前言

在Tomcat運行時你總會希望當Tomcat做什麼的時候它可以順便幫你做點什麼，最常見的莫過當ServletContext建立的時候也順便建立DB連線池資訊，讓你的服務一運行就有DB物件可供調用。

0、創建module

請參考Day05創建module

一、Event and Listener

Java EE定義了Event讓container中的物件在不同的生命週期時發布event讓註冊的對應事件的監聽器可以進行執行預定的行為，以下是在官方文件中整理出來的事件與監聽器：

(1) Event

1. javax.servlet.ServletRequestEvent
2. javax.servlet.ServletRequestAttributeEvent
3. javax.servlet.ServletContextEvent
4. javax.servlet.ServletContextAttributeEvent
5. javax.servlet.http.HttpSessionEvent
6. javax.servlet.http.HttpSessionBindingEvent
7. javax.servlet.AsyncEvent

(2) Listener

1. ServletRequestListener
   監聽ServletRequest物件創見與銷毀
2. ServletRequestAttributeListener
   監聽ServletRequest物件添加、刪除或取代Attribute
3. ServletContextListener
   監聽ServletContext物件創見與銷毀
4. ServletContextAttributeListener
   監聽ServletRequest物件添加、刪除或取代Attribute
5. HttpSessionListener
   監聽HttpSession物件創見與銷毀
6. HttpSessionAttributeListener
   監聽ServletRequest物件添加、刪除或取代Attribute
7. HttpSessionBindingListener
   監聽HttpSession物件添加或移除Listener
8. HttpSessionActivationListener
   監聽HttpSession被序列化與反序列化
9. AsyncListener
   監聽非同步Servlet的開始、完成、error、timeout

(3) 配置Listener

web.xml

  <listener>
    <listener-class>com.swj.xml.DemoListener</listener-class>
  </listener>

或是適用annotation

@WebListener

Demo

以下Demo ServletContextListener、ServletContextAttributeListener，其餘使用類似

@WebListener
public class DemoListener implements ServletContextListener, ServletContextAttributeListener {
    //ServletContextListener
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("ServletContext初始化好了");
    }
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("ServletContext銷毀了");
    }

    //ServletContextAttributeListener
    @Override
    public void attributeAdded(ServletContextAttributeEvent event) {
        System.out.println("添加屬性："+ event.getName()+",屬性值："+event.getValue());
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent event) {
        System.out.println("移除屬性："+ event.getName()+",屬性值："+event.getValue());
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent event) {
        System.out.println("取代屬性："+ event.getName()+",屬性值："+event.getValue());
    }
}

DemoServlet

@WebServlet("/DemoServlet")
public class DemoServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        getServletContext().setAttribute("msg", "Hello World1111");
        getServletContext().setAttribute("msg", "Hello World2222");
        getServletContext().removeAttribute("msg");

    }
}

訪問DemoServlet後關閉tomcat

Reference

• Handling Servlet Lifecycle Events - Java EE 6 Tutorial
• Event and Listener in Servlet - javatpoint
• ServletContextListener Servlet Listener Example - Pankaj