Singleton with a single worker thread

[Ziphyre Marco]

Hello,

In a Servlet environment I try to implement a Messenger class for the jobs of taking messages to its queue and sending them to online users. Messenger is a Singleton implementing Runnable. I can't find a proper way to .start() the inner thread for checking the queue and sending the message (in an another thread). Can you please comment on the design, possible caveats and a way to .start()

public class Messenger implements Runnable {
    private static final Log log = LogFactory.getLog(Messenger.class);
    private static Messenger ourInstance = new Messenger();

    public static synchronized Messenger getInstance() {
        return ourInstance;
    }

    private Messenger() {
        log.debug("Messenger instance created");
    }

    private Map<String, MessageBoxDto> mbList = new HashMap<String, MessageBoxDto>();
    
    public synchronized void addMessageToQueue(String sId, MessageBoxDto dto) {
        Map<String, MessageBoxDto> copyList = new HashMap<String, MessageBoxDto>(mbList);
        copyList.put(sId, dto);
        mbList = copyList;
    }

    @Override
    public void run() {
        //We don't care much about stale values of mbList.size() cause
        // 1. It's not time critical and
        // 2. This single thread is removing from it

        while (true) {
            log.debug("Messenger runs for " + mbList.size() + " message(s)");

            if (mbList.size() > 0) {
                for (Map.Entry<String, MessageBoxDto> entry : mbList.entrySet()) {
                    sendMessage(entry);
                }
            }
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }

    private void sendMessage(final Map.Entry<String, MessageBoxDto> entry) {
        Browser.withSession(entry.getKey(), new Runnable(){
            @Override
            public void run() {
                //send the message
            }
        });
    }

}

What I try is to use the Messenger class as:
Messenger.getInstance().addMessageToQueue(...);

And than the inner polling thread get messages from the queue and send them
Thanks

[Evan Caballero]

take a look at the java.util.concurrent API. Instantiate a ConcurrentLinkedQueue or something like that, and "plug" it to a ThreadPool. Each message added to the queue will be automatically consumed by the pool.