之前沒有仔細研究這個問題，學的時候别人說Servlet執行個體是在web container啟動時也即是伺服器啟動過程就new 出來。某個Servlet執行個體一旦new 出來，馬上就會執行其init(ServletConfig sc)方法。

上面這句話，前半句是錯了，後半句是正确的，這個直到今天我才發現。哇，這麼久了，現在才發現這個錯誤，對自己感到一絲悲哀啊。。好，廢話不多說，先看我做的實驗，然後再分析哈源碼。

實驗一

準備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應用，并啟動tomcat server 6.0.36，輸出如下：

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

從Tomcat的啟動日志資訊中可以發現，并沒有輸出類ServletTest的的初始化代碼，也就是Tomcat啟動過程中并沒有new 出Servlet的執行個體。

下面從浏覽器中通路位址：

http://localhost:8080/jpa_test/aaa.php

，控制台輸出如下：

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
*****************************************************

從以上過程中，就可以發現：Servet執行個體的初始化時刻并非是web container啟動的時候，而是第一次需要該Servlet處理請求的時候。

實驗二

與實驗一唯一不同的地方就是這次實驗在web.xml檔案中register servlet的時侯，多加了一個load-on-startup參數。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>

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

從以上輸出資訊可見，這次tomcat在啟動過程中new 出了Servlet的執行個體。通過上面兩個實驗也可以知道在register Servlet的時候，加了load-on-startup和沒有加這個參數的差別了。

源碼分析

做到這裡，才發現分析源碼就得分析Tomcat的源碼，這超出了我之前的預料。于是我把Tomcat的jar包也導入工程，并attach source，試圖分析源碼。但是其真的很複雜，還不能做到從tomcat入口開始分析，一步一步分析程式的執行過程。是以萌生了調試tomcat的想法。于是經過查找資料，整理出了在myeclipse2013種調試tomcat的步驟。有興趣的coder可以參考另一篇博文：

MyEclipse中調試Tomcat

，歡迎吐槽。

Servlet執行個體建立時刻實驗一實驗二源碼分析

實驗一

準備Servlet：

配置web.xml：

部署web應用，并啟動tomcat server 6.0.36，輸出如下：

實驗二

源碼分析

繼續閱讀

nginx location中斜線的位置的重要性

Apache2.4.x 配置檔案詳解Apache配置需要了解如下：開始講解：

配置apache支援PHP（win7）

27 Best Free Eclipse Plug-ins for Java Developer to be ProductiveCode Quality PluginsText Editor PluginsDependency ManagementVersion Control Integration PluginsFramework Development Continuous Integration Related PluginsOther Utility Plugins

Java String.format方法的簡單使用

neo4j之cypher使用文檔

GitHub連夜封殺！這份阿裡 10W 字内部 Java 字面試手冊到底有多強？

spark/scala關于【資源檔案】加載方法概述外部檔案加載方案測試資源檔案打包入jar包中小結

mybatis_入門程式Mybatis入門

AOP程式設計_Android優雅權限架構(1)概念基礎，2021金三銀四前言正文大綱正文

Effective Java 8:通用程式設計

OOM三種類型

工廠模式-三種類型

【遞歸】高效率求2的n次幂

win10本地scala和spark安裝安裝scala安裝spark

scala (3) Function 和 Method