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java.lang.NullPointerException有关问题，各位大侠帮忙看看，着

2012-08-29 
java.lang.NullPointerException问题，各位大侠帮忙看看，在线等着%@page pageEncodingGB18030 %%@pag

java.lang.NullPointerException问题，各位大侠帮忙看看，在线等着
<%@page pageEncoding="GB18030" %>
<%@page import=" java.sql.*" %>
<%@page import=" java.util.*" %>
<%@page import="java.io.*" %>
<%@page import="com.bjsxt.bbs.*" %>
<%!
  private void tree(Set<Article> articles,Connection conn,int id,int grade){
  String sql = "select * from atticle where pid ="+id;
  Statement stmt = DB.createStmt(conn);
  ResultSet rs = DB.executeQuery(stmt,sql);
  try{
  while(rs.next()){
  Article a = new Article();
  a.setId(rs.getInt("id"));
  a.setPid(rs.getInt("pid"));
  a.setRootId(rs.getInt("rootid"));
  a.setTitle(rs.getString("title"));
  a.setLeaf(rs.getInt("isleaf")==0? true : false);
  a.setPdate(rs.getTimestamp("pdate"));
  a.setGrade(grade);
  articles.add(a);
  if(!a.isLeaf()){
  tree(articles,conn,a.getId(),grade+1);
  }
  }
  }catch(SQLException e){
  e.printStackTrace();
  }
  }
%>
<%
  Set<Article> articles = new HashSet<Article>();
  Connection conn = DB.getConn();
  tree(articles,conn,0,0);
  DB.close(conn);
%>
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java.lang.NullPointerException请问这怎么解决啊，哪里出问题啦

[解决办法]

Java code
  tree(articles, [color=#FF0000]conn[/color],a.getId(),grade+1);  这个你都是空的.. 没有获取connection的实例....获取你先定义一下 connection          ----> private Connection conn=null;
[解决办法]
private void tree(Set<Article> articles,Connection conn[color=#FF0000][/color],int id,int grade){
 String sql = "select * from atticle where pid ="+id; 
 
 Statement stmt = DB.createStmt(conn);
 ResultSet rs = DB.executeQuery(stmt,sql);
你确定你传过来的conn 取到值了吗？
你直接不用传，改成这个试试：private void tree(Set<Article> articles,int id,int grade){
 Connection conn = DB.getConn();
 String sql = "select * from atticle where pid ="+id;
 Statement stmt = DB.createStmt(conn);
 ResultSet rs = DB.executeQuery(stmt,sql);


[解决办法]

 Connection conn = DB.getConn();
 String sql = "select * from atticle where pid ="+id;
 Statement stmt = DB.createStmt(conn);
 ResultSet rs = DB.executeQuery(stmt,sql);
这样就行
[解决办法]
探讨
private void tree(Set<Article> articles,Connection conn[color=#FF0000][/color],int id,int grade){
 String sql = "select * from atticle where pid ="+id;
 Statement stmt = DB.createStmt(conn);
 Re……

[解决办法]
在DB设几个断点看看
返回来的connection是不是已经就是一个null
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