之前没有仔细研究这个问题，学的时候别人说Servlet实例是在web container启动时也即是服务器启动过程就new 出来。某个Servlet实例一旦new 出来，马上就会执行其init(ServletConfig sc)方法。

上面这句话，前半句是错了，后半句是正确的，这个直到今天我才发现。哇，这么久了，现在才发现这个错误，对自己感到一丝悲哀啊。。好，废话不多说，先看我做的实验，然后再分析哈源码。

实验一

准备Servlet：

package com.eabax.web.action.product;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;public class ServletTest extends HttpServlet {	public ServletTest() {		System.out.println("*****************************************************");		System.out.println("new instance of class ServletTest");		System.out.println("*****************************************************");	}		public void init() throws ServletException {		super.init();		System.out.println("*****************************************************");		System.out.println("call Servlet init method");		System.out.println("*****************************************************");	}}

配置web. ：

<servlet>        <servlet-name>test</servlet-name>	<servlet-class>com.eabax.web.action.product.ServletTest</servlet-class></servlet><servlet-mapping>	<servlet-name>test</servlet-name>	<url-pattern>*.php</url-pattern></servlet-mapping>

部署web应用，并启动tomcat server 6.0.36，输出如下：

Jan 12, 2014 11:28:15 AM org.apache.catalina.core.AprLifecycleListener initINFO: The APR  d Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:Program FilesJavajdk1.7.0_45in;E:Installation ProgramsAbout JavaJava toolsTomcat serverapache-tomcat-6.0.36inJan 12, 2014 11:28:16 AM org.apache.coyote.http11.Http11Protocol initINFO: Initializing Coyote HTTP/1.1 on http-8080Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.Catalina loadINFO: Initialization processed in 1069 msJan 12, 2014 11:28:16 AM org.apache.catalina.core.StandardService startINFO: Starting service CatalinaJan 12, 2014 11:28:16 AM org.apache.catalina.core.StandardEngine startINFO: Starting Servlet Engine: Apache Tomcat/6.0.36Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDe orINFO: Deploying configuration de or host-manager. Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDe orINFO: Deploying configuration de or manager. Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory CqutSecondHandTradeSystemlog4j:WARN No appenders could be found for logger (org.spring work.core.CollectionFactory).log4j:WARN Please initialize the log4j system properly.log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.Jan 12, 2014 11:28:18 AM org.apache.catalina.core.ApplicationContext logINFO: Initializing Spring root WebApplicationContextJan 12, 2014 11:28:25 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory docsJan 12, 2014 11:28:25 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory jpa_testJan 12, 2014 11:28:25 AM org.apache.catalina.loader.WebappClassLoader validateJarFileINFO: validateJarFile(E:Installation ProgramsAbout JavaJava toolsTomcat serverapache-tomcat-6.0.36webappsjpa_testWEB-INFlibservlet-api.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.classJan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory ROOTJan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory tagJan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory userJan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory xhEditorJan 12, 2014 11:28:27 AM org.apache.coyote.http11.Http11Protocol startINFO: Starting Coyote HTTP/1.1 on http-8080Jan 12, 2014 11:28:27 AM org.apache.jk.common.ChannelSocket initINFO: JK: ajp13 listening on /0.0.0.0:8009Jan 12, 2014 11:28:27 AM org.apache.jk.server.JkMain startINFO: Jk running ID=0 time=0/44  config=nullJan 12, 2014 11:28:27 AM org.apache.catalina.startup.Catalina startINFO: Server startup in 11329 ms

下面从浏览器中访问地址：http://localhost:8080/jpa_test/aaa.php ，控制台输出如下：

Jan 12, 2014 11:28:27 AM org.apache.catalina.startup.Catalina startINFO: Server startup in 11329 ms*****************************************************new instance of class ServletTest**********************************************************************************************************call Servlet init method*****************************************************

从以上过程中，就可以发现：Servet实例的初始化时刻并非是web container启动的时候，而是第一次需要该Servlet处理请求的时候。

实验二

	<servlet>		<servlet-name>test</servlet-name>		<servlet-class>com.eabax.web.action.product.ServletTest</servlet-class>		<load-on-startup>0</load-on-startup>	</servlet>	<servlet-mapping>		<servlet-name>test</servlet-name>		<url-pattern>*.php</url-pattern>	</servlet-mapping>

Jan 13, 2014 6:12:45 PM org.apache.catalina.core.AprLifecycleListener initINFO: The APR  d Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:Program FilesJavajdk1.7.0_45in;E:Installation ProgramsAbout JavaJava toolsTomcat serverapache-tomcat-6.0.36inJan 13, 2014 6:12:45 PM org.apache.coyote.http11.Http11Protocol initINFO: Initializing Coyote HTTP/1.1 on http-8080Jan 13, 2014 6:12:45 PM org.apache.catalina.startup.Catalina loadINFO: Initialization processed in 960 msJan 13, 2014 6:12:45 PM org.apache.catalina.core.StandardService startINFO: Starting service CatalinaJan 13, 2014 6:12:45 PM org.apache.catalina.core.StandardEngine startINFO: Starting Servlet Engine: Apache Tomcat/6.0.36Jan 13, 2014 6:12:45 PM org.apache.catalina.startup.HostConfig deployDe orINFO: Deploying configuration de or host-manager. Jan 13, 2014 6:12:46 PM org.apache.catalina.startup.HostConfig deployDe orINFO: Deploying configuration de or manager. Jan 13, 2014 6:12:46 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory CqutSecondHandTradeSystemlog4j:WARN No appenders could be found for logger (org.spring work.core.CollectionFactory).log4j:WARN Please initialize the log4j system properly.log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.Jan 13, 2014 6:12:47 PM org.apache.catalina.core.ApplicationContext logINFO: Initializing Spring root WebApplicationContextJan 13, 2014 6:12:54 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory docsJan 13, 2014 6:12:54 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory jpa_testJan 13, 2014 6:12:54 PM org.apache.catalina.loader.WebappClassLoader validateJarFileINFO: validateJarFile(E:Installation ProgramsAbout JavaJava toolsTomcat serverapache-tomcat-6.0.36webappsjpa_testWEB-INFlibservlet-api.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class*****************************************************new instance of class ServletTest**********************************************************************************************************call Servlet init method*****************************************************Jan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory ROOTJan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory tagJan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory userJan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectoryINFO: Deploying web application directory xhEditorJan 13, 2014 6:12:55 PM org.apache.coyote.http11.Http11Protocol startINFO: Starting Coyote HTTP/1.1 on http-8080Jan 13, 2014 6:12:55 PM org.apache.jk.common.ChannelSocket initINFO: JK: ajp13 listening on /0.0.0.0:8009Jan 13, 2014 6:12:55 PM org.apache.jk.server.JkMain startINFO: Jk running ID=0 time=0/22  config=nullJan 13, 2014 6:12:55 PM org.apache.catalina.startup.Catalina startINFO: Server startup in 10284 ms

源码分析