本节书摘来自异步社区《Servlet、JSP和Spring MVC初学指南》一书中的第2章，第2.2节，作者：【加】Budi Kurniawan（克尼亚万） , 【美】Paul Deck著，更多章节内容可以访问云栖社区“异步社区”公众号查看
2.2 隐藏域
使用隐藏域来保持状态类似于URL重写技术，但不是将值附加到URL上，而是放到HTML表单的隐藏域中。当表单提交时，隐藏域的值也同时提交到服务器端。隐藏域技术仅当网页有表单时有效。该技术相对于URL重写的优势在于：没有字符数限制，同时无须额外的编码。但该技术同URL重写一样，不适合跨越多个界面。
清单2.3展示了如何通过隐藏域来更新客户信息。清单2.2的Customer类为客户对象模型。
清单2.2 Customer类
package app02a.hiddenfields;
public class Customer {
private int id;
private String name;
private String city;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
}
清单2.3 CustomerServlet类
package app02a.hiddenfields;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/*
* Not thread-safe. For illustration purpose only
*/
@WebServlet(name = "CustomerServlet", urlPatterns = {
"/customer", "/editCustomer", "/updateCustomer"})
public class CustomerServlet extends HttpServlet {
private static final long serialVersionUID = -20L;
private List<Customer> customers = new ArrayList<Customer>();
@Override
public void init() throws ServletException {
Customer customer1 = new Customer();
customer1.setId(1);
customer1.setName("Donald D.");
customer1.setCity("Miami");
customers.add(customer1);
Customer customer2 = new Customer();
customer2.setId(2);
customer2.setName("Mickey M.");
customer2.setCity("Orlando");
customers.add(customer2);
}
private void sendCustomerList(HttpServletResponse response)
throws IOException {
response.setContentType("text/html");
PrintWriter writer = response.getWriter();
writer.println("<html><head><title>Customers</title></head>"
+ "<body><h2>Customers </h2>");
writer.println("<ul>");
for (Customer customer : customers) {
writer.println("<li>" + customer.getName()
+ "(" + customer.getCity() + ") ("
+ "<a href='editCustomer?id=" + customer.getId()
+ "'>edit</a>)");
}
writer.println("</ul>");
writer.println("</body></html>");
}
private Customer getCustomer(int customerId) {
for (Customer customer : customers) {
if (customer.getId() == customerId) {
return customer;
}
}
return null;
}
private void sendEditCustomerForm(HttpServletRequest request,
HttpServletResponse response) throws IOException {
response.setContentType("text/html");
PrintWriter writer = response.getWriter();
int customerId = 0;
try {
customerId =
Integer.parseInt(request.getParameter("id"));
} catch (NumberFormatException e) {
}
Customer customer = getCustomer(customerId);
if (customer != null) {
writer.println("<html><head>"
+ "<title>Edit Customer</title></head>"
+ "<body><h2>Edit Customer</h2>"
+ "<form method='post' "
+ "action='updateCustomer'>");
writer.println("<input type='hidden' name='id' value='"
+ customerId + "'/>");
writer.println("<table>");
writer.println("<tr><td>Name:</td><td>"
+ "<input name='name' value='" +
customer.getName().replaceAll("'", "'")
+ "'/></td></tr>");
writer.println("<tr><td>City:</td><td>"
+ "<input name='city' value='" +
customer.getCity().replaceAll("'", "'")
+ "'/></td></tr>");
writer.println("<tr>"
+ "<td colspan='2' style='text-align:right'>"
+ "<input type='submit' value='Update'/></td>"
+ "</tr>");
writer.println("<tr><td colspan='2'>"
+ "<a href='customer'>Customer List</a>"
+ "</td></tr>");
writer.println("</table>");
writer.println("</form></body>");
} else {
writer.println("No customer found");
}
}
@Override
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String uri = request.getRequestURI();
if (uri.endsWith("/customer")) {
sendCustomerList(response);
} else if (uri.endsWith("/editCustomer")) {
sendEditCustomerForm(request, response);
}
}
@Override
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// update customer
int customerId = 0;
try {
customerId =
Integer.parseInt(request.getParameter("id"));
} catch (NumberFormatException e) {
}
Customer customer = getCustomer(customerId);
if (customer != null) {
customer.setName(request.getParameter("name"));
customer.setCity(request.getParameter("city"));
}
sendCustomerList(response);
}
}
CustomerServlet类继承自HttpServlet，其URL映射分别为/customer、/editCustomer和 /updateCustomer。前两个URL会调用Servlet的doGet方法，而/updateCustomer 会调用doPost方法。
/customer是本例的入口URL。该URL会列举出在init 方法中所初始化的类级别的列表对象customers（在真实应用中，通常是从数据库中获取用户信息），如图2.4所示。
图2.4 客户列表
如图2.4所示，每个客户信息后都有一个edit链接，每个edit链接的href属性为 /editCustomer? id=customerId。当通过/editCustomer访问servlet时，servlet会返回一个编辑表单，如图2.5所示。
图2.5 客户编辑表单
如果你点击的是第一个客户，servlet返回表单中的隐藏域如下:
<form method='post' action='updateCustomer'>
<input type='hidden' name='id' value='1'/>
<table>
<tr><td>Name:</td>
<td><input name='name' value='Donald DC.'/></td>
</tr>
<tr>
<td>City:</td><td><input name='city' value='Miami'/></td>
</tr>
<tr>
<td colspan='2' style='text-align:right'>
<input type='submit' value='Update'/>
</td>
</tr>
<tr>
<td colspan='2'><a href='customer'>Customer List</a></td>
</tr>
</table>
</form>
该隐藏域为所编辑的客户id，因此当表单提交时，服务端就知道应更新哪个客户信息。
需要强调的是，表单是通过post方式提交的，因此调用的是servlet的doPost方法。
