学习笔记 ：Java Servlet 的生命周期及其解析

  • Tomcat v9.0 + JDK 11 + Google Chrome

  • Servlet lifecycle

Servlet 2.5 示例

  • JSP program

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    <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
    <!DOCTYPE html>
    <html>
    	<head>
    		<meta charset="ISO-8859-1">
    		<title>Servlet 2.5</title>
    	</head>
    	<body>
    		<a href="ServletLifecyle">doGet : Servlet 3.0</a>
    		<!-- 注意: form的默认提交方式为get -->
    		<div>
    			<form action="ServletLifecyle" method="get"><hr>
    				<input type="submit" value="doGet " />
    			</form>
    			<form action="ServletLifecyle" method="post"><br>
    				<input type="submit" value="doPost" />
    			</form>
    		</div>
    	</body>
    </html>

  • Servlet program

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    public class ServletLifecyle extends HttpServlet {
    	
    	
    	@Override
    	public void init() throws ServletException {
    		// TODO Auto-generated method stub
    		//super.init();
    		System.out.println("(Servlet 2.5)init() ..");
    	}
    	
    	@Override
    	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		//super.doGet(req, resp);
    		System.out.println("service() -> doGet() + doPost() ..");
    	}
    	
    	@Override
    	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		//super.doPost(req, resp);
    		this.doGet(req, resp);
    	}
    	
    	@Override
    	public void destroy() {
    		// TODO Auto-generated method stub
    		//super.destroy();
    		System.out.println("destory() ..");
    	}
    }

    /*

    ### 1-第一次点击请求连接时Servlet响应的结果如下 :
    Apr 05, 2019 10:27:32 PM org.apache.catalina.core.StandardContext reload
    INFO: Reloading Context with name [/Servlet_basic_lifecycle] is completed
    (Servlet 2.5)init() ..                  <—— the result
    service() -> doGet() + doPost() ..      <—— the result


    ### 2-当返回再次点击链接请求时Servlet响应的结果如下 : 
    Apr 05, 2019 10:31:05 PM org.apache.catalina.core.StandardContext reload
    INFO: Reloading Context with name [/Servlet_basic_lifecycle] is completed
    (Servlet 2.5)init() ..					<— 第一次点击链接时Servlet的相应结果
    service() -> doGet() + doPost() ..		<— 第一次点击链接时Servlet的相应结果
    service() -> doGet() + doPost() ..		<— 第二次点击链接时Servlet的相应结果
    service() -> doGet() + doPost() ..		<— 第三次点击链接时Servlet的相应结果
    service() -> doGet() + doPost() ..		<— 第四次点击链接时Servlet的相应结果
    ...


    ### 3-上述来回来点击链接请求后且稍许时间后会发现Servlet自动调用了destory() !!!?
    Apr 05, 2019 10:31:05 PM org.apache.catalina.core.StandardContext reload
    INFO: Reloading Context with name [/Servlet_basic_lifecycle] is completed
    (Servlet 2.5)init() ..
    service() -> doGet() + doPost() ..
    service() -> doGet() + doPost() ..
    service() -> doGet() + doPost() ..
    service() -> doGet() + doPost() ..
    Apr 05, 2019 10:31:41 PM org.apache.catalina.core.StandardContext reload
    INFO: Reloading Context with name [/Servlet_basic_lifecycle] has started
    destory() ..


    ### 由上述1,2,3可得如下结论 : 
    init() :
        默认情况下第一次访问Servlet时会被执行( 只执行一次 ),扩 : 也可以通过配置web.xml文件使得Tomcat启动时自动执行.(详见下述`web.xml`示例)
    service() : 
        包括doGet()和doPost(),执行的次数与调用次数成正比.
    destory() : 
        关闭Tomcat服务时执行一次.

    */

  • web.xml

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<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>Servlet_basic_lifecycle</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  
  <!-- Servlet 2.5 -->
  <servlet>
		<servlet-name>sameName</servlet-name>
		<servlet-class>pers.huangyuhui.servlet.test.ServletLifecyle</servlet-class>
		<!-- 使得tomcat启动时自动加载init()方法,其数字代表: 当有多个Servlet时,规划其启动的顺序,默认值为`-1`. -->
		<load-on-startup>1</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>sameName</servlet-name>
		<url-pattern>/ServletLifecyle</url-pattern>
	</servlet-mapping>
</web-app>
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通过在`web.xml`配置文件中添加'<load-on-startup>1</load-on-startup>'使得Tomcat启动时自动加载init() !

INFO: 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 2.5)init() ..	<——— the result.
Apr 06, 2019 2:31:09 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-nio-8080"]
Apr 06, 2019 2:31:09 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["ajp-nio-8009"]
Apr 06, 2019 2:31:09 PM org.apache.catalina.startup.Catalina start
INFO: Server startup in [2,686] milliseconds

Servlet 3.0 示例

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/*
Servlet 3.0 较2.5可以通过注解使Tomcat启动时自动加载Servlet 中的初始化方法: init().

@WebServlet(value="/ServletTest" , loadOnStartup=1)
value : 其为一个字符串数组对象,用于接收多个Servlet,当只有一个Servlet时默认可以不写.
loadOnStartup : 当有多个Servlet时将按照数字大小顺序执行.其默认为-1.
 */

@WebServlet(value="/ServletTest" , loadOnStartup=1)
public class ServletTest extends HttpServlet {
	private static final long serialVersionUID = 1L;

    ······

总结深究 ( 待解决的问题 )

发现学习中我并没有关闭Tomcat服务器,但自动执行了销毁方法destory() ! 于是查了一些资料,至今还未发现其答案,我认为 : 应该是Tomcat新版本的特性吧(超时自动销毁Servlet?),但是自动调用Destory()时与Service()时间差也太小了吧 ! 36 秒啊 !

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### 3-上述来回来点击链接请求后且稍许时间后会发现Servlet自动调用了destory() !!!?
Apr 05, 2019 10:31:05 PM org.apache.catalina.core.StandardContext reload
INFO: Reloading Context with name [/Servlet_basic_lifecycle] is completed
(Servlet 2.5)init() ..
service() -> doGet() + doPost() ..       <—— the result
service() -> doGet() + doPost() ..       <—— the result
service() -> doGet() + doPost() ..       <—— the result
service() -> doGet() + doPost() ..       <—— the result
Apr 05, 2019 10:31:41 PM org.apache.catalina.core.StandardContext reload
INFO: Reloading Context with name [/Servlet_basic_lifecycle] has started
destory() ..    <—— the result ? what happend ! ? (I'm not close the the Tomcat server now)

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  • 注意 : <—— 是本人后期为便于大家审阅代码加上的.
  • 后期在不断学习中会完善这些我未知的问题的 ! Stay hungry,Stay foolish.