title: Android/Java Thread sync: while(true){} causing blocking
body: <p>I'm trying to better understand the behavior of threads in my android app. For some reason, when I use while(true) in one of my worker threads, code within that thread's run method that exists sequentially BEFORE the while(true) loop never executes. To be clear, I'm not sure if the code(toast messages) actually isn't executing or if the way the thread synchronization is handled by the Android OS is causing my Toast messages not to display. This behavior appears to be some sort of blocking but I can't figure out why this happens.</p>
  
  <p>My app uses 3 threads: the UI thread(default/main thread in an Android app), a thread to infinitely read data from the device's USB port during runtime, and a thread to process this data via messages from the USB-read thread. The problem seems to occur in my USBController class. When I comment out my infinite while loop, all of the Toast messages before the start of the loop display just fine. <strong>When I don't comment out my while(true), NO TOAST MESSAGES EVER DISPLAY!</strong> I'm pretty confused by this, I think i'm misunderstanding something fundamental about thread handling by the Android OS. Even if a while loop were to cause blocking, which i don't think it since it resides in a worker thread, <strong>why wouldn't the toast messages that occur before the while loop be triggered?</strong> Is this a synchronization issue? Am I misusing Android's Handler-Looper system?</p>
  
  <p>Code below. Note: I've included the relevant portion of the main activity and the entirety of the USBController class. My implementation of this class relies heavily on the USB to Serial library found here <a href="https://github.com/mik3y/usb-serial-for-android" rel="nofollow">mik3y/usb-serial-for-android</a>. I don't think it's necessary, but i've included the class that contains my third thread, SensorDataBuffer, that receives messages from the thread UsbController.</p>
  
  <p><strong>UsbController.java</strong></p>
  
  <pre><code>    public class UsbController extends Thread{
      ...
      @Override
      public void run() {
          android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_DEFAULT); //sets thread to default queing priority
          Looper.prepare();
          Toast.makeText(mContext.getApplicationContext(), "Hello from UsbController's run method!", Toast.LENGTH_SHORT).show();
  
          // **********************USB otg*******************************
          //Obtain permission to use Android device's USB intent
          PendingIntent mPermissionIntent;
          mPermissionIntent = PendingIntent.getBroadcast(mContext, 0, new Intent(ACTION_USB_PERMISSION), 0);
  
          // Find all available drivers from attached devices.
          ProbeTable customTable = new ProbeTable();
          customTable.addProduct(0x03EB, 0x2044, CdcAcmSerialDriver.class);                 
          UsbManager manager = (UsbManager) mContext.getSystemService(Context.USB_SERVICE);
          UsbSerialProber prober = new UsbSerialProber(customTable);
          List&lt;UsbSerialDriver&gt; availableDrivers = prober.findAllDrivers(manager);
  
          if (availableDrivers.isEmpty()) {
              Toast.makeText(mContext.getApplicationContext(), "No available USB drivers found",Toast.LENGTH_SHORT).show(); // Toast message for debugging
          }
          else {                                                  // open connection to first avail. driver
              UsbSerialDriver driver = availableDrivers.get(0);
              Toast.makeText(mContext.getApplicationContext(), "Driver found",Toast.LENGTH_SHORT).show(); // Toast message for debugging
              UsbDeviceConnection connection = manager.openDevice(driver.getDevice());
              Toast.makeText(mContext.getApplicationContext(), "Device Driver Opened",Toast.LENGTH_SHORT).show(); // Toast message for debugging
              if (connection == null) {           // You probably need to call UsbManager.requestPermission(driver.getDevice(), ..)
                  Toast.makeText(mContext.getApplicationContext(),"Connection to device not allowed, need permissions",Toast.LENGTH_LONG).show();
                  manager.requestPermission(driver.getDevice(),mPermissionIntent);  //conn test
                  if (manager.hasPermission(driver.getDevice())==true){
                      Toast.makeText(mContext.getApplicationContext(),"Permissions granted",Toast.LENGTH_SHORT).show();
                  }
              }
              else {                      // Read some data! Most have just one port (port 0).
                  List&lt;UsbSerialPort&gt; myPortList = driver.getPorts();
                  UsbSerialPort port = myPortList.get(0);
                  Toast.makeText(mContext.getApplicationContext(),"USB OTG Connection Established",Toast.LENGTH_SHORT).show();
                  try {
                      port.open(connection);
                      port.setParameters(9600, 8, UsbSerialPort.STOPBITS_1, UsbSerialPort.PARITY_NONE); // sets baud rate,databits, stopbits, &amp; parity
                      port.setDTR(true);                 //necessary to make Arduino Micro begin running it's program
                      Toast.makeText(mContext.getApplicationContext(),"port opened, parameters set, DTR set",Toast.LENGTH_SHORT).show();
                      byte buffer[] = new byte[16];      
                      String incompPacket = "";
                      Toast.makeText(mContext.getApplicationContext(), "hi again!"), Toast.LENGTH_LONG).show();
                      while (true){                  //continuous loop to read data
                          numBytesRead = port.read(buffer, 100);          
                          arduinoData = new String(buffer, "US-ASCII");
                          String raw = arduinoData.substring(0, numBytesRead);
                          if (numBytesRead &gt; 0) {
                              ...
                          }
                      }
                  } catch (IOException e) {
                      Toast.makeText(mContext, e.getMessage(), Toast.LENGTH_SHORT).show();
                  }
              }
          }
          Looper.loop(); 
      }
  }
  </code></pre>
  
  <p><strong>MainActivity.java</strong></p>
  
  <pre><code>...
      @Override
      protected void onCreate(Bundle savedInstanceState) {
  
          //Multi-threading
          //Create thread to handle incoming data from USB Controller thread
          SensorDataBuffer pressureDataBuffer = new SensorDataBuffer(MainActivity.this);
          Thread bufferThread = new Thread(pressureDataBuffer);
          bufferThread.start();
  
          //Create USB Serial Worker thread which will continuously receive data
          UsbController serialDataLink = new UsbController(PlayFrets.this);
          Thread sensorMonitorThread = new Thread(serialDataLink);
          sensorMonitorThread.start();
          //Toast.makeText(this, "USB Controller thread started", Toast.LENGTH_SHORT).show();
  
          //Build GUI
          super.onCreate(savedInstanceState);
          requestWindowFeature(Window.FEATURE_NO_TITLE);           //Removes action bar from display
          getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);  //Removes status bar from display
  
          //Create AsyncTask to load the note files. A splash screen will be displayed while task is executing
          new AsyncTask_NoteFileLoader(this).execute();
          }
  ...
  </code></pre>
  
  <p><strong>SensorDataBuffer.java</strong></p>
  
  <pre><code>public class SensorDataBuffer extends Thread{
  
      //Handler subclass which accepts messages one by one in
      //the main activitiy's FIFO message que called a "Looper"
      //The worker thread, sensorMonitor, runs UsbController in parallel
      //with the UI thread and continuously formats and sends pressure sensor
      //values read from the microcontroller to the Handler which updates the
      //corresponding pressure state logic variables in the UI thread.
      public void run(){
          android.os.Process.setThreadPriority(Process.THREAD_PRIORITY_URGENT_AUDIO); //TODO:priority was previously more favorable, test this to ensure UI doesn't lag
          Looper.prepare(); //create MessageQue to receive messages from USB Controller thread
          UsbController.setHandler(bufferHandler);
  
          bufferHandler = new Handler(Looper.myLooper()) {
                  //do stuff
          };
          Looper.loop();
      }
  }
  </code></pre>