Question

空指针异常org.apache.jasper.JasperException: java.lang.NullPointerException

运行JSP页面出错：org.apache.jasper.JasperException: An exception occurred processing JSP page /displayBlogList.jsp at line 14

11: <%
12: List list = (List) request.getAttribute("list");
13: Blog blog = null;
14: for (int i = 0; i < list.size(); i++) {
15: blog = (Blog) list.get(i);
16: System.out.println(blog.getId() + "," + blog.getTitle() + ","
17: + blog.getCreatedTime());
我的GetBlogListServlet.java中的
try {
// 添加博文的SQL语句
String sql = "select id,title,content,createdtime from blog order by id desc";
// DButils中核心类，生成对象时传递数据源对象
QueryRunner qr = new QueryRunner(ds);
List list=(List)qr.query(sql,new BeanListHandler(Blog.class));
request.setAttribute("List", list);
request.getRequestDispatcher("/displayBlogList.jsp").forward(request,
response);
} catch (SQLException e) {
e.printStackTrace();

Answer 1

很明显，你很粗心，大小写没有注意到。
List list = (List) request.getAttribute("list");
request.setAttribute("List", list);
第二行放进去的是List，而取出来的时候是list，所以是空指针

Answer 2

List list=(List)qr.query(sql,new BeanListHandler(Blog.class));
没有检索出数据时，返回可能是空，
那么request.setAttribute("List", list);
request中放的也会为空
jsp中的for循环，list.size(); 也就会有空指针异常了。

希望能帮到你！

Answer 3

找下有没有变量为空

Answer 4

request.setAttribute("List", list);
List list = (List) request.getAttribute("list");

好叻.自己看吧