We were badly using reflection to spy on android.bluetooth.BluetoothGatt and see while it would fail to write. Don't do this. Instead vote to have methods added/updated to use public API https://issuetracker.google.com/issues/132907122

GattDebugger.java

package com.example

import android.annotation.SuppressLint;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothGatt;
import android.bluetooth.BluetoothGattCharacteristic;
import android.bluetooth.BluetoothGattDescriptor;
import android.bluetooth.BluetoothGattService;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * Class to help debug GATT issues
 */
final class GattDebugger {

    private GattDebugger() {
        // can't instantiate helper class
    }

    /**
     * Debug reason why {@link BluetoothGatt#writeCharacteristic(BluetoothGattCharacteristic)} failed. BluetoothGatt
     * is not very helpful in explaining *why* it returned false when trying to writeCharacteristic. We can use
     * reflection to try to spy on BluetoothGatt to see if we can figure out why it returned false. This method isn't
     * super useful, but it might provide us a bit of insight to help debug problems.
     *
     * @param bluetoothGatt  Gatt client that failed
     * @param characteristic Gatt Characteristic that failed
     * @return String explaining possible reason
     */
    static String getFailingWriteCharacteristicReason(BluetoothGatt bluetoothGatt, BluetoothGattCharacteristic
            characteristic) {

        // check properties
        if ((characteristic.getProperties() & BluetoothGattCharacteristic.PROPERTY_WRITE) == 0
                && (characteristic.getProperties() & BluetoothGattCharacteristic.PROPERTY_WRITE_NO_RESPONSE) == 0) {
            return "Characteristic properties are neither PROPERTY_WRITE nor PROPERTY_WRITE_NO_RESPONSE";
        }

        // check mService == null
        try {
            Field serviceField;
            serviceField = bluetoothGatt.getClass().getDeclaredField("mService");
            serviceField.setAccessible(true);
            // use object since service is of type IBluetoothGatt which we don't have access to. It's no big deal
            // since all we need to do is null check it
            Object service = serviceField.get(bluetoothGatt);
            if (service == null) {
                return "service is null";
            }
        } catch (NoSuchFieldException | IllegalAccessException | ClassCastException e) {
            // eat it since this is just logging
        }

        // check mClientIf == 0
        try {
            Field clientIfField;
            clientIfField = bluetoothGatt.getClass().getDeclaredField("mClientIf");
            clientIfField.setAccessible(true);
            int mClientIf = (int) clientIfField.get(bluetoothGatt);
            if (mClientIf == 0) {
                return "mClientIf == 0";
            }
        } catch (NoSuchFieldException | IllegalAccessException | ClassCastException e) {
            // eat it since this is just logging
        }

        // check characteristic.getValue() is null
        if (characteristic.getValue() == null) {
            return "characteristic.getValue() == null";
        }

        // check characteristic.getService() is null
        BluetoothGattService service = characteristic.getService();
        if (service == null) {
            return "characteristic.getService() == null";
        }

        // check service.getDevice() is null
        try {
            @SuppressLint("PrivateApi")
            Method method = service.getClass().getDeclaredMethod("getDevice");
            method.setAccessible(true);
            BluetoothDevice bluetoothDevice = (BluetoothDevice) method.invoke(service);
            if (bluetoothDevice == null) {
                return "Bluetooth device is null";
            }
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            // eat it since this is just logging
        }

        // check mDeviceBusy
        try {
            Field deviceBusyField;
            deviceBusyField = bluetoothGatt.getClass().getDeclaredField("mDeviceBusy");
            deviceBusyField.setAccessible(true);
            Boolean mDeviceBusy = (Boolean) deviceBusyField.get(bluetoothGatt);
            if (mDeviceBusy) {
                return "BluetoothGatt is busy";
            }
        } catch (NoSuchFieldException | IllegalAccessException e) {
            // eat it since this is just logging
        }

        return "unknown; check logs for possible exception";
    }

    /**
     * Debug reason why {@link BluetoothGatt#writeDescriptor(BluetoothGattDescriptor)} failed. BluetoothGatt
     * is not very helpful in explaining *why* it returned false when trying to writeDescriptor. We can use
     * reflection to try to spy on BluetoothGatt to see if we can figure out why it returned false. This method isn't
     * super useful, but it might provide us a bit of insight to help debug problems.
     *
     * @param bluetoothGatt Gatt client that failed
     * @param descriptor    Gatt Descriptor that failed to write
     * @return String explaining possible reason
     */
    static String getFailingWriteDescriptorReason(BluetoothGatt bluetoothGatt,
                                                  BluetoothGattDescriptor descriptor) {
        // check mService == null
        try {
            Field serviceField;
            serviceField = bluetoothGatt.getClass().getDeclaredField("mService");
            serviceField.setAccessible(true);
            // use object since service is of type IBluetoothGatt which we don't have access to. It's no big deal
            // since all we need to do is null check it
            Object service = serviceField.get(bluetoothGatt);
            if (service == null) {
                return "service is null";
            }
        } catch (NoSuchFieldException | IllegalAccessException | ClassCastException e) {
            // eat it since this is just logging
        }

        // check mClientIf == 0
        try {
            Field clientIfField;
            clientIfField = bluetoothGatt.getClass().getDeclaredField("mClientIf");
            clientIfField.setAccessible(true);
            int mClientIf = (int) clientIfField.get(bluetoothGatt);
            if (mClientIf == 0) {
                return "mClientIf == 0";
            }
        } catch (NoSuchFieldException | IllegalAccessException | ClassCastException e) {
            // eat it since this is just logging
        }

        // check descriptor.getValue() is null
        if (descriptor.getValue() == null) {
            return "descriptor.getValue() == null";
        }

        // check descriptor.getCharacteristic() is null
        BluetoothGattCharacteristic characteristic = descriptor.getCharacteristic();
        if (characteristic == null) {
            return "descriptor.getCharacteristic() == null";
        }

        // check characteristic.getService() is null
        BluetoothGattService service = characteristic.getService();
        if (service == null) {
            return "characteristic.getService() == null";
        }

        // check service.getDevice() is null
        try {
            @SuppressLint("PrivateApi")
            Method method = service.getClass().getDeclaredMethod("getDevice");
            method.setAccessible(true);
            BluetoothDevice bluetoothDevice = (BluetoothDevice) method.invoke(service);
            if (bluetoothDevice == null) {
                return "Bluetooth device is null";

            }
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            // eat it since this is just logging
        }

        // check mDeviceBusy
        try {
            Field deviceBusyField;
            deviceBusyField = bluetoothGatt.getClass().getDeclaredField("mDeviceBusy");
            deviceBusyField.setAccessible(true);
            Boolean mDeviceBusy = (Boolean) deviceBusyField.get(bluetoothGatt);
            if (mDeviceBusy) {
                return "BluetoothGatt is busy";
            }
        } catch (NoSuchFieldException | IllegalAccessException e) {
            // eat it since this is just logging
        }

        return "unknown; check logs for possible exception";
    }
}