求解！ShoppingCart.do?bookid=4出错，空指针

Question

地址栏：http://localhost:8080/test2/ShoppingCart.do?bookid=4
提示好下错误
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.doGet(ActionServlet.java:414)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
bookstore.util.EncodingFilter.doFilter(EncodingFilter.java:75)
root cause java.lang.NullPointerException
bookstore.ShoppingCartAction.execute(ShoppingCartAction.java:53)
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.doGet(ActionServlet.java:414)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
bookstore.util.EncodingFilter.doFilter(EncodingFilter.java:75)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.13 logs.
--------------------------------------------------------------------------------

Answer 1

ShoppingCartAction.java:53  已经注释了呀 为什么出错?  
   if (CheckBookid(request, errors, bookid)) {
            List cart;
            if (session.getAttribute("cart") == null) {
                cart = new ArrayList();
                session.setAttribute("cart", cart);
            } else {
                cart = (ArrayList) session.getAttribute("cart");
            }
53          /** if (delitem.equalsIgnoreCase("del")) {
            // cart.remove(bookid); // 增加 一个删除某条购物信息的 方法*
                
}else{
*/
 cart.add(new Integer(bookid));
/** }*/
           
        } else {
            this.saveErrors(request, errors);
            return mapping.findForward("errorPage");
        }
        return mapping.findForward("bookReady");
    }

Answer 2

错误信息是值的ShoppingCartAction.java第53行.那是delitem变量是null,按你说的已经注释了,那你重新编译运行看下.

Answer 3

IDE时常犯2的
正常
弄个空格 多保存几次就好

Answer 4

java.lang.NullPointerException
bookstore.ShoppingCartAction.execute(ShoppingCartAction.java:53)通过这个注释可以看到是产生了空指针的错误了，空指针错误的产生的原因是你使用了一个null对象。可以把到work目录下把这个文件ShoppingCartAction.java 用ueEdit软件(简称UE)打开这个文件，找到上面的红色部分的行数(也就是53行)，说明就是这行产生了空指针。