read AppFuse 2-初始化续

应用程序启动时，调用StartupListener初始化ServletCOntext环境变量、Spring环境变量和取得角色信息后，接着执行UserCounterListener监听器。

 ★ UserCounterListener功能：计算使用应用的用户个数和存储用户对象。应用启动使初始化用户数变量counter，并存入ServletContext,利用HttpSessionAttributeListener监听器每当Session中的属性值有变化时，修改ServletContext中的用户个数和，用户对象。

提供了一种全局跟踪用户信息的方法。

UserCounterListener监听器实现比较简单，主要时理解ServletContextListener和HttpSessionAttributeListener监听器的使用。

★ web.xml

 <listener>

      <listener-class>org.dudu.webapp.listener.UserCounterListener</listener-class>

   </listener>

★     UserCounterListener监听器程序:

UserCounterListener出来继承了ServletContextListener监听器接口外，还继承了

HttpSessionAttributeListener监听器接口。ServletContextListener监听器接口的作用已经在“初始化”前一部分作了介绍。

★     HttpSessionAttributeListener监听器介绍：HttpSessionAttributeListener监听

HttpSession中的属性的操作。当在Session增加一个属性时，激发

attributeAdded(HttpSessionBindingEvent se) 方法；当在Session删除一个属性时，激发

attributeRemoved(HttpSessionBindingEvent se)方法；当在Session属性被重新设置时，激

发attributeReplaced(HttpSessionBindingEvent se) 方法。

       public class UserCounterListener implements ServletContextListener,

                                            HttpSessionAttributeListener {

    public static final String COUNT_KEY = "userCounter";

    public static final String USERS_KEY = "userNames";

    private final transient Log log = LogFactory.getLog(UserCounterListener.class);

    private transient ServletContext servletContext;

    private int counter;

    private Set users;

    public synchronized void contextInitialized(ServletContextEvent sce) {

        servletContext = sce.getServletContext();

        servletContext.setAttribute((COUNT_KEY), Integer.toString(counter));

    }

    public synchronized void contextDestroyed(ServletContextEvent event) {

        servletContext = null;

        users = null;

        counter = 0;

    }

    synchronized void incrementUserCounter() {

        counter =

            Integer.parseInt((String) servletContext.getAttribute(COUNT_KEY));

        counter++;

        servletContext.setAttribute(COUNT_KEY, Integer.toString(counter));

        if (log.isDebugEnabled()) {

            log.debug("User Count: " + counter);

        }

    }

    synchronized void decrementUserCounter() {

        int counter =

            Integer.parseInt((String) servletContext.getAttribute(COUNT_KEY));

        counter--;

         if (counter < 0) {

            counter = 0;

        }

        servletContext.setAttribute(COUNT_KEY, Integer.toString(counter));

        if (log.isDebugEnabled()) {

            log.debug("User Count: " + counter);

        }

    }

    synchronized void addUsername(Object user) {

        users = (Set) servletContext.getAttribute(USERS_KEY);

  if (users == null) {

            users = new HashSet();

        }

        if (log.isDebugEnabled()) {

            if (users.contains(user)) {

                log.debug("User already logged in, adding anyway...");

            }

        }

        users.add(user);

        servletContext.setAttribute(USERS_KEY, users);

        incrementUserCounter();

    }

    synchronized void removeUsername(Object user) {

        users = (Set) servletContext.getAttribute(USERS_KEY);

        if (users != null) {

            users.remove(user);

        }

        servletContext.setAttribute(USERS_KEY, users);

        decrementUserCounter();

    }

    public void attributeAdded(HttpSessionBindingEvent event) {

        if (event.getName().equals(Constants.USER_KEY)) {

            addUsername(event.getValue());

        }

    }

    public void attributeRemoved(HttpSessionBindingEvent event) {

        if (event.getName().equals(Constants.USER_KEY)) {

            removeUsername(event.getValue());

        }

    }

    public void attributeReplaced(HttpSessionBindingEvent event) {

        // I don't really care if the user changes their information

    }

}

本文引用通告地址： http://blog.csdn.net/dudutu/services/trackbacks/399553.aspx