web.xml 的加载过程

当我们去启动一个 WEB 项目的时候, 容器(包括 JBoss, Tomcat 等)首先会去读项目的 web.xml 配置文件里面的信息,

当这一步骤没有出错并且完成之后, 项目才能正常的被启动起来。

1> 首先是, 容器会先读 <context-param></context-param> 节点, 并创建一个 ServletContext 实例, 以节点的 name 作为键, value 作为值,

   存储到上下文环境中。

2> 接着, 容器会去读 <listener></listener> 节点, 根据配置的 class 类路径来创建监听。

3> 接着, 容器去读 <filter></filter> 节点, 根据指定的类路径来实例化过滤器。

以上都是在 WEB 项目还没有完全启动起来的时候就已经完成了的工作。如果系统中有用到 Servlet, 则 Servlet 是在第一次发起请求的时候被实例化的,

且一般不会被容器销毁, 它可以服务于多个用户的请求。所以, Servlet 的初始化都要比上面提到的那几个要迟。

总的来说, web.xml 的加载顺序是: context-param --> listener --> filter --> servlet

其中, 如果 web.xml 中出现了相同的节点, 则是按照在配置文件中出现的先后顺序来加载的。

下面引入一个小列子来说明:

<?xml version="1.0" encoding="UTF-8"?>
  
  <listener>
    <listener-class>net.yeah.fancydeepin.listener.AppStartListener</listener-class>
  </listener>
  
  <!-- 为了更好的说明, 特意将 context-param 放在 listener 后面 -->
  <context-param>
    <param-name>technology</param-name>
    <param-value>java,javascript,ajax,css,html</param-value>
  </context-param>
  
  <filter>
    <filter-name>ReDespatcherFilter</filter-name>
    <filter-class>net.yeah.fancydeepin.filter.ReDespatcherFilter</filter-class>
    <init-param>
      <param-name>it</param-name>
      <param-value>android, python, c</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>ReDespatcherFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  
  <filter>
    <filter-name>ReDespatcherFilter2</filter-name>
    <filter-class>net.yeah.fancydeepin.filter.ReDespatcherFilter2</filter-class>
    <init-param>
      <param-name>mail</param-name>
      <param-value>[e-mail]</param-value>
    </init-param>
  </filter>

</web-app>

package net.yeah.fancydeepin.listener;

import java.util.Arrays;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class AppStartListener implements ServletContextListener{

    public void contextInitialized(ServletContextEvent contextEvent) {
        System.out.println("********************************************");
        ServletContext context = contextEvent.getServletContext();
        List<String> params = Arrays.asList(context.getInitParameter("technology").split(","));
        for(String param : params){
            System.out.print(param + "\t");
        }
        System.out.println("\n********************************************");
    }

    public void contextDestroyed(ServletContextEvent contextEvent) {
        
    }

}

package net.yeah.fancydeepin.filter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class ReDespatcherFilter implements Filter {
    
    public void init(FilterConfig filterConfig) throws ServletException {

        System.out.println("============================================");
        System.out.println(filterConfig.getInitParameter("it"));
        System.out.println("============================================");
    }
    
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,ServletException{
        
        chain.doFilter(request, response);
    }
    
    public void destroy() {
        
    }
}

package net.yeah.fancydeepin.filter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class ReDespatcherFilter2 implements Filter {
    
    public void init(FilterConfig filterConfig) throws ServletException {

        System.out.println("++++++++++++++++++++++++++++++++++++++++++++");
        System.out.println(filterConfig.getInitParameter("mail"));
        System.out.println("++++++++++++++++++++++++++++++++++++++++++++");
    }
    
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,ServletException{
        
        chain.doFilter(request, response);
    }
    
    public void destroy() {
        
    }
}

后台启动输出结果:


转自：http://www.blogjava.net/fancydeepin