DevBytes: Bluetooth Low Energy API
Android 4.3 (API Level 18) introduces built-in platform support for Bluetooth Low Energy in the central role and provides APIs that apps can use to discover devices, query for services, and read/write characteristics. In contrast to Classic Bluetooth, Bluetooth Low Energy (BLE) is designed to provide significantly lower power consumption. This allows Android apps to communicate with BLE devices that have low power requirements, such as proximity sensors, heart rate monitors, fitness devices, and so on.
Here is a summary of key BLE terms and concepts:
Here are the roles and responsibilities that apply when an Android device interacts with a BLE device:
To understand the distinction, imagine that you have an Android phone and an activity tracker that is a BLE device. The phone supports the central role; the activity tracker supports the peripheral role (to establish a BLE connection you need one of each—two things that only support peripheral couldn't talk to each other, nor could two things that only support central).
Once the phone and the activity tracker have established a connection, they start transferring GATT metadata to one another. Depending on the kind of data they transfer, one or the other might act as the server. For example, if the activity tracker wants to report sensor data to the phone, it might make sense for the activity tracker to act as the server. If the activity tracker wants to receive updates from the phone, then it might make sense for the phone to act as the server.
In the example used in this document, the Android app (running on an Android device) is the GATT client. The app gets data from the GATT server, which is a BLE heart rate monitor that supports the Heart Rate Profile. But you could alternatively design your Android app to play the GATT server role. See {@link android.bluetooth.BluetoothGattServer} for more information.
In order to use Bluetooth features in your application, you must declare the Bluetooth permission {@link android.Manifest.permission#BLUETOOTH}. You need this permission to perform any Bluetooth communication, such as requesting a connection, accepting a connection, and transferring data.
If you want your app to initiate device discovery or manipulate Bluetooth settings, you must also declare the {@link android.Manifest.permission#BLUETOOTH_ADMIN} permission. Note: If you use the {@link android.Manifest.permission#BLUETOOTH_ADMIN} permission, then you must also have the {@link android.Manifest.permission#BLUETOOTH} permission.
Declare the Bluetooth permission(s) in your application manifest file. For example:
<uses-permission android:name="android.permission.BLUETOOTH"/> <uses-permission android:name="android.permission.BLUETOOTH_ADMIN"/>
If you want to declare that your app is available to BLE-capable devices only, include the following in your app's manifest:
<uses-feature android:name="android.hardware.bluetooth_le" android:required="true"/>
However, if you want to make your app available to devices that don't support BLE, you should still include this element in your app's manifest, but set {@code required="false"}. Then at run-time you can determine BLE availability by using {@link android.content.pm.PackageManager#hasSystemFeature PackageManager.hasSystemFeature()}:
// Use this check to determine whether BLE is supported on the device. Then
// you can selectively disable BLE-related features.
if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
Toast.makeText(this, R.string.ble_not_supported, Toast.LENGTH_SHORT).show();
finish();
}
Before your application can communicate over BLE, you need
to verify that BLE is supported on the device, and if so, ensure that it is enabled.
Note that this check is only necessary if {@code
If BLE is not supported, then you should gracefully disable any BLE features. If BLE is supported, but disabled, then you can request that the user enable Bluetooth without leaving your application. This setup is accomplished in two steps, using the {@link android.bluetooth.BluetoothAdapter}.
The {@link android.bluetooth.BluetoothAdapter} is required for any and all Bluetooth activity. The {@link android.bluetooth.BluetoothAdapter} represents the device's own Bluetooth adapter (the Bluetooth radio). There's one Bluetooth adapter for the entire system, and your application can interact with it using this object. The snippet below shows how to get the adapter. Note that this approach uses {@link android.content.Context#getSystemService getSystemService()} to return an instance of {@link android.bluetooth.BluetoothManager}, which is then used to get the adapter. Android 4.3 (API Level 18) introduces {@link android.bluetooth.BluetoothManager}:
// Initializes Bluetooth adapter.
final BluetoothManager bluetoothManager =
(BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
mBluetoothAdapter = bluetoothManager.getAdapter();
Next, you need to ensure that Bluetooth is enabled. Call {@link android.bluetooth.BluetoothAdapter#isEnabled()} to check whether Bluetooth is currently enabled. If this method returns false, then Bluetooth is disabled. The following snippet checks whether Bluetooth is enabled. If it isn't, the snippet displays an error prompting the user to go to Settings to enable Bluetooth:
private BluetoothAdapter mBluetoothAdapter;
...
