只需让Servlet根据
doGet()方法中的请求参数以所需的格式返回所需的下拉选项。这是一个在Google
Gson的帮助下以JSON格式返回的示例：
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String type = request.getParameter("type"); // Returns "country" or "state". String value = request.getParameter("value"); // Value of selected country or state. Map<String, String> options = optionDAO.find(type, id); // Do your thing to obtain them from DB. Map key is option value and map value is option label. String json = new Gson().toJson(options); // Convert Java object to JSON string. response.setContentType("application/json"); // Inform client that you're returning JSON. response.setCharacterEncoding("UTF-8"); // important if you want world domination. response.getWriter().write(json); // Write JSON string to response.}假设上述servlet映射到的
url-pattern上
/options，则可以在jQuery的帮助下，在以下JSP示例中使用它。我建议使用jQuery，因为否则，此“简单”任务所需的Javascript代码将达到10倍。
<%@ page pageEncoding="UTF-8" %><%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %><!DOCTYPE html><html lang="en"> <head> <title>SO question 3983929</title> <script src="http://pre.jquery.com/jquery-latest.min.js"></script> <script> $(document).ready(function() { $('#country').change(function() { fillOptions('state'); }); $('#state').change(function() { fillOptions('city'); }); }); function fillOptions(dropdownId) { var dropdown = $('#' + dropdownId); $.getJSON('options?type=' + dropdownId + '&value=' + $(this).val(), function(opts) { $('>option', dropdown).remove(); // Clean old options first. if (opts) { $.each(opts, function(key, value) { dropdown.append($('<option/>').val(key).text(value)); }); } else { dropdown.append($('<option/>').text('Please select ' + dropdownId)); } }); } </script> </head> <body> <form> <select id="country" name="country"> <c:forEach items="${countries}" var="country"> <option value="${country.key}" ${param.country == option.key ? 'selected' : ''}>${country.value}</option> </c:forEach> </select> <select id="state" name="state"> <option>Please select country</option> </select> <select id="city" name="city"> <option>Please select state</option> </select> </form> </body></html>在这里，我假设您已经 在servlet中预先将${countries}其填充为Map<String,String>
，该servlet已对该JSP的请求进行了预处理以进行初始显示。
