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JavaWeb学习之Servlet(二)----Servlet的生命周期、继承结构、修改Servlet模板
一、http协议回顾:
在上一篇文章中:JavaWeb学习之Servlet(一)----MyEclipse及Tomcat的配置,我们通过在浏览器输入url,就能看到在MyEclipse中编写的Servlet资源,效果如下:
上图中,整个过程是这样的:浏览器中输入url后,会通过hosts文件/dns服务器解析为IP地址,进而找到对应ip地址的服务器。
在这期间,浏览器会通过http协议发出请求。服务器端收到请求后,做了下面这些事:
(1)分析出当前请求的是哪台虚拟主机:
- 查看Host请求头分析出访问的是哪台虚拟主机
- 如果没有Host请求头(在浏览器地址栏直接输入ip地址而不是url),则访问缺省虚拟主机
(2)分析当前请求访问的是当前虚拟主机的哪个Web应用:
- 从请求行中请求的资源部分来分析
(3)分析当前请求要访问的是这个Web应用的哪个资源:
- 从请求行的资源部分分析出访问的是哪个资源
(4)查找web.xml文件,查看有没有对应的虚拟路径,如果有则用这个虚拟路径对应的资源做响应
(5)服务器从response对象中获取之前写入的数据(这就是写在Servlet当中的java代码),组织成http响应消息发给浏览器。
注:第(5)句话便是本文要学习的重点。
二、Servet的运行过程及生命周期
Servlet程序是由WEB服务器调用,web服务器收到客户端的ServletWeb服务器首先检查是否已经装载并创建了该Servlet的实例对象。如果是,则直接执行第(4)步,否则,执行第(2)步。
访问请求后:
(1)装载并创建该Servlet的一个实例对象。
(2)调用Servlet实例对象的init()方法。
(3)创建一个用于封装HTTP请求消息的HttpServletRequest对象和一个代表HTTP响应消息的HttpServletResponse对象,然后调用Servlet的service()方法并将请求和响应对象作为参数传递进去。
(4)WEB应用程序被停止或重新启动之前,Servlet引擎将卸载Servlet,并在卸载之前调用Servlet的destroy()方法。
Servet的生命周期:
Servlet 的生命周期定义了一个Servlet如何被加载、初始化,以及它怎样接收请求、响应请求、提供服务。
生命周期如下:
- (1)通常情况下,服务器会在Servlet第一次被调用时创建该Servlet类的实例对象(servlet出生),创建出对象后立即调用init()方法做初始化操作;
- (2)一旦被创建出来,该Servlet实例就会驻留在内存中,为后续对这个Servlet的请求做服务,每次对这个Servlet的访问都会导致Servlet中Service方法执行;
- (3)当web应用被移除容器或者关闭服务器时,随着web应用的销毁,Servlet也会被销毁(servlet死亡)。在销毁之前服务器会调用Servlet的destroy方法做一些善后的工作。
有3个方法代表了Servlet的生命周期:
- init方法,负责初始化Servlet对象。
- service方法,负责响应客户的请求(调用doGet或doPost等方法)。
- destroy方法,当Servlet对象退出生命周期时,负责释放占用的资源。
注:在Servlet的整个生命周期内,Servlet的init方法只有在servlet被创建时被调用一次,每次对这个Servlet的访问都会导致Servlet中Service方法执行。
例如:现在浏览器连续访问Servlet 10次,内存中只有一个Sevlet对象。Servlet对象由服务器创建(创建一次),request和response由Servlet容器创建(创建10次)
来看一段代码:
1 package com.vae.servlet; 2 import java.io.IOException; 3 import java.io.PrintWriter; 4 import javax.servlet.ServletException; 5 import javax.servlet.http.HttpServlet; 6 import javax.servlet.http.HttpServletRequest; 7 import javax.servlet.http.HttpServletResponse; 8 //一般实现一个Servlet,只要继承HttpServlet类即可 9 public class MyServlet extends HttpServlet {10 //Servlet初始化时调用的方法11 @Override12 public void init() throws ServletException {13 super.init();14 System.out.println("init....");15 }16 17 //Servlet被销毁时调用的方法18 @Override19 public void destroy() {20 super.destroy();21 System.out.println("destroy...");22 }23 //-------doGet/doPost 核心的业务处理方法(由service方法来调用)------------ 24 @Override25 protected void doGet(HttpServletRequest req, HttpServletResponse resp)26 throws ServletException, IOException {27 //super.doGet(req, resp);28 doPost(req, resp);29 System.out.println("do service...");30 }31 32 @Override33 protected void doPost(HttpServletRequest req, HttpServletResponse resp)34 throws ServletException, IOException {35 //super.doPost(req, resp);36 }37 }
运行程序,输入url,此时,一按回车,马上就会在后台打出日志:
然后连续刷新三次网页,日志如下:
可以看到,Servelet只会初始化一次,之后的话,我们多次访问的是同一个Sevlet对象。此时,即使关掉网页,Servlet也不会销毁,只有关掉Tomcat服务器才会销毁Servlet。
需要注意的是,前台可能有get和post两种请求,但是在后台做的处理是一样的。例如:前台输入用户名密码,在后台验证的时候是不区分哪一种请求方式的。于是,如果在doGet()方法中写了代码内容,我们可以在doPost()方法中加一句:"doGet(req,resp);"即可,就可以进行重复利用(毕竟执行的都是同一段逻辑)。
三、Servlet的继承结构:
- Servlet接口:定义了Servlet应该具有的基本方法
- GenericServlet:抽象类,实现了Servlet接口。通用基本Servlet实现,对于不常用的方法在这个实现类中进行了基本的实现,将Service设计为了抽象方法,需要子类去实现
- HttpServlet:抽象类,继承了GenericServlet类。在通用Servlet的基础上基于HTTP协议进行了进一步的强化:复写了GenericServlet中的Service方法,Service方法体内的代码会自动判断用户的请求方式,如为GET请求,则调用HttpServlet的doGet方法,如为Post请求,则调用doPost方法。因此,开发人员在编写Servlet时,通常只需要继承HttpServlet,然后覆写doGet或doPost方法,而不要去覆写service方法。
四、修改Servlet模板:
使用MyEclipse创建Servlet时,根据默认的Servlet模板生成的Servlet代码如下:
