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;
@Override
public void run() {
System.out.println("Servlet refreshed: " + ++i + " times.");
}
}
class TimerTest02 extends TimerTask {
private static int i = 0;
@Override
public void run() {
System.out.println("Listener refreshed: " + ++i + " times.");
}
}
[html]
<?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">
<filter>
<filter-name>HelloFilter</filter-name>
<filter-class>com.tom.servlet.HelloFilter</filter-class>
<init-param>
<param-name>seconds</param-name>
<param-value>5</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>HelloFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<servlet>
<servlet-name>HelloServlet</servlet-name>
<servlet-class>com.tom.servlet.HelloServlet</servlet-class>
<load-on-startup>0</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/HelloServlet</url-pattern>
</servlet-mapping>
<listener>
<listener-class>com.tom.servlet.HelloListener</listener-class>
</listener>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>
以上代码是纯web项目下的示例，不依赖当下流行的任何开发框架；当然项目初始化还有一些其他的解决方案，比如在Struts1.X下的ActionServlet、Struts2.X下的FilterDispatcher，都可以解决，本例不再赘述，喜欢的可以去研究一下。
本例中代码供大家参考，欢迎交流并提出批评指正！
为了方便大家在IDE中阅读，特意将示例项目打包，下载地址：http://www.2cto.com/uploadfile/2012/0424/20120424015209986.zip
摘自 TOM4J
