浅谈Listener、Filter、Servlet初始化顺序

Listener、Filter、Servlet都有一个初始化的过程，对应的方法分别为：

contextInitialized(ServletContextEvent arg0); // Listener    init(FilterConfig filterConfig); // Filter    init(ServletConfig config); // Servlet

那么它们的初始化顺序是什么呢？

Listener > Filter > Servlet

TestServlet.java：

package com.king;  import java.io.IOException;  import javax.servlet.GenericServlet;  import javax.servlet.ServletConfig;  import javax.servlet.ServletException;  import javax.servlet.ServletRequest;  import javax.servlet.ServletResponse;    public class TestServlet extends GenericServlet {           @Override          public void init(ServletConfig config){               System.out.println("Servlet 初始化 。。。");           }           @Override          public void service(ServletRequest arg0, ServletResponse arg1) throws ServletException, IOException {               System.out.println("Servlet service 。。。");           }       }

TestFilter.java：

package com.king;       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 TestFilter implements Filter{           public void init(FilterConfig filterConfig) throws ServletException {               System.out.println("Filter 初始化。。。");           }           public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {               System.out.println("doFilter 。。。");               chain.doFilter(request, response);           }           public void destroy() {               System.out.println("Filter 销毁。。。");           }       }

TestListener.java：

package com.king;       import javax.servlet.ServletContextEvent;       import javax.servlet.ServletContextListener;         public class TestListener implements ServletContextListener{           public void contextInitialized(ServletContextEvent arg0) {               System.out.println("Listener 初始化。。。");           }           public void contextDestroyed(ServletContextEvent arg0) {               System.out.println("Listener 销毁。。。");           }       }

web.xml：

<xml version="1.0" encoding="UTF-8"?>      <web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">          <servlet>              <servlet-name>s<servlet-name>              <servlet-class>com.cos.TestServlet<servlet-class>          <servlet>          <servlet-mapping>              <servlet-name>s<servlet-name>              <url-pattern>/login<url-pattern>          <servlet-mapping>          <filter>              <filter-name>f<filter-name>              <filter-class>com.cos.TestFilter<filter-class>          <filter>          <filter-mapping>              <filter-name>f<filter-name>              <url-pattern>/*<url-pattern>          <filter-mapping>          <listener>              <listener-class>com.cos.TestListener<listener-class>          <listener>      <web-app>

启动tomcat后，在控制台里以此打印出：

Listener 初始化。。。
Filter 初始化。。。
信息: Server startup in 675 ms

可以看出Listener的初始化最早，Filter次之。他俩的初始化都是在容器启动完成之前初始化的。Servlet没有初始化，原因是没有匹配的请求进来。初始化的顺序跟Listener、Filter、Servlet在web.xml中的顺序无关。

而多个Filter或多个Servlet的时候，谁的mapping在前面，谁先初始化。