Listener、Filter、Servlet与Java Web项目初始化的工作

通常JavaWeb项目启动时我们需要做一些初始化工作，比如配置缓存、定时任务等等。举个例子：我在国内某公司曾经负责维护公司内部的全球通信系统的服务端，公司员工在10W人以上，需要初始化的内容就是将这10W多的用户信息，在系统启动时就初始化到缓存中，用户通过客户端登陆，像服务端发出请求后，直接从缓存里面验证用户的信息，这样做的好处是减轻服务端和数据库的压力；还有一个好处就是，当员工从客户端登陆时，系统需要经过一系列的鉴权，过程非常复杂，而处理鉴权的过程很耗费时间，这样就非常影响员工使用客户端的体验，在服务器端配置缓存，能很好的解决这些麻烦。
下面代码是模拟初始化的一个示例，可以在控制台看到程序的输出，和Listener、Filter、Servlet的启动先后顺序，强烈建议跟我一样喜欢动手的Coder操作一下，废话不多说，直接上代码了。
[java] package com.tom.servlet;

import java.util.Timer;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class HelloListener implements ServletContextListener {

    public HelloListener() {
        System.out.println("Listener() ----------");
    }

    public void contextDestroyed(ServletContextEvent arg0) {
        System.out.println("Listener contextDestroyed()-----------------");
    }

    public void contextInitialized(ServletContextEvent arg0) {
        System.out.println("Listener contextInitialized()-----------------");
        Timer timer = new Timer();
        timer.schedule(new TimerTest02(), 0, 4000L);
    }

}

[java]
package com.tom.servlet;
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 HelloFilter implements Filter {

    private Long seconds;

    public HelloFilter() {
        System.out.println("+++++ Filter() +++++");
    }

    public void init(FilterConfig config) throws ServletException {
        System.out.println("+++++ FilterInit() +++++");
        this.seconds = Long.parseLong(config.getInitParameter("seconds"));
        TimerTest.initTimer(seconds);
    }

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        System.out.println("+++++ doFilter() +++++");
    }

    public void destroy() {
        this.seconds = null;
    }

    public Long getSeconds() {
        return seconds;
    }

}

[java]
package com.tom.servlet;

import java.io.IOException;
import java.util.Timer;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloServlet extends HttpServlet {

    private static final long serialVersionUID = -7693562670980406523L;

    public HelloServlet() {
        System.out.println("----- Servlet() -----");
    }

    public void init() throws ServletException {
        System.out.println("----- ServletInit() -----");
        Timer timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTest01(), 0, 6000L);
    }

    public void destroy() {
        System.out.println("----- ServletDestroy() -----");
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
    }

}

[java]
package com.tom.servlet;

import java.util.TimerTask;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class TimerTest {

    private static int i = 0;

    public static void initTimer(Long seconds) {
        TimerTest.schedule(new TimerTask() {
            public void run() {
                callUsers();
            }
        }, 0, seconds*1000);

    }
    public static void callUsers() {
        System.out.println("Filter refreshed: " + ++i + " times.");
    }

    public static void schedule(TimerTask task, long delay, long period) {
        Executors.newScheduledThreadPool(1).scheduleAtFixedRate(task, delay, period,
                TimeUnit.MILLISECONDS);
    }

}

class TimerTest01 extends TimerTask {
    private static int i = 0;

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补充：Web开发 , Jsp ,