使用 IF 块后 JSP Servlet 不起作用

Question

这个 servlet 在没有使用 if 块时可以正常工作，但现在我需要使用 if 块添加更多选项。 请问谁能告诉我这是为什么？

    public class SearchServlet extends HttpServlet {
        private static final long serialVersionUID = 1L;
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        response.setContentType("text/html");

        FileReader fr = null;
        BufferedReader br = null;

        try {

            ArrayList<Contact> contactList = new ArrayList<>();

            String query = request.getParameter("q");

            String path = getServletContext().getRealPath("pb.txt");
            File file = new File(path);
            fr = new FileReader(file);
            br = new BufferedReader(fr);

            String byPhone = request.getParameter("byPhone");
            String byAddress = request.getParameter("byAddress");
            String byEmail = request.getParameter("byEmail");

            out.println(byPhone);

            String data = null;

            if (byPhone.equals("on")) {
                out.print("By Phone:"+byPhone);
                while ((data = br.readLine()) != null) {
                    String[] token = data.split(":");
                    if (token[1].toLowerCase().startsWith(query.toLowerCase())) {
                        Contact c = new Contact(token[0], token[1], token[2], token[3]);
                        contactList.add(c);
                    }
                }
            }


                out.print("Else");
                while ((data = br.readLine()) != null) {
                    String[] token = data.split(":");
                    if (token[0].toLowerCase().startsWith(query.toLowerCase())) {
                        Contact c = new Contact(token[0], token[1], token[2], token[3]);
                        contactList.add(c);
                    }
                }


            out.print("<h1>Results</h1>");

            out.print("<table>");
            out.print("<tr>");
            out.print("<td>Name</td>");
            out.print("<td>Phone No</td>");
            out.print("<td>Email</td>");
            out.print("<td>Address</td>");
            out.print("</tr>");
            for (Contact c : contactList) {
                out.print("<tr>");
                out.print("<td>" + c.getName() + "</td>");
                out.print("<td>" + c.getPhoneNo() + "</td>");
                out.print("<td>" + c.getEmail() + "</td>");
                out.print("<td>" + c.getAddress() + "</td>");
                out.print("</tr>");
            }

            out.print("</table>");
            out.print("<a href=\"search.html\">Back</a>");

        } finally {
            if (fr != null) {
                fr.close();
            }
            if (br != null) {
                br.close();
            }
            out.close();
        }
    }
}

您可以在 String data = null; 正下方看到 if 块；

谢谢！

Answer 1

它可能失败的一个明显方式似乎是在请求中未提供参数byPhone，留下变量byPhone = null，导致if语句出现NullPointerException。

两种解决方法：

// Explicit null check
if (byPhone != null && byPhone.equals("on")) {

// Reverse equals check
if ("on".equals(byPhone)) {