JavaWeb 監聽器實戰 (一)：監聽 ServletContext 與 HttpSession 生命週期

監聽器類型對照

1. 監聽 ServletContext (應用層)

public final class MyServletContextListener
    implements ServletContextListener, ServletContextAttributeListener {

    private ServletContext context = null;

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        this.context = sce.getServletContext();
        log("ServletContext 初始化");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        log("ServletContext 銷毀");
        this.context = null;
    }

    @Override
    public void attributeAdded(ServletContextAttributeEvent scae) {
        log("Context 新增屬性: " + scae.getName() + "=" + scae.getValue());
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent scae) {
        log("Context 移除屬性: " + scae.getName());
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent scae) {
        log("Context 替換屬性: " + scae.getName() + "=" + scae.getValue());
    }

    private void log(String msg) {
        try (PrintWriter out = new PrintWriter(new FileOutputStream("E:\\contextLog.txt", true))) {
            out.println(new Date() + " --> " + msg);
        } catch (Exception e) { e.printStackTrace(); }
    }
}

2. 監聽 HttpSession (會話層)

public final class MySessionListener
    implements HttpSessionListener, HttpSessionAttributeListener {

    private ServletContext context;
    private int userCount = 0;

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        userCount++;
        log("Session 建立: " + se.getSession().getId() + "，當前用戶數: " + userCount);
        context.setAttribute("users", userCount);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        userCount--;
        log("Session 銷毀: " + se.getSession().getId() + "，當前用戶數: " + userCount);
        context.setAttribute("users", userCount);
    }

    @Override
    public void attributeAdded(HttpSessionBindingEvent e) {
        log("Session 新增屬性: " + e.getName() + "=" + e.getValue());
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent e) {
        log("Session 移除屬性: " + e.getName());
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent e) {
        log("Session 替換屬性: " + e.getName() + "=" + e.getValue());
    }

    // Context 監聽 (可選，用於獲取 ServletContext)
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        this.context = sce.getServletContext();
        log("ServletContext 初始化 (從 Session 監聽器)");
    }
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        log("ServletContext 銷毀");
        this.context = null;
    }

    private void log(String msg) {
        try (PrintWriter out = new PrintWriter(new FileOutputStream("E:\\sessionLog.txt", true))) {
            out.println(new Date() + " --> " + msg);
        } catch (Exception e) { e.printStackTrace(); }
    }
}

3. 前端 JSP 測試頁

context_test.jsp - 測試 Context 屬性操作

<%@ page contentType="text/html;charset=UTF-8"%>
<%
    out.println("新增屬性");
    getServletContext().setAttribute("userName", "Smith");
    out.println("替換屬性");
    getServletContext().setAttribute("userName", "Kate");
    out.println("移除屬性");
    getServletContext().removeAttribute("userName");
%>

session_test.jsp - 測試 Session 屬性與用戶計數

<%@ page contentType="text/html;charset=UTF-8"%>
執行操作：
session.setAttribute("userName","Smith")<br>
<% session.setAttribute("userName","Smith"); %>
session.setAttribute("userName","Kate")<br>
<% session.setAttribute("userName","Kate"); %>
session.removeAttribute("userName")<br>
<% session.removeAttribute("userName"); %>
目前有 <%=getServletContext().getAttribute("users")%> 個用戶。<br>
銷毀 Session <br>
<% session.invalidate(); %>
目前有 <%=getServletContext().getAttribute("users")%> 個用戶。

4. web.xml 註冊 (Servlet 3.0+ 可用 @WebListener 替代)

<listener>
    <listener-class>com.examp.ch9.MyServletContextListener</listener-class>
</listener>
<listener>
    <listener-class>com.examp.ch9.MySessionListener</listener-class>
</listener>