// Ensures Bluetooth is available on the device and it is enabled. If not,
// displays a dialog requesting user permission to enable Bluetooth.
if (mBluetoothAdapter == null || !mBluetoothAdapter.isEnabled()) {
Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
To find BLE devices, you use the {@link android.bluetooth.BluetoothAdapter#startLeScan startLeScan()} method. This method takes a {@link android.bluetooth.BluetoothAdapter.LeScanCallback} as a parameter. You must implement this callback, because that is how scan results are returned. Because scanning is battery-intensive, you should observe the following guidelines:
The following snippet shows how to start and stop a scan:
/**
* Activity for scanning and displaying available BLE devices.
*/
public class DeviceScanActivity extends ListActivity {
private BluetoothAdapter mBluetoothAdapter;
private boolean mScanning;
private Handler mHandler;
// Stops scanning after 10 seconds.
private static final long SCAN_PERIOD = 10000;
...
private void scanLeDevice(final boolean enable) {
if (enable) {
// Stops scanning after a pre-defined scan period.
mHandler.postDelayed(new Runnable() {
@Override
public void run() {
mScanning = false;
mBluetoothAdapter.stopLeScan(mLeScanCallback);
}
}, SCAN_PERIOD);
mScanning = true;
mBluetoothAdapter.startLeScan(mLeScanCallback);
} else {
mScanning = false;
mBluetoothAdapter.stopLeScan(mLeScanCallback);
}
...
}
...
}
If you want to scan for only specific types of peripherals, you can instead call {@link android.bluetooth.BluetoothAdapter#startLeScan startLeScan(UUID[], BluetoothAdapter.LeScanCallback)}, providing an array of {@link java.util.UUID} objects that specify the GATT services your app supports.
Here is an implementation of the {@link android.bluetooth.BluetoothAdapter.LeScanCallback}, which is the interface used to deliver BLE scan results:
private LeDeviceListAdapter mLeDeviceListAdapter;
...
// Device scan callback.
private BluetoothAdapter.LeScanCallback mLeScanCallback =
new BluetoothAdapter.LeScanCallback() {
@Override
public void onLeScan(final BluetoothDevice device, int rssi,
byte[] scanRecord) {
runOnUiThread(new Runnable() {
@Override
public void run() {
mLeDeviceListAdapter.addDevice(device);
mLeDeviceListAdapter.notifyDataSetChanged();
}
});
}
};
Note: You can only scan for Bluetooth LE devices or scan for Classic Bluetooth devices, as described in Bluetooth. You cannot scan for both Bluetooth LE and classic devices at the same time.
The first step in interacting with a BLE device is connecting to it—
more specifically, connecting to the GATT server on the device. To
connect to a GATT server on a BLE device, you use the
{@link android.bluetooth.BluetoothDevice#connectGatt connectGatt()} method.
This method takes three parameters: a {@link android.content.Context} object,
autoConnect (boolean indicating whether to automatically connect to
the BLE device as soon as it becomes available), and a reference to a
{@link android.bluetooth.BluetoothGattCallback}:
mBluetoothGatt = device.connectGatt(this, false, mGattCallback);
This connects to the GATT server hosted by the BLE device, and returns a {@link android.bluetooth.BluetoothGatt} instance, which you can then use to conduct GATT client operations. The caller (the Android app) is the GATT client. The {@link android.bluetooth.BluetoothGattCallback} is used to deliver results to the client, such as connection status, as well as any further GATT client operations.
In this example, the BLE app provides an activity
(DeviceControlActivity) to connect,
display data, and display GATT services and characteristics
supported by the device. Based on user input, this activity communicates with a
{@link android.app.Service} called {@code BluetoothLeService},
which interacts with the BLE device via the Android BLE API:
// A service that interacts with the BLE device via the Android BLE API.
