java web应用ServletContext及ServletContextListener 学习记录

java官方文档中这样描述ServletContext：

There is one context per "web application" per Java Virtual Machine.

这意味着：每一个web应用在每一个JVM中只有一个 ServletContext  (上下文)。

Several Web Apps on one Server (one JVM):

 

Web App A - Server/JVM 1 - Context 1
Web App B - Server/JVM 1 - Context 2
Web App C - Server/JVM 1 - Context 3

Same Web App on several Servers (several JVMs):

Web App A - Server/JVM 1 - Context 1
Web App B - Server/JVM 1 - Context 2
Web App C - Server/JVM 1 - Context 3
Web App C - Server/JVM 2 - Context 4

当一个应用被部署(deploying)时,ServletContext将会被创建。一旦ServletContext 被创建，它可以被同一个应用下的所有其他servlets和jsp文件共享。在web应用中，ServletContext 也被当做应用级别的变量（ called as the application scope variables）。

ServletContextListener 有以下两个方法:

public void contextInitialized(ServletContextEvent event)
public void contextDestroyed(ServletContextEvent event)

以下例子创建一个继承ServletContextListener 的类MyAppContextListener ，它实现了ServletContextListener 中的两个方法。

 

package com.javaer.tools.config;
import java.io.File;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyAppContextListener implements ServletContextListener {

    // NOTE: static references are not a great idea, shown here for simplicity
    public static File xsltFolder;
    public static File xmlIndentXsltFile;
    ServletContext context;

    public void contextInitialized(ServletContextEvent e) {
        xsltFolder = new File(e.getServletContext().getInitParameter("xsltFolder"));
        xmlIndentXsltFile = new File(xsltFolder, "xml-indent.xslt");

        context = e.getServletContext();
        // set variable to servlet context
        context.setAttribute("TEST", "TEST_VALUE");

        System.out.println("xsltFolder:"+xsltFolder);//xsltFolder:\WEB-INF\xslt
        System.out.println("TEST:"+context.getAttribute("TEST"));//TEST:TEST_VALUE
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        //当web服务器shut down或者crash时执行
    }
}

上面的程序读取了web.xml文档中的以下内容：

 

<context-param>
        <param-name>xsltFolder</param-name>
               <param-value>/WEB-INF/xslt</param-value>
</context-param>

为了让应用部署时加载这个类，我们需要在web.xml中添加以下内容：

 

<listener>
        <listener-class>com.javaer.tools.config.MyAppContextListener</listener-class>
</listener>

完整的web.xml如下(与本例无关内容已经注释)：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="" xmlns="" xmlns:web="/web-app_2_5.xsd" xsi:schemaLocation=" /web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>javer</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-list>
      <!--
      <servlet>
        <servlet-name>mvc-dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>mvc-dispatcher</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/servlet-context.xml</param-value>
    </context-param>
      -->
    <context-param>
        <param-name>xsltFolder</param-name>
               <param-value>/WEB-INF/xslt</param-value>
    </context-param>
    <!--
        <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
        -->
    <listener>
        <listener-class>com.javaer.tools.config.MyAppContextListener</listener-class>
    </listener>
</web-app>

当应用deploy时就会输出：

 

xsltFolder:\WEB-INF\xslt
TEST:TEST_VALUE

参考：