求助，开发一论坛，打开一网页出现一个空指针异常，自己找了很久没找出问题，求帮助

[V非凡]

这是错误信息：
message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: An exception occurred processing JSP page /article.jsp at line 7

4: <%!
5: private void tree(List<Article> articles, Connection conn, int id, int grade) {
6: String sql = "select * from article where pid = " + id;
7: Statement stmt = DB.createStmt(conn);
8: ResultSet rs = DB.executeQuery(stmt, sql);
9: try {
10: while(rs.next()) {


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:524)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:435)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)


root cause

java.lang.NullPointerException
com.bjsxt.bbs.DB.createStmt(DB.java:27)
org.apache.jsp.article_jsp.tree(article_jsp.java:16)
org.apache.jsp.article_jsp._jspService(article_jsp.java:88)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.14 logs.

[-江沐风-]

conn = null;

[-江沐风-]

DB是？ Statement stmt=conn.createStatement();

[V非凡]

内容有点多，麻烦大家帮帮忙，谢谢：这是article.jsp部分内容

这是DB.java的内容：

[V非凡]

引用 1 楼 u012724379 的回复:

conn = null;

具体内容在下面

[V非凡]

引用 2 楼 u012724379 的回复:

DB是？ Statement stmt=conn.createStatement();

DB.java在下面

[-江沐风-]

把conn = null添加到DB.java的25行；

[V非凡]

引用 6 楼 u012724379 的回复:

把conn = null添加到DB.java的25行；

还是不行，还是有异常：type Exception report message description The server encountered an internal error () that prevented it from fulfilling this request. exception org.apache.jasper.JasperException: java.lang.NullPointerException org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:541) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:435) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266) javax.servlet.http.HttpServlet.service(HttpServlet.java:803) root cause java.lang.NullPointerException com.bjsxt.bbs.DB.createStmt(DB.java:27) org.apache.jsp.article_jsp.tree(article_jsp.java:17) org.apache.jsp.article_jsp._jspService(article_jsp.java:89) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:803) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266) javax.servlet.http.HttpServlet.service(HttpServlet.java:803) note The full stack trace of the root cause is available in the Apache Tomcat/6.0.14 logs.

[-江沐风-]

报的是空指针错误，应该是你获取了一个null值；单步调试一下；

[-江沐风-]

感觉应该是你的conn没有执行 conn = DriverManager.getConnection(URL,userName,userPwd); 你把这句放到26行的try中吧；

[V非凡]

引用 9 楼 u012724379 的回复:

感觉应该是你的conn没有执行 conn = DriverManager.getConnection(URL,userName,userPwd); 你把这句放到26行的try中吧；

是这样，原来我没有在项目中添加mysql连接的.jar，添加后没错，谢谢你啦

[-江沐风-]

引用 10 楼 gj716 的回复:

[quote=引用 9 楼 u012724379 的回复:] 感觉应该是你的conn没有执行 conn = DriverManager.getConnection(URL,userName,userPwd); 你把这句放到26行的try中吧；

是这样，原来我没有在项目中添加mysql连接的.jar，添加后没错，谢谢你啦[/quote]

[V非凡]

呵呵，犯了一个常识性错误，哎。。。。太不小心啦