public class BluetoothLeService extends Service {
private final static String TAG = BluetoothLeService.class.getSimpleName();
private BluetoothManager mBluetoothManager;
private BluetoothAdapter mBluetoothAdapter;
private String mBluetoothDeviceAddress;
private BluetoothGatt mBluetoothGatt;
private int mConnectionState = STATE_DISCONNECTED;
private static final int STATE_DISCONNECTED = 0;
private static final int STATE_CONNECTING = 1;
private static final int STATE_CONNECTED = 2;
public final static String ACTION_GATT_CONNECTED =
"com.example.bluetooth.le.ACTION_GATT_CONNECTED";
public final static String ACTION_GATT_DISCONNECTED =
"com.example.bluetooth.le.ACTION_GATT_DISCONNECTED";
public final static String ACTION_GATT_SERVICES_DISCOVERED =
"com.example.bluetooth.le.ACTION_GATT_SERVICES_DISCOVERED";
public final static String ACTION_DATA_AVAILABLE =
"com.example.bluetooth.le.ACTION_DATA_AVAILABLE";
public final static String EXTRA_DATA =
"com.example.bluetooth.le.EXTRA_DATA";
public final static UUID UUID_HEART_RATE_MEASUREMENT =
UUID.fromString(SampleGattAttributes.HEART_RATE_MEASUREMENT);
// Various callback methods defined by the BLE API.
private final BluetoothGattCallback mGattCallback =
new BluetoothGattCallback() {
@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status,
int newState) {
String intentAction;
if (newState == BluetoothProfile.STATE_CONNECTED) {
intentAction = ACTION_GATT_CONNECTED;
mConnectionState = STATE_CONNECTED;
broadcastUpdate(intentAction);
Log.i(TAG, "Connected to GATT server.");
Log.i(TAG, "Attempting to start service discovery:" +
mBluetoothGatt.discoverServices());
} else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
intentAction = ACTION_GATT_DISCONNECTED;
mConnectionState = STATE_DISCONNECTED;
Log.i(TAG, "Disconnected from GATT server.");
broadcastUpdate(intentAction);
}
}
@Override
// New services discovered
public void onServicesDiscovered(BluetoothGatt gatt, int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
broadcastUpdate(ACTION_GATT_SERVICES_DISCOVERED);
} else {
Log.w(TAG, "onServicesDiscovered received: " + status);
}
}
@Override
// Result of a characteristic read operation
public void onCharacteristicRead(BluetoothGatt gatt,
BluetoothGattCharacteristic characteristic,
int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
}
}
...
};
...
}
When a particular callback is triggered, it calls the appropriate {@code broadcastUpdate()} helper method and passes it an action. Note that the data parsing in this section is performed in accordance with the Bluetooth Heart Rate Measurement profile specifications:
private void broadcastUpdate(final String action) {
final Intent intent = new Intent(action);
sendBroadcast(intent);
}
private void broadcastUpdate(final String action,
final BluetoothGattCharacteristic characteristic) {
final Intent intent = new Intent(action);
// This is special handling for the Heart Rate Measurement profile. Data
// parsing is carried out as per profile specifications.
if (UUID_HEART_RATE_MEASUREMENT.equals(characteristic.getUuid())) {
int flag = characteristic.getProperties();
int format = -1;
if ((flag & 0x01) != 0) {
format = BluetoothGattCharacteristic.FORMAT_UINT16;
Log.d(TAG, "Heart rate format UINT16.");
} else {
format = BluetoothGattCharacteristic.FORMAT_UINT8;
Log.d(TAG, "Heart rate format UINT8.");
}
final int heartRate = characteristic.getIntValue(format, 1);
Log.d(TAG, String.format("Received heart rate: %d", heartRate));
intent.putExtra(EXTRA_DATA, String.valueOf(heartRate));
} else {
// For all other profiles, writes the data formatted in HEX.
final byte[] data = characteristic.getValue();
if (data != null && data.length > 0) {
final StringBuilder stringBuilder = new StringBuilder(data.length);
for(byte byteChar : data)
stringBuilder.append(String.format("%02X ", byteChar));
intent.putExtra(EXTRA_DATA, new String(data) + "\n" +
stringBuilder.toString());
}
}
sendBroadcast(intent);
}
Back in DeviceControlActivity, these events are handled by a
{@link android.content.BroadcastReceiver}:
// Handles various events fired by the Service.
// ACTION_GATT_CONNECTED: connected to a GATT server.
// ACTION_GATT_DISCONNECTED: disconnected from a GATT server.
// ACTION_GATT_SERVICES_DISCOVERED: discovered GATT services.
