Bluetooth with Qt on Android. Calling java classes through jni on abstract class

Question

Possible duplicate here, but the accepted answer is very short and not of much help.

I am using Qt 5.2 for Android. I want to use my android device Bluetooth functionality which is currently not directly supported by the Qt framework. Therefore I have started using JNI to access Android Java classes and methods.

I am able to create an object of the AudioTimestamp class by doing so:

QAndroidJniObject audioTimestamp2("android/media/AudioTimestamp");

if(!audioTimestamp2.isValid())
{
    qDebug() << "audioTimestamp2 is not a valid object";
    return false;
}

The first line calls the default constructor so this works fine. This was just a test to see if I were able to create a valid object.

When starting with the Bluetooth implementation I read from developer.android.com that I need to get the bluetooth adapter of the device by:

BluetoothManager bluetoothManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
BluetoothAdapter mBluetoothAdapter = bluetoothManager.getAdapter();

so I tried doing it like this:

//Create Java string used to obtain bluetooth adapter.
QAndroidJniObject systemService = QAndroidJniObject::fromString("BLUETOOTH_SERVICE");

//Create Context Object.
QAndroidJniObject context("android/content/Context");

if(!context.isValid())
{
    qDebug() << "context is not a valid object";
    return false;
}

//Call getSystemService method on context object and return manager object.
QAndroidJniObject bluetoothManager = context.callObjectMethod("getSystemService", "(Ljava/lang/String;)Landroid/bluetooth/BluetoothManager;", systemService.object<jstring>());

if(!bluetoothManager.isValid())
{
    qDebug() << "bluetoothManager is not a valid object";
    return false;
}

//Call getAdapter() on manager.
QAndroidJniObject bluetoothAdapter = bluetoothManager.callObjectMethod("getAdapter", "()Landroid/bluetooth/BluetoothAdapter;");

//More code

if (!bluetoothAdapter.callMethod<jboolean>("isEnabled"))
{
    qDebug() << "Bluetooth is off";

    //Code to ask user to turn bluetooth on here...
}

What I soon figured out is that "getSystemService(Context.BLUETOOTH_SERVICE)" seems to be called out of nowhere. It belongs to the abstract class Context.java. How can I use Qt JNI calls on an abstract class to get the bluetoothManager?

EDIT:

I tried doing this as well:

QAndroidJniObject context("android/app/Service");

but still get:

D/Qt      (22736): ..\AndroidTest\bluetooth.cpp:33 (bool Bluetooth::start()): context is not a valid object 

Answer 1

getSystemService() is a method of android.content.Context. I think you get "context is not a valid object" because it is an abstract class which cannot be instantiated this way. The same for android.app.Service.

"figured out is that "getSystemService(Context.BLUETOOTH_SERVICE)" seems to be called out of nowhere" - in your sample from developer.android.com it is called on this, which is likely a class derived from Activity and, as a result, Context.

You need a context object to call getSystemService().

In addition, you seem to be calling object method with wrong signature: "(Ljava/lang/String;)Ljava/lang/Object;"

Answer 2

just for others who may end up here, you can get main context using

QtAndroid::androidActivity()

this is how you use it in your case:

// from https://developer.android.com/reference/android/content/Context.html#BLUETOOTH_SERVICE    
// note that the actual string value is "bluetooth" not "BLUETOOTH_SERVICE", Context.BLUETOOTH_SERVICE is a constant that holds "bluetooth" string value
QAndroidJniObject service_name = QAndroidJniObject::fromString("bluetooth");     

// this is your context
QAndroidJniObject activity = QtAndroid::androidActivity(); 

// now you get your bluetooth manager
QAndroidJniObject bluetoothManager = activity.callObjectMethod("getSystemService","(Ljava/lang/String;)Ljava/lang/Object;", service_name.object<jstring>());