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

 <!-- Servlet生命周期演示 -->
   <servlet>
     <servlet-name>LifeCycleServlet</servlet-name>
     <servlet-class>com.servlet.LifeCycleServlet</servlet-class>
   </servlet>
 <servlet-mapping>
     <servlet-name>LifeCycleServlet</servlet-name>
     <url-pattern>/servlet/LifeCycleServlet</url-pattern>
   </servlet-mapping>

2.LifeCycleServlet

 package com.servlet;

 import java.io.IOException;
 import java.io.PrintWriter;

 import javax.servlet.ServletConfig;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

 public class LifeCycleServlet extends HttpServlet {

     private double startPoint;

     /**
      * Destruction of the servlet. <br>
      */
     public void destroy() {
         this.log("执行destroy()方法...");
         // Put your code here
     }

     public void doGet(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
         this.log("执行doGet()方法...");
         response.setContentType("text/html");
         response.setCharacterEncoding("UTF-8");
         PrintWriter out = response.getWriter();
         out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
         out.println("<HTML>");
         out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
         out.println("  <BODY>");
         out.println("<form method='post'>");
         out.println("<div align='center'>");
         out.print("<h1>Servlet 生命周期(1)</h1></br>");
         out.println("<input type='submit' value='点我'>");
         out.println("</div>");
         out.println("</form>");
         out.println("  </BODY>");
         out.println("</HTML>");
         out.flush();
         out.close();

     }

     public void doPost(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
         this.log("执行doPost()方法...");
         response.setContentType("text/html");
         response.setCharacterEncoding("UTF-8");
         PrintWriter out = response.getWriter();
         out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
         out.println("<HTML>");
         out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
         out.println("  <BODY>");
         out.println("<div align='center'>");
         out.print("<h1>Servlet 生命周期(2)</h1></br>");
         out.println("<input type='button' onclick='history.go(-1)' value='返回上一级'>");
         out.println("</div>");
         out.println("  </BODY>");
         out.println("</HTML>");
         out.flush();
         out.close();

     }

     public void service(HttpServletRequest request,HttpServletResponse response)
             throws ServletException, IOException {
         this.log("执行service()方法...");
         super.service(request, response);
     }

     public void init() throws ServletException {
         // Put your code here
         this.log("执行init()方法...");
     }

 }

3.效果图

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控制台输出：