Javascript jax正以所需的值发送过来。 function Fill() { var facno = document.getElementById("facname").value; var xhttp = new XMLHttpR

jax正以所需的值发送过来。

function Fill() {

        var facno = document.getElementById("facname").value;
        var xhttp = new XMLHttpRequest();
        xhttp.overrideMimeType("text/html;charset=UTF-8");
        xhttp.onreadystatechange = function() {

            if (xhttp.readyState === 4 && xhttp.status === 200) {
                document.getElementById("subname").innerHTML = xhttp.responseText;

            }
        };

        xhttp.open("POST", "DropFill?facnoajax=" + facno, true);
        xhttp.send();
    }

<%
                    Connection con = null;
                    con = DBConnection.createConnection();
                    PreparedStatement ps = null;
                    try {
                        String sql = "SELECT fac_no, fac_name FROM faculty_info";
                        ps = con.prepareStatement(sql);
                        ResultSet rs = ps.executeQuery();
                %>



                <div class="form-group row">
                    <label class="col-form-label col-md-3" for="facname">Faculty:</label>
                    <div class="col-md-4">
                        <select name="facname" id="facname" class="form-control"
                            onchange="Fill()" required>
                            <option value="" selected>Choose any:</option>

                            <%
                                while (rs.next()) {
                                        String facname = rs.getString("fac_name");
                                        String facno = rs.getString("fac_no");
                            %>
                            <option value="<%=facno%>"><%=facname%></option>
                            <%
                                }
                            %>
                        </select>
                    </div>
                </div>


                <%
                    } catch (SQLException sqe) {
                        out.println(sqe);
                    }
                %>

<div class="form-group row" id="dd2">
                    <label class="col-form-label col-md-3" for="subname">Subject:</label>
                    <div class="col-md-4">
                        <select id="subname" name="subname" class="form-control" required>
                        <option value="" selected>Choose any:</option>

                        </select>
                    </div>
                </div>

public class DropFill extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public DropFill() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html;charset=UTF-8");


        try (PrintWriter out = response.getWriter()) {

            ArrayList<String> subname = new ArrayList<String>();
            ArrayList<String> subno = new ArrayList<String>();

            Connection con = null;
            con = DBConnection.createConnection();
            PreparedStatement ps = null;
            String facno = request.getParameter("facnoajax");
            try {
                String sql = "SELECT sub_name FROM subject_info WHERE fac_no='" + facno + "'";
                ps = con.prepareStatement(sql);
                ResultSet rs = ps.executeQuery();

                while (rs.next()) {
                    subname.add(rs.getString("sub_name"));
                    subno.add(rs.getString("sub_no"));
                }

                for (int i = 0; i < subname.size(); i++) {
                    for (int j = 0; j <= subno.size(); j++)
                        response.getWriter().write("<select> <option value=" + subno.get(j) + ">" + subname.get(i)
                                + "</option> </select>");
                }

            }

            catch (SQLException sqe) {
                out.println(sqe);
            }

        }

    }
}

<td><select name="class_main" id="class_main"
                        onchange="this.form.submit();">
                            <option value="0">Select class</option>
                            <%
                                try {
                                    String url = "jdbc:mysql://localhost:3306/online_exam?useTimezone=ture&serverTimezone=UTC";
                                    Connection com = DriverManager.getConnection(url, "root", "");
                                    PreparedStatement ps = com.prepareStatement("select * from class");
                                    ResultSet rs = ps.executeQuery();
                                    while (rs.next()) {
                            %>
                            <option value="<%=rs.getInt("classid")%>"
                                <%if (request.getParameter("class_main") != null) {
                        if (rs.getInt("classid") == Integer.parseInt(request.getParameter("class_main"))) {
                            out.print("selected");
                        }
                    }%>><%=rs.getString("classname")%></option>
                            <%
                                }
                                    classid = request.getParameter("class_main");
                                    System.out.println("From class " + classid);
                                } catch (Exception e) {
                                    System.out.println(e.getMessage());
                                }
                            %>
                    </select></td>
                </tr>

                <tr>
                    <td><label>Choose Subject :</label></td>

                    <td><select name="subject" id="subject"
                        onchange="this.form.submit();">
                            <option value="0">Select Subject</option>
                            <%
                                try {
                                    String url = "jdbc:mysql://localhost:3306/online_exam?useTimezone=ture&serverTimezone=UTC";
                                    Connection com = DriverManager.getConnection(url, "root", "");
                                    PreparedStatement ps = com.prepareStatement("select * from subject where classid = ?");
                                    ps.setString(1, request.getParameter("class_main"));
                                    ResultSet rs = ps.executeQuery();
                                    while (rs.next()) {
                            %>
                            <option value="<%=rs.getInt("subjectid")%>"
                                <%if (request.getParameter("subject") != null) {
                        if (rs.getInt("subjectid") == Integer.parseInt(request.getParameter("subject"))) {
                            out.print("selected");
                        }
                    }%>><%=rs.getString("subjectname")%></option>
                            <%
                                }
                                    subjectid = request.getParameter("subject");
                                    System.out.println("from subject " + subjectid);
                                } catch (Exception e) {
                                    System.out.println(e.getMessage());
                                }
                            %>

                    </select></td>
                </tr>
                <tr>
                    <td><label>Choose Chapter :</label></td>

                    <td><select name="chapter" id="chapter"
                        onchange="this.form.submit();">
                            <option value="0">Select Chapter</option>

                            <%
                                try {
                                    String url = "jdbc:mysql://localhost:3306/online_exam?useTimezone=ture&serverTimezone=UTC";
                                    Connection com = DriverManager.getConnection(url, "root", "");
                                    PreparedStatement ps = com
                                            .prepareStatement("select * from chapter where classid = ? and subjectid = ?");
                                    ps.setString(1, request.getParameter("class_main"));
                                    ps.setString(2, request.getParameter("subject"));
                                    ResultSet rs = ps.executeQuery();
                                    while (rs.next()) {
                            %>
                            <option value="<%=rs.getInt("chapterid")%>"
                                <%if (request.getParameter("chapter") != null) {
                        if (rs.getInt("chapterid") == Integer.parseInt(request.getParameter("chapter"))) {
                            out.print("selected");
                        }
                    }%>><%=rs.getString("chaptername")%></option>
                            <%
                                }
                                    chapterid = request.getParameter("chapter");
                                    System.out.println("From chapter " + chapterid);
                                } catch (Exception e) {
                                    System.out.println(e.getMessage());
                                }
                            %>

</select></td>