How to programmatically tell if a Bluetooth device is connected?

Add bluetooth permission to your AndroidManifest,

<uses-permission android:name="android.permission.BLUETOOTH" />

Then use intent filters to listen to the ACTION_ACL_CONNECTED, ACTION_ACL_DISCONNECT_REQUESTED, and ACTION_ACL_DISCONNECTED broadcasts:

public void onCreate() {
    ...
    IntentFilter filter = new IntentFilter();
    filter.addAction(BluetoothDevice.ACTION_ACL_CONNECTED);
    filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECT_REQUESTED);
    filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECTED);
    this.registerReceiver(mReceiver, filter);
}

//The BroadcastReceiver that listens for bluetooth broadcasts
private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

        if (BluetoothDevice.ACTION_FOUND.equals(action)) {
           ... //Device found
        }
        else if (BluetoothDevice.ACTION_ACL_CONNECTED.equals(action)) {
           ... //Device is now connected
        }
        else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {
           ... //Done searching
        }
        else if (BluetoothDevice.ACTION_ACL_DISCONNECT_REQUESTED.equals(action)) {
           ... //Device is about to disconnect
        }
        else if (BluetoothDevice.ACTION_ACL_DISCONNECTED.equals(action)) {
           ... //Device has disconnected
        }           
    }
};

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A few notes:

• There is no way to retrieve a list of connected devices at application startup. The Bluetooth API does not allow you to QUERY, instead it allows you to listen to CHANGES.
• A hoaky work around to the above problem would be to retrieve the list of all known/paired devices... then trying to connect to each one (to determine if you're connected).
• Alternatively, you could have a background service watch the Bluetooth API and write the device states to disk for your application to use at a later date.

In my use case I only wanted to see if a Bluetooth headset is connected for a VoIP app. The following solution worked for me:

public static boolean isBluetoothHeadsetConnected() {
    BluetoothAdapter mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    return mBluetoothAdapter != null && mBluetoothAdapter.isEnabled()
            && mBluetoothAdapter.getProfileConnectionState(BluetoothHeadset.HEADSET) == BluetoothHeadset.STATE_CONNECTED;
} 

Of course you'll need the Bluetooth permission:

<uses-permission android:name="android.permission.BLUETOOTH" />

---

Big thanks to Skylarsutton for his answer. I'm posting this as a response to his, but because I'm posting code I can't reply as a comment. I already upvoted his answer so am not looking for any points. Just paying it forward.

For some reason BluetoothAdapter.ACTION_ACL_CONNECTED could not be resolved by Android Studio. Perhaps it was deprecated in Android 4.2.2? Here is a modification of his code. The registration code is the same; the receiver code differs slightly. I use this in a service which updates a Bluetooth-connected flag that other parts of the app reference.

    public void onCreate() {
        //...
        IntentFilter filter = new IntentFilter();
        filter.addAction(BluetoothDevice.ACTION_ACL_CONNECTED);
        filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECT_REQUESTED);
        filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECTED);
        this.registerReceiver(BTReceiver, filter);
    }

    //The BroadcastReceiver that listens for bluetooth broadcasts
    private final BroadcastReceiver BTReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

        if (BluetoothDevice.ACTION_ACL_CONNECTED.equals(action)) {
            //Do something if connected
            Toast.makeText(getApplicationContext(), "BT Connected", Toast.LENGTH_SHORT).show();
        }
        else if (BluetoothDevice.ACTION_ACL_DISCONNECTED.equals(action)) {
            //Do something if disconnected
            Toast.makeText(getApplicationContext(), "BT Disconnected", Toast.LENGTH_SHORT).show();
        }
        //else if...
    }
};