當我們去啟動一個 WEB 項目的時候, 容器(包括 JBoss, Tomcat 等)首先會去讀項目的 web.xml 配置文件里面的信息,
當這一步驟沒有出錯并且完成之后, 項目才能正常的被啟動起來。
1> 首先是, 容器會先讀 <context-param></context-param> 節點, 并創建一個 ServletContext 實例, 以節點的 name 作為鍵, value 作為值,
存儲到上下文環境中。
2> 接著, 容器會去讀 <listener></listener> 節點, 根據配置的 class 類路徑來創建監聽。
3> 接著, 容器去讀 <filter></filter> 節點, 根據指定的類路徑來實例化過濾器。
以上都是在 WEB 項目還沒有完全啟動起來的時候就已經完成了的工作。如果系統中有用到 Servlet, 則 Servlet 是在第一次發起請求的時候被實例化的,
且一般不會被容器銷毀, 它可以服務于多個用戶的請求。所以, Servlet 的初始化都要比上面提到的那幾個要遲。
總的來說, web.xml 的加載順序是: context-param --> listener --> filter --> servlet
其中, 如果 web.xml 中出現了相同的節點, 則是按照在配置文件中出現的先后順序來加載的。
下面引入一個小列子來說明:
<?xml version="1.0" encoding="UTF-8"?>
<listener>
<listener-class>net.yeah.fancydeepin.listener.AppStartListener</listener-class>
</listener>
<!-- 為了更好的說明, 特意將 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>fancydeepin@yeah.net</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() {
}
}
后臺啟動輸出結果:
posted on 2013-03-30 23:30
fancydeepin 閱讀(4744)
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