Cometd : It seems that ServerChannel lose some subscribers - cometd

Question

I used cometd to realize notification push, but i found out the following issue :
After log in the system, at the beginning, the client can receive message from server, but after wait pretty long time or do some other operation, the client may not receive message from server any more. Did anyone else encountered this problem? Thanks in Advance.
Blow is my code :
1. Client Code
var cometd = dojox.cometd;cometd.websocketEnabled = false;
cometd.init(url);
cometd.subscribe("/foo/new", function(message) {
......The business logic......
}
);
2. The ServletContextAttributeListener that integrate with AbstractService
public class BayeuxInitializerListener implements ServletContextAttributeListener {
private static final String CLIENT_CHANNEL = "/foo/new";
#Override
public void attributeAdded(ServletContextAttributeEvent event) {
if(BayeuxServer.ATTRIBUTE.equals(event.getName())) {
BayeuxServer bayeuxServer = (BayeuxServer) event.getValue();
boolean isCreated = bayeuxServer.createIfAbsent(CLIENT_CHANNEL, new ConfigurableServerChannel.Initializer() {
#Override
public void configureChannel(ConfigurableServerChannel channel) {
channel.setPersistent(true);
}
});
new MyService(bayeuxServer);
}
}
3. Service
public class MyService extends AbstractService {
private static final Logger logger = Logger.getLogger(MyService .class);
private static final String CLIENT_CHANNEL = "/foo/new";
private static final String LISTENER_CHANNEL = "/service/notification";
public MyService(BayeuxServer bayeuxServer) {
super(bayeuxServer, "notification");
this.addService(LISTENER_CHANNEL, "processNotification");
}
public void processNotification(ServerSession serverSession, Map<String, Object> data) {
LocalSession localSession = this.getLocalSession();
if(logger.isDebugEnabled()) {
logger.debug("Local Session : " + localSession.getId() + ".");
}
ServerChannel serverChannel = this.getBayeux().getChannel(CLIENT_CHANNEL)
Set<ServerSession> subscribers = serverChannel.getSubscribers();
if(0 == subscribers.size()) {
logger.info("There are no subcribers for " + CLIENT_CHANNEL + ".");
}
for(ServerSession subscriber : subscribers) {
logger.info("The subscriber for " + CLIENT_CHANNEL + " : " + subscriber.getId() + ".");
}
serverChannel.publish(localSession, data, null);
}