Java开发之Servlet生命周期

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：