1 import java.io.IOException; 2 import java.io.PrintWriter; 3 4 import javax.servlet.ServletException; 5 import javax.servlet.http.HttpServlet; 6 import javax.servlet.http.HttpServletRequest; 7 import javax.servlet.http.HttpServletResponse; 8 9 10 public class Servlet2 extends HttpServlet {11 12 /**13 * The doGet method of the servlet. <br>14 *15 * This method is called when a form has its tag value method equals to get.16 * 17 * @param request the request send by the client to the server18 * @param response the response send by the server to the client19 * @throws ServletException if an error occurred20 * @throws IOException if an error occurred21 */22 public void doGet(HttpServletRequest request, HttpServletResponse response)23 throws ServletException, IOException {24 25 response.setContentType("text/html");26 PrintWriter out = response.getWriter();27 out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");28 out.println("<HTML>");29 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");30 out.println(" <BODY>");31 out.print(" This is ");32 out.print(this.getClass());33 out.println(", using the GET method");34 out.println(" </BODY>");35 out.println("</HTML>");36 out.flush();37 out.close();38 }39 40 /**41 * The doPost method of the servlet. <br>42 *43 * This method is called when a form has its tag value method equals to post.44 * 45 * @param request the request send by the client to the server46 * @param response the response send by the server to the client47 * @throws ServletException if an error occurred48 * @throws IOException if an error occurred49 */50 public void doPost(HttpServletRequest request, HttpServletResponse response)51 throws ServletException, IOException {52 53 response.setContentType("text/html");54 PrintWriter out = response.getWriter();55 out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");56 out.println("<HTML>");57 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");58 out.println(" <BODY>");59 out.print(" This is ");60 out.print(this.getClass());61 out.println(", using the POST method");62 out.println(" </BODY>");63 out.println("</HTML>");64 out.flush();65 out.close();66 }67 68 }
在实际开发中,这些生成的代码和注释一般我们都用不到的,每次都要手工删除这些注释和代码,很麻烦,因此可以根据开发的实际情况修改Servlet的模板代码,改成符合实际开发需求的模板代码。
MyEclipse 10修改Servlet模板的步骤如下:
关闭MyEclipse,找到MyEclipse安装目录下的\Common\plugins文件夹,比如:D:\MyEclipse10\Common\plugins,然后找到com.genuitec.eclipse.wizards_9.0.0.me201108091322.jar这个jar文件,如下图所示:
用压缩工具打开,注意是打开而不是解压这个jar包,如下图所示:
上图中,打开Jar包中的Templates文件夹中的Servlet.java文件,可以看到里面的模板代码:
1 #---------------------------------------------# 2 # <aw:description>Template for Servlet</aw:description> 3 # <aw:version>1.1</aw:version> 4 # <aw:date>04/05/2003</aw:date> 5 # <aw:author>Ferret Renaud</aw:author> 6 #---------------------------------------------# 7 8 <aw:import>java.io.IOException</aw:import> 9 <aw:import>java.io.PrintWriter</aw:import> 10 11 <aw:import>javax.servlet.ServletException</aw:import> 12 <aw:import>javax.servlet.http.HttpServlet</aw:import> 13 <aw:import>javax.servlet.http.HttpServletRequest</aw:import> 14 <aw:import>javax.servlet.http.HttpServletResponse</aw:import> 15 16 <aw:parentClass>javax.servlet.http.HttpServlet</aw:parentClass> 17 18 <aw:constructor name="c1"> 19 /** 20 * Constructor of the object. 21 */ 22 public <aw:className/>() { 23 super(); 24 } 25 26 </aw:constructor> 27 28 <aw:method name="doGet"> 29 /** 30 * The doGet method of the servlet. <br> 31 * 32 * This method is called when a form has its tag value method equals to get. 33 * 34 * @param request the request send by the client to the server 35 * @param response the response send by the server to the client 36 * @throws ServletException if an error occurred 37 * @throws IOException if an error occurred 38 */ 39 public void doGet(HttpServletRequest request, HttpServletResponse response) 40 throws ServletException, IOException { 41 response.setContentType("text/html"); 