Question

I’m trying implement server code of Server-Sent Events in a generic way that any Object of my application could send a message to client, so I’ve decided implement a specific Servlet just for SSE. The initial test codes worked like a charm, but wasn’t flexible enought to send messages from different parts of my application. So I’ve rewrite the code in a way that all objects that has a reference to Servlet object could send a message to the clients:

public class PushServlet extends HttpServlet {

    private Thread threadServlet;
    private boolean processando=true;
    private MensagemSSEBean mensagem;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        threadServlet=Thread.currentThread();
        response.setContentType("text/event-stream; charset=utf-8");
        while (processando){
            if(!pausarThread())
                break;
            enviarMensagemParaOCliente(response.getWriter());
        }
        enviarMensagemDeFechamento(response.getWriter());
    }

    private void enviarMensagemParaOCliente(PrintWriter saida) {
        ConversorMensagemSSE conversor = new ConversorMensagemSSE();
        saida.print(conversor.converter(mensagem));
        saida.flush();
    }

    private synchronized void enviarMensagemDeFechamento(PrintWriter saida) {
        mensagem.setMensagem("#FECHAR_CONEXAO#");
        enviarMensagemParaOCliente(saida);
        saida.close();
    }

    public synchronized void enviarMensagem(MensagemSSEBean mensagem) throws IOException {
        this.mensagem=mensagem;
        threadServlet.notifyAll();
    }

    public synchronized void finalizar(){
        processando=false;
    }

    private boolean pausarThread() {
        try {
            threadServlet.wait();
            return true;
        } catch (InterruptedException e) {
            e.printStackTrace();  
        }
        return false;
    }
}

As you can see, I pause the Servlet Thread until something call “enviarMensagem”. I didn’t tested this code, basically cause I don’t know how I can get this Servlet object. Could someone explain me how could I get this Servlet object from any Object?? Another important question, is this the ideal approach for this kind of problem??

Answer 1

Finally I implemented it in a generic way. The servlet class now send keep-alive every ten seconds or the messages in a shared queue:

public class PushServlet extends HttpServlet {

    private boolean processing = true;
    private HttpServletResponse response;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.response = response;
        configureAndStart();
        while (processing) {
            try {
                sendMessages();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private void configureAndStart() throws IOException {
        processing = true;
        response.setContentType("text/event-stream; charset=utf-8");
        sendMessage(new SSEMessageBean(SSEEventType.START));
    }

    private void sendMessages() throws IOException, InterruptedException {
        SSEMessageBean message = MessageQueueController.getInstance().getNextMessage();
        while (message != null) {
            sendMessage(message);
            message = MessageQueueController.getInstance().getNextMessage();
            if (message.getEventType() != SSEEventType.END)
                return;
        }
        Thread.sleep(10000);
        sendMessage(new SSEMessageBean(SSEEventType.KEEP_ALIVE));
    }

    public void sendMessage(SSEMessageBean message) throws IOException {
        SSEMessageConverter converter = new SSEMessageConverter();
        PrintWriter out = response.getWriter();
        out.print(converter.convert(message));
        out.flush();
        if (message.getEventType() == SSEEventType.END) {
            processing = false;
            out.close();
        }
    }
}

The objects that want send events to clients simply write in shared queue.