// ACTION_DATA_AVAILABLE: received data from the device. This can be a
// result of read or notification operations.
private final BroadcastReceiver mGattUpdateReceiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
final String action = intent.getAction();
if (BluetoothLeService.ACTION_GATT_CONNECTED.equals(action)) {
mConnected = true;
updateConnectionState(R.string.connected);
invalidateOptionsMenu();
} else if (BluetoothLeService.ACTION_GATT_DISCONNECTED.equals(action)) {
mConnected = false;
updateConnectionState(R.string.disconnected);
invalidateOptionsMenu();
clearUI();
} else if (BluetoothLeService.
ACTION_GATT_SERVICES_DISCOVERED.equals(action)) {
// Show all the supported services and characteristics on the
// user interface.
displayGattServices(mBluetoothLeService.getSupportedGattServices());
} else if (BluetoothLeService.ACTION_DATA_AVAILABLE.equals(action)) {
displayData(intent.getStringExtra(BluetoothLeService.EXTRA_DATA));
}
}
};
Once your Android app has connected to a GATT server and discovered services, it can read and write attributes, where supported. For example, this snippet iterates through the server's services and characteristics and displays them in the UI:
public class DeviceControlActivity extends Activity {
...
// Demonstrates how to iterate through the supported GATT
// Services/Characteristics.
// In this sample, we populate the data structure that is bound to the
// ExpandableListView on the UI.
private void displayGattServices(List<BluetoothGattService> gattServices) {
if (gattServices == null) return;
String uuid = null;
String unknownServiceString = getResources().
getString(R.string.unknown_service);
String unknownCharaString = getResources().
getString(R.string.unknown_characteristic);
ArrayList<HashMap<String, String>> gattServiceData =
new ArrayList<HashMap<String, String>>();
ArrayList<ArrayList<HashMap<String, String>>> gattCharacteristicData
= new ArrayList<ArrayList<HashMap<String, String>>>();
mGattCharacteristics =
new ArrayList<ArrayList<BluetoothGattCharacteristic>>();
// Loops through available GATT Services.
for (BluetoothGattService gattService : gattServices) {
HashMap<String, String> currentServiceData =
new HashMap<String, String>();
uuid = gattService.getUuid().toString();
currentServiceData.put(
LIST_NAME, SampleGattAttributes.
lookup(uuid, unknownServiceString));
currentServiceData.put(LIST_UUID, uuid);
gattServiceData.add(currentServiceData);
ArrayList<HashMap<String, String>> gattCharacteristicGroupData =
new ArrayList<HashMap<String, String>>();
List<BluetoothGattCharacteristic> gattCharacteristics =
gattService.getCharacteristics();
ArrayList<BluetoothGattCharacteristic> charas =
new ArrayList<BluetoothGattCharacteristic>();
// Loops through available Characteristics.
for (BluetoothGattCharacteristic gattCharacteristic :
gattCharacteristics) {
charas.add(gattCharacteristic);
HashMap<String, String> currentCharaData =
new HashMap<String, String>();
uuid = gattCharacteristic.getUuid().toString();
currentCharaData.put(
LIST_NAME, SampleGattAttributes.lookup(uuid,
unknownCharaString));
currentCharaData.put(LIST_UUID, uuid);
gattCharacteristicGroupData.add(currentCharaData);
}
mGattCharacteristics.add(charas);
gattCharacteristicData.add(gattCharacteristicGroupData);
}
...
}
...
}
It's common for BLE apps to ask to be notified when a particular characteristic changes on the device. This snippet shows how to set a notification for a characteristic, using the {@link android.bluetooth.BluetoothGatt#setCharacteristicNotification setCharacteristicNotification()} method:
private BluetoothGatt mBluetoothGatt;
BluetoothGattCharacteristic characteristic;
boolean enabled;
...
mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);
...
BluetoothGattDescriptor descriptor = characteristic.getDescriptor(
UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
mBluetoothGatt.writeDescriptor(descriptor);
Once notifications are enabled for a characteristic, an {@link android.bluetooth.BluetoothGattCallback#onCharacteristicChanged onCharacteristicChanged()} callback is triggered if the characteristic changes on the remote device:
@Override
// Characteristic notification
public void onCharacteristicChanged(BluetoothGatt gatt,
BluetoothGattCharacteristic characteristic) {
broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
}
Once your app has finished using a BLE device, it should call {@link android.bluetooth.BluetoothGatt#close close()} so the system can release resources appropriately:
public void close() {
if (mBluetoothGatt == null) {
return;
}
mBluetoothGatt.close();
mBluetoothGatt = null;
}