Servlet实例创建时刻
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.xml：
<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 init INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:\Program Files\Java\jdk1.7.0_45\bin;E:\Installation Programs\About Java\Java tools\Tomcat server\apache-tomcat-6.0.36\bin Jan 12, 2014 11:28:16 AM org.apache.coyote.http11.Http11Protocol init INFO: Initializing Coyote HTTP/1.1 on http-8080 Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.Catalina load INFO: Initialization processed in 1069 ms Jan 12, 2014 11:28:16 AM org.apache.catalina.core.StandardService start INFO: Starting service Catalina Jan 12, 2014 11:28:16 AM org.apache.catalina.core.StandardEngine start INFO: Starting Servlet Engine: Apache Tomcat/6.0.36 Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDescriptor INFO: Deploying configuration descriptor host-manager.xml Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDescriptor INFO: Deploying configuration descriptor manager.xml Jan 12, 2014 11:28:16 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory CqutSecondHandTradeSystem log4j:WARN No appenders could be found for logger (org.springframework.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 log INFO: Initializing Spring root WebApplicationContext Jan 12, 2014 11:28:25 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory docs Jan 12, 2014 11:28:25 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory jpa_test Jan 12, 2014 11:28:25 AM org.apache.catalina.loader.WebappClassLoader validateJarFile INFO: validateJarFile(E:\Installation Programs\About Java\Java tools\Tomcat server\apache-tomcat-6.0.36\webapps\jpa_test\WEB-INF\lib\servlet-api.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class Jan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory ROOT Jan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory tag Jan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory user Jan 12, 2014 11:28:26 AM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory xhEditor Jan 12, 2014 11:28:27 AM org.apache.coyote.http11.Http11Protocol start INFO: Starting Coyote HTTP/1.1 on http-8080 Jan 12, 2014 11:28:27 AM org.apache.jk.common.ChannelSocket init INFO: JK: ajp13 listening on /0.0.0.0:8009 Jan 12, 2014 11:28:27 AM org.apache.jk.server.JkMain start INFO: Jk running ID=0 time=0/44 config=null Jan 12, 2014 11:28:27 AM org.apache.catalina.startup.Catalina start INFO: Server startup in 11329 ms从Tomcat的启动日志信息中可以发现，并没有输出类ServletTest的的初始化代码，也就是Tomcat启动过程中并没有new 出Servlet的实例。
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 start INFO: 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
配置web.xml：
与实验一唯一不同的地方就是这次实验在web.xml文件中register servlet的时侯，多加了一个load-on-startup参数。如下：
<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>
部署web应用，并启动tomcat server 6.0.36，输出如下：
Jan 13, 2014 6:12:45 PM org.apache.catalina.core.AprLifecycleListener init INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:\Program Files\Java\jdk1.7.0_45\bin;E:\Installation Programs\About Java\Java tools\Tomcat server\apache-tomcat-6.0.36\bin Jan 13, 2014 6:12:45 PM org.apache.coyote.http11.Http11Protocol init INFO: Initializing Coyote HTTP/1.1 on http-8080 Jan 13, 2014 6:12:45 PM org.apache.catalina.startup.Catalina load INFO: Initialization processed in 960 ms Jan 13, 2014 6:12:45 PM org.apache.catalina.core.StandardService start INFO: Starting service Catalina Jan 13, 2014 6:12:45 PM org.apache.catalina.core.StandardEngine start INFO: Starting Servlet Engine: Apache Tomcat/6.0.36 Jan 13, 2014 6:12:45 PM org.apache.catalina.startup.HostConfig deployDescriptor INFO: Deploying configuration descriptor host-manager.xml Jan 13, 2014 6:12:46 PM org.apache.catalina.startup.HostConfig deployDescriptor INFO: Deploying configuration descriptor manager.xml Jan 13, 2014 6:12:46 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory CqutSecondHandTradeSystem log4j:WARN No appenders could be found for logger (org.springframework.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 log INFO: Initializing Spring root WebApplicationContext Jan 13, 2014 6:12:54 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory docs Jan 13, 2014 6:12:54 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory jpa_test Jan 13, 2014 6:12:54 PM org.apache.catalina.loader.WebappClassLoader validateJarFile INFO: validateJarFile(E:\Installation Programs\About Java\Java tools\Tomcat server\apache-tomcat-6.0.36\webapps\jpa_test\WEB-INF\lib\servlet-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 deployDirectory INFO: Deploying web application directory ROOT Jan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory tag Jan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory user Jan 13, 2014 6:12:55 PM org.apache.catalina.startup.HostConfig deployDirectory INFO: Deploying web application directory xhEditor Jan 13, 2014 6:12:55 PM org.apache.coyote.http11.Http11Protocol start INFO: Starting Coyote HTTP/1.1 on http-8080 Jan 13, 2014 6:12:55 PM org.apache.jk.common.ChannelSocket init INFO: JK: ajp13 listening on /0.0.0.0:8009 Jan 13, 2014 6:12:55 PM org.apache.jk.server.JkMain start INFO: Jk running ID=0 time=0/22 config=null Jan 13, 2014 6:12:55 PM org.apache.catalina.startup.Catalina start INFO: Server startup in 10284 ms
从以上输出信息可见，这次tomcat在启动过程中new 出了Servlet的实例。通过上面两个实验也可以知道在register Servlet的时候，加了load-on-startup和没有加这个参数的区别了。
源码分析
做到这里，才发现分析源码就得分析Tomcat的源码，这超出了我之前的预料。于是我把Tomcat的jar包也导入工程，并attach source，试图分析源码。但是其真的很复杂，还不能做到从tomcat入口开始分析，一步一步分析程序的执行过程。所以萌生了调试tomcat的想法。于是经过查找资料，整理出了在myeclipse2013种调试tomcat的步骤。有兴趣的coder可以参考另一篇博文： MyEclipse中调试Tomcat，欢迎吐槽。