Java HTTP状态500-在第104行处理JSP页面/header.JSP时发生异常

我得到以下错误：

org.apache.jasper.jaspereException:处理时发生异常 第104行的JSP page/header.JSP

根本原因

java.lang.NullPointerException
org.apache.jsp.header_jsp._jspService(header_jsp.java:235)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

这是我的分类

  public ArrayList<Category> getListCategory() throws SQLException, ClassNotFoundException {
    Connection cons = DBconnect.getConnecttion();
    String sql = "SELECT * FROM category";
    ArrayList<Category> list = new ArrayList<>();
    PreparedStatement ps = null;//cai
    try {
        try {
            ps = (PreparedStatement) cons.prepareStatement(sql);
        } catch (Exception e) {

            return null;
        }
        ResultSet rs = ps.executeQuery();
        while (rs.next()) {
            Category category = new Category();
            category.setCategoryID(rs.getLong("category_id"));
            category.setCategoryName(rs.getString("category_name"));
            list.add(category);
        }
        cons.close();
    } catch (SQLException e) {
         cons.close();
        return null;

    }
    return list;
}

我在CategoryDAO中检查了getListCategory（），它仍然有效。 我的代码怎么了？请帮帮我。谢谢大家。

if（categoryDAO.getListCategory（）！=null）{
if (categoryDAO.getListCategory() != null) {
          for (Category ds : categoryDAO.getListCategory()) {
  out.write("\n");
  out.write("<li><a href=\"products.jsp?category=");
  out.print(ds.getCategoryID());
  out.write(" \"> ");
  out.print(ds.getCategoryName());
  out.write("</a></li>\n");
  out.write("\n");
  out.write("                                    ");

对于（类别ds:categoryDAO.getListCategory（））{
输出。写入（“\n”）；
out.写出（“
\n”）；
输出。写入（“\n”）；
请写出（“”）；

第235行是：for（类别ds:categoryDAO.getListCategory（））

---

有什么问题吗？Ramanlfc。

根本原因表明错误出现在第

235行
not
104
，发布jspDo not pick and choose bit of the stack trace to show---在一个代码块中显示整个堆栈跟踪，完成所有CausedBy部分。另外，不要以任何方式修改堆栈跟踪。将其从日志复制/粘贴到问题。对不起，吉姆，这是我第一次发帖。下面第235行，Ramanlfc
  public ArrayList<Category> getListCategory() throws SQLException, ClassNotFoundException {
    Connection cons = DBconnect.getConnecttion();
    String sql = "SELECT * FROM category";
    ArrayList<Category> list = new ArrayList<>();
    PreparedStatement ps = null;//cai
    try {
        try {
            ps = (PreparedStatement) cons.prepareStatement(sql);
        } catch (Exception e) {

            return null;
        }
        ResultSet rs = ps.executeQuery();
        while (rs.next()) {
            Category category = new Category();
            category.setCategoryID(rs.getLong("category_id"));
            category.setCategoryName(rs.getString("category_name"));
            list.add(category);
        }
        cons.close();
    } catch (SQLException e) {
         cons.close();
        return null;

    }
    return list;
}

 static final String DBNAME = "shop";
static final String USERNAME = "root";
static final String PASSWORD = "vancao";
private static ResultSet rs = null;
private static Connection connection = null;

public static Connection getConnecttion() throws ClassNotFoundException, SQLException {
    if (connection != null) {
        return connection;
    } else {
        Class.forName("com.mysql.jdbc.Driver");
        String connectionURL = "jdbc:mysql://" + HOSTNAME + ":3306/" + DBNAME + "?useUnicode=true&characterEncoding=UTF-8&useSSL=false";
        connection = DriverManager.getConnection(connectionURL, USERNAME,
                PASSWORD);
        return connection;
    }
}

if (categoryDAO.getListCategory() != null) {
          for (Category ds : categoryDAO.getListCategory()) {
  out.write("\n");
  out.write("<li><a href=\"products.jsp?category=");
  out.print(ds.getCategoryID());
  out.write(" \"> ");
  out.print(ds.getCategoryName());
  out.write("</a></li>\n");
  out.write("\n");
  out.write("                                    ");