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[Java教程]Java Web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.

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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