一、轉向（Forward）
1.要點說明
轉向是通過RequestDispatcher對象的forward（）方法來實現的。RequestDispatcher可以通過HttpServletRequest的getRequestDispatcher()方法獲得。getRequestDispatcher（）方法的參數必須以“/”開始。Forward不僅可以跳轉到本應用的另一個Servlet、JSP頁面，也可以跳轉到另一個文件，甚至WEB-INF文件夾下的文件。
轉向通過request.setAttribute()把結果放到request中，然後forward到jsp中顯示。
注意：當執行forward動作的時候不能有任何輸出到客戶端（例如：System.out.println()）,否則會拋出IllegalStateException。也就是在forward之前盡量不要使用out.println()語句向客戶端輸出。
2.代碼實踐
web.xml
1 <!-- 轉向（Forward） --> 2 <servlet> 3 <servlet-name>ForwardServlet</servlet-name> 4 <servlet-class>com.servlet.ForwardServlet</servlet-class> 5 </servlet> 6 <servlet-mapping> 7 <servlet-name>ForwardServlet</servlet-name> 8 <url-pattern>/servlet/ForwardServlet</url-pattern> 9 </servlet-mapping>
ForwardServlet.java
1 package com.servlet;
2
3 import java.io.IOException;
4 import java.io.PrintWriter;
5
6 import javax.servlet.RequestDispatcher;
7 import javax.servlet.ServletException;
8 import javax.servlet.http.HttpServlet;
9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11
12 public class ForwardServlet extends HttpServlet {
13
14 public void doGet(HttpServletRequest request, HttpServletResponse response)
15 throws ServletException, IOException {
16
17 request.setCharacterEncoding("UTF-8");
18 String destination = request.getParameter("destination");
19 if("file".equals(destination)){//跳轉到/WEB-INF/web.xml
20 RequestDispatcher d = request.getRequestDispatcher("/WEB-INF/web.xml");
21 d.forward(request, response);
22 }else if("jsp".equals(destination)){//跳轉到UploadFile.jsp
23 RequestDispatcher dispatcher = request.getRequestDispatcher("/page/UploadFile.jsp");
24 dispatcher.forward(request, response);
25 }else if("servlet".equals(destination)){//跳轉到另一個Servlet
26 RequestDispatcher dispatcher = request.getRequestDispatcher("/servlet/LifeCycleServlet");
27 dispatcher.forward(request, response);
28 }else{
29 response.setCharacterEncoding("UTF-8");
30 response.setContentType("text/html");
31 response.getWriter().println("缺少參數。用法："+request.getRequestURI()+
32 "?destination=jsp或者file或者servlet");
33 }
34
35 }
36
37 }
Forward.jsp
1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%> 2 <% 3 String path = request.getContextPath(); 4 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; 5 %> 6 7 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 8 <html> 9 <head> 10 <base href="<%=basePath%>"> 11 12 <title>My JSP 'Forward.jsp' starting page</title> 13 14 <meta http-equiv="pragma" content="no-cache"> 15 <meta http-equiv="cache-control" content="no-cache"> 16 <meta http-equiv="expires" content="0"> 17 <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> 18 <meta http-equiv="description" content="This is my page"> 19 <!-- 20 <link rel="stylesheet" type="text/css" href="styles.css"> 21 --> 22 23 </head> 24 25 <body> 26 <div align="center"> 27 <input type="button" onclick='location="<%=basePath%>servlet/ForwardServlet" 28 +"?destination=servlet";' value='跳轉到LifeCycleServlet'> 29 <input type="button" onclick='location="<%=basePath%>servlet/ForwardServlet" 30 +"?destination=file";' value='跳轉到web.xml'> 31 <input type="button" onclick='location="<%=basePath%>servlet/ForwardServlet" 32 +"?destination=jsp";' value='跳轉到JSP'> 33 </div> 34 </body> 35 </html>
