Servlet会在服务器启动或第一次请求该Servlet的时候开始生命周期，在服务器结束的时候结束生命周期。无论请求多少次Servlet，最多只有一个Servlet实例。多个客户端并发请求Servlet时，服务器会启动多个线程分别执行该Servlet的service()方法。
一、重点方法说明
1.init(ServletConfig conf)
加载Servlet时运行此方法，如果load-on-startup为0，则客户端第一次请求该Servlet时加载Servlet，如果load-on-startup配置为1，则容器启动时加载Servlet。
2.Service(ServletRequest req, ServletResponse res)
客户端每次请求Servlet都会运行该方法，该方法判断访问类型，然后根据HttpServletRequest的getMethod()返回结果判断是执行doGet还是doPost，doPut等。
3.destroy()
卸载Servlet时执行该方法，当容器关闭时会先卸载所有的Servlet，因此容器关闭时会执行destroy方法。
二、例子实践
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("执行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("执行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='点我'>");
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("执行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='返回上一级'>");
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("执行service()方法...");
71 super.service(request, response);
72 }
73
74 public void init() throws ServletException {
75 // Put your code here
76 this.log("执行init()方法...");
77 }
78
79 }
3.效果图
浏览器：
控制台输出：
