Android send data from main UI thread to another thread

Question

I need to send some data from the main thread to another thread. I've already read a lot of materials on threads, asynctasks and handlers but maybe they created some confusion to me. I read that I need to create a Handler to my 'second thread' so that I can send messages from the main thread to it (for now I'm not worrying about send back anything to the main thread).

I need the second thread to connect to a server (via socket) and send some date on some user events. I'm trying to do it in an efficiently manner (do not open/close unnecessary socket connections). So I'm wondering where should I put my open socket command? Also, in my handler's handleMessage() method I need a reference to the socket output stream in order to send data to server.

I have currently the following code:

protected void initThread(){
    this.thread = new HandlerThread(WorkerHandler.class.getCanonicalName()){        

        @Override
        public void run() {
            super.run();
            try{
                handler = new WorkerHandler(getLooper());
            }catch(Exception e){
                e.printStackTrace();
            }               
        }

    };
    this.thread.start();
}

The method initThread() is called in the onCreate() method of my activity.

Here follows the code of my custom handler class:

public class WorkerHandler extends Handler {

protected Socket socket;
protected BufferedWriter writer;

public WorkerHandler(Looper looper) throws Exception{
    super(looper);      
    this.socket = new Socket("192.168.1.7", 5069);
    this.writer = new BufferedWriter(new OutputStreamWriter(this.socket.getOutputStream(), "utf-8"));

}

public BufferedWriter getWriter(){
    return this.writer;
}

public Socket getSocket(){
    return this.socket;
}

@Override
public void handleMessage(Message msg) {
    Draw draw = (Draw) msg.obj;
    if (draw != null){          
        if (getWriter() != null){
            try{
                getWriter().write(DrawUtil.toJson(draw)+"\n");
                getWriter().flush();
            }catch(IOException e){
                e.printStackTrace();
            }
        }
    }
}

}

And again, in my activity, I trigger the sendDataToServer() method

protected void sendDataToServer(){
    Draw draw = new Draw(getFlowType(), getID(), getSeq(), Calendar.getInstance(), startX, startY, endX, endY);
    if (getWorkerHandler() != null){
        Message msg = getWorkerHandler().obtainMessage();
        msg.obj = draw;
        getWorkerHandler().sendMessage(msg);
    }       
}

But my reference to the WorkerHandler object is always null. I'm pretty sure I misunderstood some concept... Could you please give me some hints?

Thanks a lot!

Answer 1

You can't do it this way. You've created your second Thread by using HandlerThread. A HandlerThread is a Thread that has a Looper. So that is what is going on in the run() method of HandlerThread. It is running the looper loop. That means that the run() method in the HandlerThread will only complete when the Looper exits.

In your initThread() method you wrote:

    @Override
    public void run() {
        super.run(); // <-- This call runs the Looper loop and doesn't complete!!
        try{
            handler = new WorkerHandler(getLooper());
        }catch(Exception e){
            e.printStackTrace();
        }               
    }

You can see that your overridden run() method first calls super.run(). This runs the looper loop and doesn't complete. So the rest of your code in initThread() never executes.

If you want to use a HandlerThread() then you can't mess with its run() method. If you want it to do stuff for you then you'll need to post messages (or Runnables) to it, and do your work there. Here's an example:

    HandlerThread handlerThread = new HandlerThread("myHandlerThread");
    handlerThread.start();
    // Now get the Looper from the HandlerThread so that we can create a Handler that is attached to
    // the HandlerThread
    // NOTE: This call will block until the HandlerThread gets control and initializes its Looper
    Looper looper = handlerThread.getLooper();
    // Create a handler attached to the background message processing thread
    handler = new Handler(looper, this);

Now you can post messages and Runnables to the "handler". In this example, the messages will be processed by an overridden handleMessage() method of the creating class.

EDIT: Provide code example for the Handler callback

You can use your WorkerHandler class to handle the callbacks if you modify it like this (I've changed the name to Worker because it isn't really a Handler, it just implements the Handler.Callback interface):

public class Worker implements Handler.Callback {

    protected Socket socket;
    protected BufferedWriter writer;

    public Worker() throws Exception{    
        this.socket = new Socket("192.168.1.7", 5069);
        this.writer = new BufferedWriter(new OutputStreamWriter(this.socket.getOutputStream(), "utf-8"));
    }

    public BufferedWriter getWriter(){
        return this.writer;
    }

    public Socket getSocket(){
        return this.socket;
    }

    @Override
    public void handleMessage(Message msg) {
        Draw draw = (Draw) msg.obj;
        if (draw != null){          
            if (getWriter() != null){
                try{
                    getWriter().write(DrawUtil.toJson(draw)+"\n");
                    getWriter().flush();
                }catch(IOException e){
                    e.printStackTrace();
                }
            }
        }
    }
}

Now you need to create an instance of this Worker class and pass it as the callback argument when you create the Handler. In your activity do:

    HandlerThread handlerThread = new HandlerThread("myHandlerThread");
    handlerThread.start();
    Looper looper = handlerThread.getLooper();
    // Create an instance of the class that will handle the messages that are posted
    //  to the Handler
    Worker worker = new Worker();
    // Create a Handler and give it the worker instance to handle the messages
    handler = new Handler(looper, worker);