Servlet生命周期详解

概述

 servlet的生命周期：servlet在容器中开始实例化到实例销毁的整个过程 

servlet生命周期的四个过程

1、实例化servlet对象

我们的OOP(面向对象)思想中，总是先创建对象，通过对象调用成员，那么servlet如何实例化呢？

• 第一种方式：通过配置文件web.xml进行实例化

public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } } 

 
    <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">  
    <servlet> <servlet-name>testServlet 
     servlet-name> <servlet-class>org.softeem.controller.ActionServlet 
      servlet-class>  
       servlet> <servlet-mapping> <servlet-name>testServlet 
        servlet-name> <url-pattern>/friend_demo/action 
         url-pattern>  
          servlet-mapping>  
           web-app> 

• 第二种方式：注解

//注解方式，效果等同于web.xml中的配置 @WebServlet("/friend_demo/action") public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } } 

• 通过以上两种方式中的一种，此时运行tomcat会出现如下结果

23-Jan-2021 21:52:28.552 信息 [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["ajp-nio-8009"] 23-Jan-2021 21:52:28.554 信息 [main] org.apache.catalina.startup.Catalina.start Server startup in 39 ms Connected to server [2021-01-23 09:52:29,482] Artifact ZhenaiDemo:war exploded: Artifact is being deployed, please wait... 23-Jan-2021 21:52:29.779 警告 [RMI TCP Connection(3)-127.0.0.1] org.apache.tomcat.util.descriptor.web.WebXml.setVersion Unknown version string [4.0]. Default version will be used. 23-Jan-2021 21:52:30.540 信息 [RMI TCP Connection(3)-127.0.0.1] org.apache.jasper.servlet.TldScanner.scanJars At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. servlet对象实例化中..... 

• 通过上面的运行结果可以发现，确实调用了ActionServlet 类中的无参构造器。但是，出现该结果是在我们在浏览器客户端中输入了当前servlet对应的 /friend_demo/action 请求后才在控制台中输出 “servlet对象实例化中…”，由此可以发现只有当我们在使用对应的servlet时才会进行对象的创建。有时我们需要某个servlet随着容器加载时就创建servlet对象，而不是在使用时才创建。这是就可以通过设定servlet对应的优先级来决定servlet实例化的时机

设置对应servlet的优先级(loadOnStartup)

设定servlet优先级可以使用loadOnStartup属性，对应的属性值越小优先级别越高。当我们需要servlet与tomcat容器一起加载时而被实例化，即将loadOnStartup属性值设定为1，设定的方式有两种

• 配置文件方式

 
    <servlet> <servlet-name>testServlet 
     servlet-name> <servlet-class>org.softeem.controller.ActionServlet 
      servlet-class>  
      <load-on-startup>1 
       load-on-startup>  
        servlet> <servlet-mapping> <servlet-name>testServlet 
         servlet-name> <url-pattern>/friend_demo/action 
          url-pattern>  
           servlet-mapping> 

• 注解方式
  注意：若同时设定请求协议与优先级时，需要将协议赋值给value属性，且中间使用逗号隔开,如下
  
  

@WebServlet(value="/friend_demo/action",loadOnStartup = 1) public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } } 

2、初始化参数配置

servlet实例化后就会调用init方法，被设计为只能调用一次，且是在进行第一次实例化servlet对象时调用。当用户在调用servlet时，就会创建一个servlet实例，每一次在客户端发送的请求servlet就会产生一个新的线程，从而调用对应的doGet 或 doPost 方法。init方法的作用在于初始化一些配置参数(比如编码设置)，并写这些参数将被贯穿整个servlet生命周期。配置参数的参数有两种，如下

• 通过配置文件web.xml方式配置

 
    <servlet> <servlet-name>testServlet 
     servlet-name> <servlet-class>org.softeem.controller.ActionServlet 
      servlet-class>  
      <load-on-startup>1 
       load-on-startup>  
        
       <init-param> <param-name>encoding 
        param-name> <param-value>utf-8 
         param-value>  
          init-param>  
          <init-param> <param-name>userName 
           param-name> <param-value>上进的小仓鼠 
            param-value>  
             init-param>  
              servlet> <servlet-mapping> <servlet-name>testServlet 
               servlet-name> <url-pattern>/friend_demo/action 
                url-pattern>  
                 servlet-mapping> 

@WebServlet(value="/friend_demo/action",loadOnStartup = 1) public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } //2、初始化 @Override public void init(ServletConfig config) throws ServletException { 
    System.out.println("servlet初始化中......"); //获取配置的参数值 String encoding = config.getInitParameter("encoding"); String userName = config.getInitParameter("userName"); System.out.println(encoding + "," + userName); } 

• 通过注解方式配

@WebServlet(value="/friend_demo/action",loadOnStartup = 1, initParams = { 
    @WebInitParam(name="encoding",value = "utf-8"), @WebInitParam(name="userName",value = "上进的小仓鼠") } ) public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } //2、初始化 @Override public void init(ServletConfig config) throws ServletException { 
    System.out.println("servlet初始化中......"); //获取配置的参数值 String encoding = config.getInitParameter("encoding"); String userName = config.getInitParameter("userName"); System.out.println(encoding + "," + userName); } } 

