Pushlet 推送數據庫消息到前臺指定用戶

       在項目中遇到需要實現類似 facebook 消息推送的功能, 也就是服務器端將未讀的系統消息條數推送給前臺用戶(秒級), 即用戶在前臺頁面可以實時的看到最新的系統消息, 經過調研決定使用 Pushlet (http://www.pushlets.com/)  來實現.

      首先, 由於 Pushlet 自己產生的 sessionid 是個隨機數, 當後臺向前臺推送消息時無法和當前登錄用戶聯繫起來, 因此我們需要修改 Pushlet 產生 sessionid 的方法. 具體來說就是在

      nl.justobjects.pushlet.core.SessionManager 中修改方法 public Session createSession(Event anEvent) throws PushletException

      修改後的代碼爲

public Session createSession(Event anEvent) throws PushletException {
		// Trivial
		//return Session.create(createSessionId());
		//return Session.create(createSessionId(), anEvent);
		return Session.create(anEvent.getField("userid", "visitor")); 
	}

      

       其次, 在前臺的 ajax-pushlet-client.js 增加幾行代碼用來向後臺傳用戶登錄 id

      在此代碼後面加上

       // Construct base URL for GET
       var url = PL.pushletURL + '?p_event=' + anEvent;

var userid = getMenuValue("userid");
if (anEvent == 'join' || anEvent == 'join-listen') {  
 url = url + '&userid=' + userid; 
}

      對 Pushlet 的源代碼改造完成, 需要對其重新編譯生成 jar 文件後加入到項目 lib 中去

      接下來就是編寫 servlet 來獲取當前登錄用戶有多少未讀消息

public class NotifyPushletServlet extends HttpServlet{
	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	private static SystemService systemService;

	public SystemService getSystemService() {
		return systemService;
	}

	public void setSystemService(SystemService systemService) {
		NotifyPushletServlet.systemService = systemService;
	}


	@Override
	public void init(ServletConfig config) throws ServletException {
		// TODO Auto-generated method stub
		super.init(config);
		ServletContext servletContext = this.getServletContext();  
		WebApplicationContext wac = null;   
		wac = WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext);  
		this.setSystemService((SystemService) wac.getBean("systemService"));
	}

	static public class Pushlet extends EventPullSource {
		@Override
		protected long getSleepTime() {
			return 5000;
		}

		@Override
		protected Event pullEvent() {
			System.out.println(new Date());
			Event event = Event.createDataEvent("/notifynum");
			Session[] sessions = SessionManager.getInstance().getSessions();  
			if (sessions.length > 0) {
				StringBuilder str = new StringBuilder("");
				for (int i = 0; i < sessions.length; i++) {
					if (sessions[i].getId().equals("guest")) continue;
					if (!str.toString().equals("")) str.append(","); 
					str.append("'" + sessions[i].getId() + "'");
				}
				if (!str.toString().equals("")) {
					List<?> list = systemService.findNotReadNotificationNumOnline(str.toString());
					if (list != null && list.size() > 0) {
						Object[] obj = null;
						String userid = null, num = null;
						for (int i = 0; i < list.size(); i++) {
							obj = (Object[]) list.get(i);
							userid = obj[0].toString();
							num = obj[1].toString();
							System.out.println("userid = " + userid + ", num = " + num);
							event.setField(userid, num);
						}
					}
				}
			}
			
			return event;
		}

	}
}

       最後在前臺只接受並顯示當前用戶自己對應的未讀的消息條數

   <script type="text/javascript">
    	var userid = getMenuValue("userid");
 		PL._init(); 
		PL.joinListen('/notifynum');
		function onData(event) { 
			var num = event.get(userid);
			if (num > 0) {
				document.getElementById('desktopnotify').innerHTML = num;
				document.getElementById('desktopnotify').style.display='';
			} else {
				document.getElementById('desktopnotify').style.display='none';
			}
		}
	</script>