Java开发之Servlet生命周期

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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 here22     }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 here76         this.log("执行init()方法...");77     }78 79 }

3.效果图

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