Simple Bluetooth Communication in J2ME

Simple Bluetooth Communication

In this article, I will try to explain the simple Bluetooth communication standards and show how you can create a simple wrapper class around Bluetooth technology. This article is for those peoples who want to write a J2ME Bluetooth application by understanding its API and protocols.

Wireless Technologies

The most famous wireless technologies are infraree, Bluetooth, WiFi, and Zigbee. Infrared is the technology that you can see in TV remote controls or air conditioner remotes where the communication should be pointed to the target device. WiFi technology is used for strong and wide area communication where wireless communication can be made. Zigbee is the most recent technology; it's cheaper than all the other wireless media. Bluetooth technology is the most used temporary communication technology, especially inside mobile devices, palm tops, pocket PCs, and so forth. It can be used to exchange objects, packets, or a simple stream.

Bluetooth Communication Types

There are three types of communication protocols defined inside Bluetooth technology:

  • OBEX: The "Object Exchange" communication protocol is used to exchange physical data such as files, images, and so on in binary format.
  • L2CAP: The "Logical Link Control and Adaptation Protocol" used to send packets between host and client.
  • RFCOMM: The "Radio Frequency COMMunication" is very easy and uncomplicated; it is used to stream simple data.

Java Bluetooth API

Sun Java has introduced the Bluetooth JSR82 API package. The JSR82 API has capability to provide all three kinds of communications: either Obex, L2CAP, or RFCOMM. This article will focus on the simplest protocol, RFCOMM, and send only string data between the devices.

Client and Server

The technique to communicate any device will follow the good old-fashioned rule of Client and Server. You will open the Server and then wait for the client to connect; after that, the server and client both can communicate with each other easily. In Bluetooth, you have to do the same technique; the application must allow the user to select it as either the server or client.

Code and Explanation

1. Server

Every Bluetooth device contains the local Bluetooth object that helps communicate between devices. In JSR82, the LocalDevice.getLocalDevice(); function returns the object of the local Bluetooth device. The local device object should call the setDiscoverable(DiscoveryAgent.GIAC); function, in which the mode is set as GIAC. In simple words, by doing this you give permission to the current device to find other devices.

To open the Bluetooth connection, you have to build a Bluetooth URL string that will be called inside the Connector.open(URL) function; this function will return the StreamConnectionNotifier Object. The URL actually is the way to initialize the communication protocol for Bluetooth, just like on an Internet Explorer search box. You just type http://www.address.com, where http:// is the protocol and the rest is the address of the target place. In Bluetooth, you will do something like this:

URL = "btspp://localhost:" + UUID + ";name=rfcommtest;authorize=true";

Here, you have btspp:// just like the http:// protocol. The rest has an uniquely identified ID so that it will have its unique address.

After the StreamConnectionNotifier has been initialized, it has to call the final acceptAndOpen(); function that simply opens the communication and returns the StreamConnection object. But, unless no client connection is found, it will block the other processes and wait.

Now, you can use two functions by StreamConnectionb' object: openOutputStream() or openInputStream(). Both are used as a way to send and receive data.

m_strUrl= "btspp://localhost:" + RFCOMM_UUID + ";
   name=rfcommtest;authorize=true";

// m_StrmConn = BTFACADE.waitForClient(SERVICE_NBR);

try
{
   m_LclDevice = LocalDevice.getLocalDevice();

   m_LclDevice.setDiscoverable(DiscoveryAgent.GIAC);

   m_StrmNotf = (StreamConnectionNotifier)Connector.open(m_strUrl);

   //Now it will start waiting for the client connection
   m_StrmConn = m_StrmNotf.acceptAndOpen();

   m_bInitServer = true;

   m_Output = m_StrmConn.openOutputStream();
   m_Input  = m_StrmConn.openInputStream();
}
catch (BluetoothStateException e)
{
   System.err.println( "BluetoothStateException: " + e.getMessage() );
}
catch (IOException ex)
{
   ex.printStackTrace();
}
catch(Exception e)
{
   System.err.println( "Exception: " + e.getMessage() );
}

2. Client

To create the client, the UPI has to follow some rules to obtain your goal, which is to implement the DiscoveryListener interface. It has four, pure-virtual functions:

  • void deviceDiscovered(RemoteDevice btDevice, DeviceClass cod)
  • void servicesDiscovered(int transID, ServiceRecord[] records)
  • void serviceSearchCompleted(int transID, int respCode)
  • void inquiryCompleted(int discType)

At first, you have to search the available Bluetooth devices around you. You have to get the local device information and, through it, retrieve the DiscoveryAgent object to start enquiry about available devices.

public void SearchAvailDevices()
{
   try
   {
      //First, get the local device and obtain the discovery agent.
      m_LclDevice = LocalDevice.getLocalDevice();

      m_DscrAgent=  m_LclDevice.getDiscoveryAgent();

      m_DscrAgent.startInquiry(DiscoveryAgent.GIAC,this);
   }
   catch (BluetoothStateException ex)
   {
      System.out.println("Problem in searching the Bluetooth devices");
      ex.printStackTrace();
   }

}

For the client, these four methods of DiscoveryListener must to be override in the class. According to their name, they do work; for example, deviceDiscovered suddenly gets triggered when any Bluetooth device is found. After that, it is your responsibility to find the services of devices such as OBEX, RFCOMM, or L2CAP.

