本节书摘来自异步社区《Servlet、JSP和Spring MVC初学指南》一书中的第1章，第1.10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
1.10 处理HTML表单
一个Web应用程序中几乎总会包含一个或者多个HTML表单，供用户输入值。你可以轻松地将一个HTML表单从一个Servlet发送到浏览器。当用户提交表单时，在表单元素中输入的值就会被当作请求参数发送到服务器。
HTML输入域（文本域、隐藏域或者密码域）或者文本区的值，会被当作字符串发送到服务器。空的输入域或者文本区会发送空的字符串。因此，有输入域名称的，ServletRequest.getParameter绝对不会返回null。
HTML的select元素也向header发送了一个字符串。如果select元素中没有任何选项被选中，那么就会发出所显示的这个选项值。
包含多个值的select元素（允许选择多个选项并且用<select multiple>表示的select元素）发出一个字符串数组，并且必须通过SelectRequest.getParameterValues进行处理。
复选框比较奇特。核查过的复选框会发送字符串“on”到服务器。未经核查的复选框则不向服务器发送任何内容，ServletRequest.getParameter(fieldName)返回null。
单选框将被选中按钮的值发送到服务器。如果没有选择任何按钮，将没有任何内容被发送到服务器，并且ServletRequest.getParameter(fieldName)返回null。
如果一个表单中包含多个输入同名的元素，那么所有值都会被提交，并且必须利用ServletRequest.getParameterValues来获取它们。ServletRequest.getParameter将只返回最后一个值。
清单1.4中的FormServlet类示范了如何处理HTML表单。它的doGet方法将一个Order表单发送到浏览器。它的doPost方法获取到所输入的值，并将它们输出。这个Servlet就是app01b应用程序的一部分。
清单1.4 FormServlet类
package app01b;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(name = "FormServlet", urlPatterns = { "/form" })
public class FormServlet extends HttpServlet {
private static final long serialVersionUID = 54L;
private static final String TITLE = "Order Form";
@Override
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter writer = response.getWriter();
writer.println("<html>");
writer.println("<head>");
writer.println("<title>" + TITLE + "</title></head>");
writer.println("<body><h1>" + TITLE + "</h1>");
writer.println("<form method='post'>");
writer.println("<table>");
writer.println("<tr>");
writer.println("<td>Name:</td>");
writer.println("<td><input name='name'/></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Address:</td>");
writer.println("<td><textarea name='address' "
+ "cols='40' rows='5'></textarea></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Country:</td>");
writer.println("<td><select name='country'>");
writer.println("<option>United States</option>");
writer.println("<option>Canada</option>");
writer.println("</select></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Delivery Method:</td>");
writer.println("<td><input type='radio' " +
"name='deliveryMethod'"
+ " value='First Class'/>First Class");
writer.println("<input type='radio' " +
"name='deliveryMethod' "
+ "value='Second Class'/>Second Class</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Shipping Instructions:</td>");
writer.println("<td><textarea name='instruction' "
+ "cols='40' rows='5'></textarea></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td> </td>");
writer.println("<td><textarea name='instruction' "
+ "cols='40' rows='5'></textarea></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Please send me the latest " +
"product catalog:</td>");
writer.println("<td><input type='checkbox' " +
"name='catalogRequest'/></td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td> </td>");
writer.println("<td><input type='reset'/>" +
"<input type='submit'/></td>");
writer.println("</tr>");
writer.println("</table>");
writer.println("</form>");
writer.println("</body>");
writer.println("</html>");
}
@Override
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter writer = response.getWriter();
writer.println("<html>");
writer.println("<head>");
writer.println("<title>" + TITLE + "</title></head>");
writer.println("</head>");
writer.println("<body><h1>" + TITLE + "</h1>");
writer.println("<table>");
writer.println("<tr>");
writer.println("<td>Name:</td>");
writer.println("<td>" + request.getParameter("name")
+ "</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Address:</td>");
writer.println("<td>" + request.getParameter("address")
+ "</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Country:</td>");
writer.println("<td>" + request.getParameter("country")
+ "</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Shipping Instructions:</td>");
writer.println("<td>");
String[] instructions = request
.getParameterValues("instruction");
if (instructions != null) {
for (String instruction : instructions) {
writer.println(instruction + "<br/>");
}
}
writer.println("</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Delivery Method:</td>");
writer.println("<td>"
+ request.getParameter("deliveryMethod")
+ "</td>");
writer.println("</tr>");
writer.println("<tr>");
writer.println("<td>Catalog Request:</td>");
writer.println("<td>");
if (request.getParameter("catalogRequest") == null) {
writer.println("No");
} else {
writer.println("Yes");
}
writer.println("</td>");
writer.println("</tr>");
writer.println("</table>");
writer.println("<div style='border:1px solid #ddd;" +
"margin-top:40px;font-size:90%'>");
writer.println("Debug Info<br/>");
Enumeration<String> parameterNames = request
.getParameterNames();
while (parameterNames.hasMoreElements()) {
String paramName = parameterNames.nextElement();
writer.println(paramName + ": ");
String[] paramValues = request
.getParameterValues(paramName);
for (String paramValue : paramValues) {
writer.println(paramValue + "<br/>");
}
}
writer.println("</div>");
writer.println("</body>");
writer.println("</html>");
}
}
利用下面的URL，可以调用FormServlet：http://localhost:8080/app01b/form
被调用的doGet方法会被这个HTML表单发送给浏览器：
<form method='post'>
<input name='name'/>
<textarea name='address' cols='40' rows='5'></textarea>
<select name='country'>");
<option>United States</option>
<option>Canada</option>
</select>
<input type='radio' name='deliveryMethod' value='First Class'/>
<input type='radio' name='deliveryMethod' value='Second Class'/>
<textarea name='instruction' cols='40' rows='5'></textarea>
<textarea name='instruction' cols='40' rows='5'></textarea>
<input type='checkbox' name='catalogRequest'/>
<input type='reset'/>
<input type='submit'/>
</form>
表单的方法设为post，确保当用户提交表单时，使用HTTP POST方法。它的action属性默认，表示该表单会被提交给请求它时用的相同的URL。
图1.6展示了一个空的Order表单。
图1.6 一个空的Order表单
现在，填写表单，并单击Submit按钮。在表单中输入的值，将利用HTTP POST方法被发送给服务器，这样就会调用Servlet的doPost方法。因此，将会看到图1.7所示的那些值。
图1.7 在Order表单中输入的值
