javax.servlet.http package
通常情况下，servlet app处理的是HTTP请求。 javax.servlet.http主要用于提供HTTP相关特性的处理。
HttpServlet
继承GenericServlet，重写service方法，并提供一个新的service方法，其参数是HttpServletRequest和HttpServletResponse类型。
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, java.io.IOException
servlet Container仍然会调用javax.servlet.Servlet中定义的service方法，该方法在HttpServlet中的实现如下
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
if(req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
HttpServletRequest request = (HttpServletRequest)req;
HttpServletResponse response = (HttpServletResponse)res;
this.service(request, response);
} else {
throw new ServletException("non-HTTP request or response");
}
}
首先将servlet Container传递的ServletRequest, ServletResponse向下转型为HttpServletRequest，HttpServletResponse。然后再调用HttpServlet定义的service方法。
HttpServlet自定义的service方法实现如下
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String method = req.getMethod();
long errMsg;
if(method.equals("GET")) {
errMsg = this.getLastModified(req);
if(errMsg == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if(ifModifiedSince < errMsg) {
this.maybeSetLastModified(resp, errMsg);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if(method.equals("HEAD")) {
errMsg = this.getLastModified(req);
this.maybeSetLastModified(resp, errMsg);
this.doHead(req, resp);
} else if(method.equals("POST")) {
this.doPost(req, resp);
} else if(method.equals("PUT")) {
this.doPut(req, resp);
} else if(method.equals("DELETE")) {
this.doDelete(req, resp);
} else if(method.equals("OPTIONS")) {
this.doOptions(req, resp);
} else if(method.equals("TRACE")) {
this.doTrace(req, resp);
} else {
String errMsg1 = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[]{method};
errMsg1 = MessageFormat.format(errMsg1, errArgs);
resp.sendError(501, errMsg1);
}
}
首先通过request.getMethod获取Http发送请求的方法，然后调用如下七种方法之一：
doGet, doPost, doHead, doPut, doTrace, doOptions, and doDelete.
其中最常用的两种方式为doGet, doPost。也就是说servlet开发者只需要根据需求重写doGet方法或doPost方法即可。
总结 ：HttpServlet相对GenericServlet提供如下两点特性
servlet开发者需要实现的方法是doGet，doPost（少数情况下是其他五中方法）而不再是service方法
servlet需要处理的是HttpServletRequest 和 HttpServletResponse, 而不再是ServletRequest 和 ServletResponse.
HttpServletRequest
对于HTTP请求，HttpServletRequest用于代表servlet request。
HttpServletRequest继承于javax.servlet.ServletRequest接口，并添加了一些方法。它添加的一些比较常用的方法如下
java.lang.String getContextPath()
Cookie[] getCookies()
java.lang.String getHeader(java.lang.String name)
java.lang.String getMethod()
java.lang.String getQueryString()
HttpSession getSession()
HttpSession getSession(boolean create)
HttpServletResponse
对于HTTP请求，HttpServletResponse用于代表servlet response。
HttpServletResponse继承于javax.servlet.ServletResponse接口，并添加了一些方法。它添加的一些比较常用的方法如下
void addCookie(Cookie cookie)
void addHeader(java.lang.String name, java.lang.String value)
void sendRedirect(java.lang.String location)
HTML Forms
input field（(a text field, a hidden field, or a password field)）或text area发送给server的是string，empty input field或text area发送给server的是empty string，而不是null。
select 发送给server的也是string
- multiple-value select发送给server的是string数组，由ServletRequest.getParameterValues获取
- checkbox由ServletRequest.getParameter(fieldName)获取，若没选中则返回null
- Radio buttons发送选中的value给server，如果没有选择则ServletRequest.getParameter(fieldName) 返回null
- 如果一个form有多个名字相同的input，则可通过ServletRequest.getParameterValues 获取，如果通过ServletRequest.getParameter获取则得到的是最后一个。
例
@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("<!DOCTYPE html>"); 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>"); } }