Servlet會在服務器啟動或第一次請求該Servlet的時候開始生命周期，在服務器結束的時候結束生命周期。無論請求多少次Servlet，最多只有一個Servlet實例。多個客戶端並發請求Servlet時，服務器會啟動多個線程分別執行該Servlet的service()方法。
一、重點方法說明
1.init(ServletConfig conf)
加載Servlet時運行此方法，如果load-on-startup為0，則客戶端第一次請求該Servlet時加載Servlet，如果load-on-startup配置為1，則容器啟動時加載Servlet。
2.Service(ServletRequest req, ServletResponse res)
客戶端每次請求Servlet都會運行該方法，該方法判斷訪問類型，然後根據HttpServletRequest的getMethod()返回結果判斷是執行doGet還是doPost，doPut等。
3.destroy()
卸載Servlet時執行該方法，當容器關閉時會先卸載所有的Servlet，因此容器關閉時會執行destroy方法。
二、例子實踐
1.web.xml
1 <!-- Servlet生命周期演示 --> 2 <servlet> 3 <servlet-name>LifeCycleServlet</servlet-name> 4 <servlet-class>com.servlet.LifeCycleServlet</servlet-class> 5 </servlet> 6 <servlet-mapping> 7 <servlet-name>LifeCycleServlet</servlet-name> 8 <url-pattern>/servlet/LifeCycleServlet</url-pattern> 9 </servlet-mapping>
2.LifeCycleServlet
1 package com.servlet;
2
3 import java.io.IOException;
4 import java.io.PrintWriter;
5
6 import javax.servlet.ServletConfig;
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 LifeCycleServlet extends HttpServlet {
13
14 private double startPoint;
15
16 /**
17 * Destruction of the servlet. <br>
18 */
19 public void destroy() {
20 this.log("執行destroy()方法...");
21 // Put your code here
22 }
23
24 public void doGet(HttpServletRequest request, HttpServletResponse response)
25 throws ServletException, IOException {
26 this.log("執行doGet()方法...");
27 response.setContentType("text/html");
28 response.setCharacterEncoding("UTF-8");
29 PrintWriter out = response.getWriter();
30 out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
31 out.println("<HTML>");
32 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");
33 out.println(" <BODY>");
34 out.println("<form method='post'>");
35 out.println("<div align='center'>");
36 out.print("<h1>Servlet 生命周期(1)</h1></br>");
37 out.println("<input type='submit' value='點我'>");
38 out.println("</div>");
39 out.println("</form>");
40 out.println(" </BODY>");
41 out.println("</HTML>");
42 out.flush();
43 out.close();
44
45 }
46
47 public void doPost(HttpServletRequest request, HttpServletResponse response)
48 throws ServletException, IOException {
49 this.log("執行doPost()方法...");
50 response.setContentType("text/html");
51 response.setCharacterEncoding("UTF-8");
52 PrintWriter out = response.getWriter();
53 out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
54 out.println("<HTML>");
55 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");
56 out.println(" <BODY>");
57 out.println("<div align='center'>");
58 out.print("<h1>Servlet 生命周期(2)</h1></br>");
59 out.println("<input type='button' onclick='history.go(-1)' value='返回上一級'>");
60 out.println("</div>");
61 out.println(" </BODY>");
62 out.println("</HTML>");
63 out.flush();
64 out.close();
65
66 }
67
68 public void service(HttpServletRequest request,HttpServletResponse response)
69 throws ServletException, IOException {
70 this.log("執行service()方法...");
71 super.service(request, response);
72 }
73
74 public void init() throws ServletException {
75 // Put your code here
76 this.log("執行init()方法...");
77 }
78
79 }
3.效果圖
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