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

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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