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://code.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。
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