42 PrintWriter out = response.getWriter(); 43 out.println( 44 "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">"); 45 out.println("<HTML>"); 46 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); 47 out.println(" <BODY>"); 48 out.print(" This is "); 49 out.print(this.getClass()); 50 out.println(", using the GET method"); 51 out.println(" </BODY>"); 52 out.println("</HTML>"); 53 out.flush(); 54 out.close(); 55 } 56 57 </aw:method> 58 59 <aw:method name="doPost"> 60 /** 61 * The doPost method of the servlet. <br> 62 * 63 * This method is called when a form has its tag value method equals to post. 64 * 65 * @param request the request send by the client to the server 66 * @param response the response send by the server to the client 67 * @throws ServletException if an error occurred 68 * @throws IOException if an error occurred 69 */ 70 public void doPost(HttpServletRequest request, HttpServletResponse response) 71 throws ServletException, IOException { 72 response.setContentType("text/html"); 73 PrintWriter out = response.getWriter(); 74 out.println( 75 "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">"); 76 out.println("<HTML>"); 77 out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); 78 out.println(" <BODY>"); 79 out.print(" This is "); 80 out.print(this.getClass()); 81 out.println(", using the POST method"); 82 out.println(" </BODY>"); 83 out.println("</HTML>"); 84 out.flush(); 85 out.close(); 86 } 87 88 </aw:method> 89 90 <aw:method name="doPut"> 91 /** 92 * The doPut method of the servlet. <br> 93 * 94 * This method is called when a HTTP put request is received. 95 * 96 * @param request the request send by the client to the server 97 * @param response the response send by the server to the client 98 * @throws ServletException if an error occurred 99 * @throws IOException if an error occurred100 */101 public void doPut(HttpServletRequest request, HttpServletResponse response)102 throws ServletException, IOException {103 104 // Put your code here105 106 }107 108 </aw:method>109 110 <aw:method name="doDelete">111 /**112 * The doDelete method of the servlet. <br>113 *114 * This method is called when a HTTP delete request is received.115 * 116 * @param request the request send by the client to the server117 * @param response the response send by the server to the client118 * @throws ServletException if an error occurred119 * @throws IOException if an error occurred120 */121 public void doDelete(HttpServletRequest request, HttpServletResponse response)122 throws ServletException, IOException {123 124 // Put your code here125 126 }127 128 </aw:method>129 130 <aw:method name="init">131 /**132 * Initialization of the servlet. <br>133 *134 * @throws ServletException if an error occurs135 */136 public void init() throws ServletException {137 // Put your code here138 }139 140 </aw:method>141 142 <aw:method name="destroy">143 /**144 * Destruction of the servlet. <br>145 */146 public void destroy() {147 super.destroy(); // Just puts "destroy" string in log148 // Put your code here149 }150 151 </aw:method>152 153 <aw:method name="getServletInfo">154 /**155 * Returns information about the servlet, such as 156 * author, version, and copyright. 157 *158 * @return String information about this servlet159 */160 public String getServletInfo() {161 return "This is my default servlet created by Eclipse";162 }163 164 </aw:method>
代码模板中,删除doGet和doPost上方的注释和方法里面的代码,并在doPost方法里面添加一行doGet(request,response);效果如下:
修改好之后,保存,重启MyEclipse,就可以使用新的模板代码了:
package com.vae.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class Servlet2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); }}
JavaWeb学习之Servlet(二)----Servlet的生命周期、继承结构、修改Servlet模板