3.效果截圖
Forward.jsp:
跳轉到servlet：
跳轉到web.xml:
跳轉到jsp:
二、重定向（Redirect）
1.重點說明
重定向是利用服務器返回的狀態碼來實現的。客戶端浏覽器請求服務器的時候，服務器端會返回一個狀態碼。服務器端通過HttpServletResponse的setStatus（）方法設置狀態碼。
2.代碼實踐
web.xml
1 <!-- 重定向（Redirect） --> 2 <servlet> 3 <servlet-name>RedirectServlet</servlet-name> 4 <servlet-class>com.servlet.RedirectServlet</servlet-class> 5 </servlet> 6 <servlet-mapping> 7 <servlet-name>RedirectServlet</servlet-name> 8 <url-pattern>/servlet/RedirectServlet</url-pattern> 9 </servlet-mapping>
RedirectServlet.java
1 package com.servlet;
2
3 import java.io.IOException;
4 import java.io.PrintWriter;
5 import java.util.HashMap;
6 import java.util.Map;
7 import java.util.Map.Entry;
8
9 import javax.servlet.ServletException;
10 import javax.servlet.http.HttpServlet;
11 import javax.servlet.http.HttpServletRequest;
12 import javax.servlet.http.HttpServletResponse;
13
14 public class RedirectServlet extends HttpServlet {
15
16 Map<String,Integer> map = new HashMap<String, Integer>();
17
18 public void destroy() {
19 super.destroy(); // Just puts "destroy" string in log
20 // Put your code here
21 map = null;
22 }
23
24 public void doGet(HttpServletRequest request, HttpServletResponse response)
25 throws ServletException, IOException {
26
27 String filename = request.getParameter("filename");
28 if(filename != null){//統計下載次數
29 int hit = map.get(filename);//取下載次數
30 map.put(filename, ++hit);//下載次數+1後保存
31 response.sendRedirect(request.getContextPath()+filename);//重定向到文件
32 }else{//顯示下載次數
33 response.setCharacterEncoding("UTF-8");
34 response.setContentType("text/html");
35 PrintWriter out = response.getWriter();
36 out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
37 out.println("<HTML>");
38 out.println(" <HEAD><TITLE>文件下載</TITLE></HEAD>");
39 out.println(" <BODY>");
40 out.println("<table width=100%>");
41 out.println("<tr>");
42 out.println("<td><b>文件名</b></td>");
43 out.println("<td><b>下載次數</b></td>");
44 out.println("<td><b>下載</b></td>");
45 out.println("</tr>");
46 for(Entry<String, Integer> entry : map.entrySet()){//遍歷map
47 out.println("<tr>");
48 out.println("<td>"+entry.getKey()+"</td>");
49 out.println("<td>"+entry.getValue()+"</td>");
50 out.println("<td><a href='"+request.getRequestURI()+
51 "?filename="+entry.getKey()+"' target='blank' "+
52 "onclick='location=location;'>下載</a></td>");
53 out.println("</tr>");
54 }
55 out.println("</table>");
56 out.println("</legend>");
57 out.println(" </BODY>");
58 out.println("</HTML>");
59 out.flush();
60 out.close();
61 }
62 }
63
64 public void init() throws ServletException {
65 // Put your code here
66 map.put("/uploadfile/1.txt", 0);
67 map.put("/uploadfile/2.txt", 0);
68 map.put("/uploadfile/3425.jpg", 0);
69 }
70
71 }
3.效果截圖
三、自動刷新（Refresh）
1.重點說明
自動刷新不僅可以實現一段時間之後自動跳轉到另一個頁面，還可以實現一段時間之後自動刷新本頁面。Servlet中通過HttpServletResponse對象設置Header屬性實現自動刷新效果。
2.代碼實踐
1 response.setHeader("Refresh", "1000; URL=http://localhost:8080/King/servlet/Forward.jsp");
其中1000為時間，單位毫秒。當URL設置的路徑為Servlet自己的路徑時，就會定時自動刷新本頁面。
