DB.getRs(DB.java:66)

Answer 5

public ResultSet getRs(String sql){
System.out.println(sql);
    if(sql==null)sql="";
try{
stm=getStmed();
rs=stm.executeQuery(sql);
}
catch(SQLException e){e.printStackTrace();}
return rs;
}

Answer 6

先测试一下是否可以得到connection和statement