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。