public void deviceDiscovered(RemoteDevice btDevice, DeviceClass cod)
{
   try
   {
      // Device information
      System.out.println("Major Device Class and information : " +
                         cod.getMajorDeviceClass() +
                         " Minor Device Class: " +
                         cod.getMinorDeviceClass());
      System.out.println("Bluetooth Address of the device: " +
                         btDevice.getBluetoothAddress());
      System.out.println("Friendly Name: " +
                         btDevice.getFriendlyName(true));

      // Now its our responsibility to search its services
      UUID uuidSet[] = new UUID[1];
      uuidSet[0]     = RFCOMM_UUID;
      int searchID  = m_DscrAgent.searchServices(null,uuidSet,
                                                 btDevice,this);
   }
   catch (Exception e)
   {
      System.out.println("Device Discovered Error: " + e);
   }

}

Here, m_DscrAgent is the DiscoveryAgent object that searches the available services of the first device you found.

public void servicesDiscovered(int transID, ServiceRecord[] records)
{

   for (int i = 0; i < records.length; i++)
   {
      m_strUrl = records[i].getConnectionURL(ServiceRecord.
         AUTHENTICATE_ENCRYPT, false);

      System.out.println(m_strUrl);
   //we have found our service protocol
   if(m_strUrl.startsWith("btspp"))
   {
      m_bServerFound = true;
      m_bInitClient=true;
      break;
   }

}

The ServicesDiscovered function above is triggered when services of that device are found. Here, you stop the loop on the first protocol that you found as Bluetooth.

Simple Bluetooth Communication in J2ME

After the Services Search function is complete, serviceSearchCompleted(int transID, int respCode) is triggered. From it, you can do further initialization such as when you found the service that you were looking for. Now, it's time to open input/output variables.

public void serviceSearchCompleted(int transID, int respCode)
{
   if(m_bServerFound)
   {
      try
      //lets the communication start by setting the URL and sending
      //the client response
      {
         m_StrmConn = (StreamConnection) Connector.open(m_strUrl);

         m_Output   = m_StrmConn.openOutputStream();
         m_Input    = m_StrmConn.openInputStream();

         m_Output.write(CLIENT_RESPONSE.length());
         m_Output.write(CLIENT_RESPONSE.getBytes());

         System.out.println("serviceSearchCompleted");
      }
      catch (IOException ex)
      {
         ex.printStackTrace();
      }

   }
}

Here, you open the connection with the server Connector.open(m_strUrl); and then you open the Input/Output stream for further communication with the server. To send any data to the current server, you have to send the buffer length or data length first, and then the total bytes of the string. This way, the server or client will know what length the upcoming text will have.

m_Output.write(CLIENT_RESPONSE.length());
m_Output.write(CLIENT_RESPONSE.getBytes());

m_Output is used to send data to the connected person; CLIENT_RESPONSE is just string data. On the server side, the acceptAndOpen() function lets the process close from waiting of any client and start communication.

Send and Receive Data

When the client and server both start working, the communication is held by the two wrapper functions: SendMessage() and ReceiveMessage().

public void SendMessages(String v_strData)
{
   if((m_bInitClient) || (m_bInitServer) )
   {
      try
      {
         m_Output.write(v_strData.length());
         m_Output.write(v_strData.getBytes());

      }
      catch (IOException ex)
      {
         ex.printStackTrace();
      }

   }

}


public String  RecieveMessages()
{
   byte[] data = null;

   try
   {

      int length = m_Input.read();
      data       = new byte[length];
      length     = 0;

      while (length != data.length)
      {
         int ch = m_Input.read(data, length, data.length - length);

         if (ch == -1)
         {
            throw new IOException("Can't read data");
         }
         length += ch;
      }


   }
   catch (IOException e)
   {
      System.err.println(e);
   }


   return new String(data);
}

Any client or server can use these functions easily without any problem. The Receive message function simply decodes the data that was received and returns it in a human-readable format.

How the Wrapper Works

The ClientServer class is a very flexible class that can be used as a client or server dynamically. If it needs to be a server, then simply initialize the object as follow:

ClientServer Obj = new ClientServer(true);

Here, true means that this object is a server; making it a client just passes the value as false. It is better to initialize the Server inside a thread so that it will not block at the time of waiting for a client.

The rest is to use SendMessage() and RecieveMessage() for communication as a client or a server. Before closing down the midlet, always remember to call Obj.CloseAll(); so that everything will remain smooth.

public void destroyApp(boolean unconditional)
{
   m_bRunThread=false;
   m_BlueObj.CloseAll();
}


public void run()
{
   while(m_bRunThread)
   {
      try
      {
         if(m_BlueObj==null)
         {
            //it should be called once when object is null
            m_BlueObj=new ClientServer(m_bIsServer);
         }

         String str = m_BlueObj.RecieveMessages();

         System.out.println(str);

         if(m_bIsServer)
            m_BlueObj.SendMessages("Hi there, it's Mr. Server");
         else
            m_BlueObj.SendMessages("Hi there, it's Mr. Client");

         Thread.sleep(100);
      }
      catch(Exception ex)
      {
         System.out.println(ex.getMessage());
      }

   }    //end while

  }

Summary

English is not my native language but I have tried to explain the usage of Bluetooth with a very simple way using the wrapper class; it can be extended by implementing new ideas in it. I hope I have shared the knowledge between learners, as a learner.

Farhan Hameed Khan, from Pakistan



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