Question

每次我点击指向其他页面的链接时，都会出现Null Pointer Exception错误，我不知道是什么问题。

这是错误

05-Jul-2014 17:27:25.941 SEVERE [http-nio-8084-exec-14] org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [itemServlet] in context with path [/servlet] threw exception
 java.lang.NullPointerException
    at com.etc.servlet.itemServlet.showProductList(itemServlet.java:172)
    at com.etc.servlet.itemServlet.doPost(itemServlet.java:94)
    at com.etc.servlet.itemServlet.doGet(itemServlet.java:67)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    at   org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:301)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:136)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:74)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:516)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1015)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:652)
    at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1575)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1533)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

这是抛出NullPointerException

的servlet的部分代码

public class itemServlet extends HttpServlet {

private memberDao usrdao = new memberDao();
private itemDao itmdao = new itemDao();
private noticeDao ntcdao=new noticeDao();
private articleDao atcdao=new articleDao();
private List<item> listItm;
private List<notice> listNtc;
private List<article> listAtc;
private List<member> listUsr;
private List<item> listpiece;
/**
 * Constructor of the object.
 */
public itemServlet() {
    super();
}

/**
 * Destruction of the servlet. <br>
 */
public void destroy() {
    super.destroy(); // Just puts "destroy" string in log
    // Put your code here
}

/**
 * The doGet method of the servlet. <br>
 *
 * This method is called when a form has its tag value method equals to get.
 * 
 * @param request the request send by the client to the server
 * @param response the response send by the server to the client
 * @throws ServletException if an error occurred
 * @throws IOException if an error occurred
 */
public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    doPost(request,response);
}

/**
 * The doPost method of the servlet. <br>
 *
 * This method is called when a form has its tag value method equals to post.
 * 
 * @param request the request send by the client to the server
 * @param response the response send by the server to the client
 * @throws ServletException if an error occurred
 * @throws IOException if an error occurred
 */
public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    //1 中文处理
    request.setCharacterEncoding("gbk");
    response.setContentType("text/html;charset=gbk");

    //2 读取参数op
    String op = request.getParameter("op");

    //3 根据op调用不同的业务方法
    if("add".equals(op)){
        additem(request,response);//
    }else if("findall".equals(op)){
        showProductList(request,response);//展现所有商品
    }else if("finddetail".equals(op)){
        showProductDetail(request,response);//展现商品的细节
    }else if("addcart".equals(op)){
        addcart(request,response);//添加购物车
    }

}

    private void addcart(HttpServletRequest request,HttpServletResponse response)throws           ServletException, IOException//展示商品细节
{
    int itmID=Integer.parseInt(request.getParameter("itemID"));
    System.out.println("获取本页的商品编号"+itmID);
    item additem = itmdao.findbyitmmID(itmID);
    HttpSession session =request.getSession();
    if(request.getSession().getAttribute("listpiece")==null){

        listpiece=new ArrayList<item>();
        listpiece.add(additem);
        System.out.println("listpiece为空。将查到的商品作为单件加入listpiece的列表");
        session.setAttribute("listpiece", listpiece);           
    }
    else{
        List<item> listpiece=(List<item>)      request.getSession().getAttribute("listpiece");
        listpiece.add(additem);
        System.out.println("listpiece不为空，将查到的商品作为单件加入listpiece的列表");
        session.setAttribute("listpiece", listpiece);
    }
    if(listItm==null)
    {
        response.getWriter().print("查询失败！");
        return;                     
    }

    int itid = Integer.parseInt(request.getParameter("itemID"));

    for(item it:listItm)
    {
        if(itid==it.getItemID())//找出该对象
        {
            request.setAttribute("it", it);
              request.getRequestDispatcher("../itemdetail.jsp").forward(request, response);
            return;             
        }

    }
}

private void showProductDetail(HttpServletRequest request,HttpServletResponse     response)throws ServletException, IOException//展示商品细节
{
    if(listItm==null)
    {
        response.getWriter().print("查询失败！");
        return;                     
    }

    int itid = Integer.parseInt(request.getParameter("itemID"));

    for(item it:listItm)
    {
        if(itid==it.getItemID())//找出该对象
        {
            request.setAttribute("it", it);
            request.getRequestDispatcher("../itemdetail.jsp").forward(request, response);
            return;             
        }

    }
}

private void showProductList(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException//商品成列
{
    listItm = itmdao.findAll();
    listNtc = ntcdao.findSix();
    listAtc = atcdao.findTen();
    request.setAttribute("listNtc", listNtc);
    request.setAttribute("listAtc", listAtc);
    request.setAttribute("listItm", listItm);
    request.getRequestDispatcher("../showlist.jsp").forward(request, response);
}

private void additem(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException//添加一件新的商品
{
    //1 图片上传
    SmartUpload sm = new SmartUpload();
    sm.initialize(this.getServletConfig(), request, response);
    sm.setAllowedFilesList("jpg,jpeg,png,gif");

    try 
    {
        sm.upload();
        sm.save("/pic");//在pic路径下按原来的名字进行保存
    } catch (SmartUploadException e) 
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
        response.getWriter().print("上传失败！添加失败！");
        return;                     
    }

