Getting Android Bluetooth Adapter Name from JNI/C++

Question

The Android API in question is android.bluetooth.BluetoothAdapter, which has a member function getName() which returns the adapter's user friendly name.

In java: BluetoothAdapter.getDefaultAdapter().getName()

I know I can wrap this in a java-function, which I call through jni, but, how can I achieve the same in C++, with only jni/android-ndk?

Answer 1

First of all, you need to have permission to read this value (which you would need regardless of it being native).

Add to AndroidManifest.xml:

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

In native jni-land, things are a bit cumbersome. In short, this is what you need:

1. Get class android.bluetooth.BluetoothAdapter
2. Get static method BluetoothAdapter.getDefaultAdapter()
3. Get method BluetoothAdapter.getName()

In order to:

4. Call 2 on 1 to get the default BluetoothAdapter instance
5. Call getName() on instance from 4. to get the adapter's name.

This is the same as the java one-liner, just broken down.

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The code (assuming you already have a JNIEnv object):

// 1. Get class
// The path matches the package hierarchy of
// 'android.bluetooth.BluetoothAdapter'
jclass classBta = env->FindClass("android/bluetooth/BluetoothAdapter");

// 2. Get id of the static method, getDefaultAdapter()
// Search the web for 'jni function signature' to understand
// the third argument. In short it's a function that takes no arguments,
// hence '()', and returns an object of type
// android.bluetooth.BluetoothAdapter, which uses syntax "L{path};"
jmethodID methodIdGetAdapter =
    env->GetStaticMethodID(classBta,
                           "getDefaultAdapter",
                           "()Landroid/bluetooth/BluetoothAdapter;");

// 3. Get id of the non-static method, getName()
// The third argument is the getName function signature,
// no arguments, and returns a java.lang.String object.
jmethodID methodIdGetName =
    env->GetMethodID(classBta,
                     "getName",
                     "()Ljava/lang/String;");

// 4. We get the instance returned by getDefaultAdapter()
jobject objBta = (jobject)
    env->CallStaticObjectMethod(classBta, methodIdGetAdapter);

// 5. Call member method getName on instance
jstring strName = (jstring)
    env->CallObjectMethod(objBta, methodIdGetName);

// Convert jstring to a regular string
const char *result = env->GetStringUTFChars(strName, 0);
std::string blueToothName(result);

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For the sake of clarity, I've omitting sensible checks to see if the various functions have succeeded, and also cleanup:

env->DeleteLocalRef(classBta);
env->DeleteLocalRef(objBta);
env->DeleteLocalRef(strName);
env->ReleaseStringUTFChars(strName, result);