配置完参数运行结果如下：

23-Jan-2021 22:42:05.584 信息 [main] org.apache.catalina.startup.Catalina.start Server startup in 29 ms Connected to server [2021-01-23 10:42:05,899] Artifact ZhenaiDemo:war exploded: Artifact is being deployed, please wait... 23-Jan-2021 22:42:06.062 警告 [RMI TCP Connection(3)-127.0.0.1] org.apache.tomcat.util.descriptor.web.WebXml.setVersion Unknown version string [4.0]. Default version will be used. 23-Jan-2021 22:42:06.657 信息 [RMI TCP Connection(3)-127.0.0.1] org.apache.jasper.servlet.TldScanner.scanJars At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. servlet对象实例化中..... servlet初始化中...... utf-8,上进的小仓鼠 

3、就绪状态

• 案例代码如下

@WebServlet(value="/friend_demo/action",loadOnStartup = 1, initParams = { 
    @WebInitParam(name="encoding",value = "utf-8"), @WebInitParam(name="userName",value = "上进的小仓鼠") } ) public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } //2、初始化 @Override public void init(ServletConfig config) throws ServletException { 
    System.out.println("servlet初始化中......"); //获取配置的参数值 String encoding = config.getInitParameter("encoding"); String userName = config.getInitParameter("userName"); System.out.println(encoding + "," + userName); } //3、就绪状态 @Override public void service(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { 
    System.out.println("servlet:service()就绪中..."); } } 

程序运行结果如下

23-Jan-2021 22:42:15.575 信息 [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory D:\apache\apache-tomcat-8.0.45\webapps\manager 23-Jan-2021 22:42:15.594 信息 [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory D:\apache\apache-tomcat-8.0.45\webapps\manager has finished in 18 ms [2021-01-23 10:54:32,996] Artifact ZhenaiDemo:war exploded: Artifact is being deployed, please wait... servlet实例销毁中.... servlet对象实例化中..... servlet初始化中...... utf-8,小仓鼠 [2021-01-23 10:54:34,124] Artifact ZhenaiDemo:war exploded: Artifact is deployed successfully [2021-01-23 10:54:34,124] Artifact ZhenaiDemo:war exploded: Deploy took 1,128 milliseconds servlet:service()就绪中... 

4、销毁状态

servlet生命周期结束时将会调用destroy()方法，并且只会调用一次。destroy方法可以让我们的servlet关闭数据库连接，把后台线程关闭，清楚Cookie数据等。当调用了destroy()方法后，当前servlet实例将会被标记为回收垃圾，会对servlet实例进行清除，案例代码如下

@WebServlet(value="/friend_demo/action",loadOnStartup = 1, initParams = { 
    @WebInitParam(name="encoding",value = "utf-8"), @WebInitParam(name="userName",value = "上进小仓鼠") } ) public class ActionServlet extends HttpServlet { 
    //1、构造器 public ActionServlet(){ 
    System.out.println("servlet对象实例化中....."); } //2、初始化 @Override public void init(ServletConfig config) throws ServletException { 
    System.out.println("servlet初始化中......"); //获取配置的参数值 String encoding = config.getInitParameter("encoding"); String userName= config.getInitParameter("userName"); System.out.println(encoding + "," + userName); } //3、就绪状态 @Override public void service(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { 
    System.out.println("servlet:service()就绪中..."); } //4、销毁状态 @Override public void destroy() { 
    System.out.println("servlet实例销毁中...."); } } 

执行程序后的运行效果

[2021-01-23 10:54:32,996] Artifact ZhenaiDemo:war exploded: Artifact is being deployed, please wait... servlet实例销毁中.... servlet对象实例化中..... servlet初始化中...... utf-8,小仓鼠 [2021-01-23 10:54:34,124] Artifact ZhenaiDemo:war exploded: Artifact is deployed successfully [2021-01-23 10:54:34,124] Artifact ZhenaiDemo:war exploded: Deploy took 1,128 milliseconds servlet:service()就绪中... [2021-01-23 11:06:03,372] Artifact ZhenaiDemo:war exploded: Artifact is being deployed, please wait... servlet实例销毁中.... 

servlet生命周期总结

• 实例化：servlet容器创建servlet对象。默认创建servlet实例的时机：当我们发送servlet对应的请求时(在使用时创建)。类似单例模式中的懒加载方式。希望容器一旦启动，就自动创建servlet实例通过load-on-startup=1设置，正数数值越低优先级别越高，优先实例化
• 初始化：servlet实例一旦创建，就开始初始化一些参数配置，我们可以做一些参数配置，比如编码，可以在web.xml或注解中配置
• 就绪状态：当发送对应的servlet请求时，会调用service()方法，注意此时不会重新创建servlet实例，也不会调用init()方法
• 销毁状态：调用了destroy()方法后，当前servlet实例将会被标记为回收垃圾，会对servlet实例进行清除处理