    //2 读取表单的其他数据，插入数据库

    String itemName = sm.getRequest().getParameter("itemName");
   float itemPrice = Float.parseFloat(sm.getRequest().getParameter("price"));
     float itemPrice = Float.parseFloat(sm.getRequest().getParameter("itemPrice"));     
     String itemStatus = sm.getRequest().getParameter("itemStatus");        
     String itemBrief = sm.getRequest().getParameter("itemBrief");      
     int typeID =Integer.parseInt(sm.getRequest().getParameter("typeID"));
     item newitem = new item(-1,itemName,itemPrice,itemStatus,itemBrief,typeID );

    if (!itmdao.add(newitem))
    {
        response.getWriter().print("添加失败！");
        return;         
    }
    request.getRequestDispatcher("itemServlet?op=findall").forward(request, response);
}

/**
 * Initialization of the servlet. <br>
 *
 * @throws ServletException if an error occurs
 */
public void init() throws ServletException {
    // Put your code here
}

}

空指针是否可能来自数据库？

itmDao.findall（）

public List<item> findAll() 
{
    System.out.println("itemDao的findAll被调用");
    List<item> list = new ArrayList();
    ResultSet rs =  DBUtils.doQuery("select * from itemTable  order by itemID desc");
    try {
        if (rs == null)
            return list;
        while (rs.next()) {
            item item = new item(rs.getInt("itemID"),
                    rs.getString("itemName"),      rs.getFloat("itemPrice"),rs.getString("itemStatus"),rs.getString("itembrief")
                    ,rs.getInt("typeID"));
            list.add(item);

        }
    } catch (Exception e) 
    {
        e.printStackTrace();
    }
    finally
    {
        try 
        {
            DBUtils.close(rs, rs.getStatement(), rs.getStatement()
                    .getConnection());
        } catch (Exception e2) 
        {
            e2.printStackTrace();
        }
    }
    return list;

}

ntddao.findsix（）

  public List<notice> findSix() //用在首页的  查找前六条的公告
{
    System.out.println("noticeDao的findSix函数被调用");
    List<notice> list = new ArrayList();
    ResultSet rs =  DBUtils.doQuery("select * from notice order by ntID desc");
    try {
        if (rs == null)
            return list;
        int i=1;
        while (rs.next()&&i<=6) {
            notice notice = new notice(rs.getInt("ntID"),
                    rs.getString("ntTitle"),    rs.getString("ntTime"),rs.getString("ntText"),rs.getInt("adID"));
            list.add(notice);
            i++;
        }
    } catch (Exception e) 
    {
        e.printStackTrace();
    }
    finally
    {
        try 
        {
            DBUtils.close(rs, rs.getStatement(), rs.getStatement()
                    .getConnection());
        } catch (Exception e2) 
        {
            e2.printStackTrace();
        }
    }
    return list;

}

atcdao.findten（）

public List<article> findTen() //用在首页的  查找前10条的公告
{
    System.out.println("articleDao的findTen函数被调用");
    List<article> list = new ArrayList();
    ResultSet rs =  DBUtils.doQuery("select * from article order by atID desc");
    try {
        if (rs == null)
            return list;
        int i=1;
        while (rs.next()&&i<=10) {
            article article = new article(rs.getInt("atID"),
                    rs.getString("atTitle"), rs.getString("atPic"),rs.getString("atTime"),rs.getString("atText"),rs.getInt("adID"));
            list.add(article);
            i++;
        }
    } catch (Exception e) 
    {
        e.printStackTrace();
    }
    finally
    {
        try 
        {
            DBUtils.close(rs, rs.getStatement(), rs.getStatement()
                    .getConnection());
        } catch (Exception e2) 
        {
            e2.printStackTrace();
        }
    }
    return list;

}

Answer 1

您提供给getRequestDispatcher的路径必须位于当前的servlet上下文中。这意味着你不能使用像"../showlist.jsp"这样的东西。您将不得不以不同方式组织目录结构。

Answer 2

你可以在那里检查以下对象是否为空？

listItm = itmdao.findAll();
listNtc = ntcdao.findSix();
listAtc = atcdao.findTen();

我假设其中一个（或多个）为null，因此setAttribute会抛出异常。

否则我强烈建议不要强制使用在内存中作为单例运行的servlet中的字段，并且多个用户请求可能会破坏数据的一致性。我在谈论这些领域：

private List<item> listItm;
private List<notice> listNtc;
private List<article> listAtc;
private List<member> listUsr;
private List<item> listpiece;

我的另一个建议是遵循java命名约定。例如，将类的第一个字符更改为capital：itemServlet - ＆gt; ItemServlet。

这是更标准的方式，有助于我们更好地理解您的代码：）

Java空指针异常错误

2 个答案: