Are Android's BroadcastReceivers started in a new thread?

The onReceive() method is always called on the main thread (which is also referred to as "UI thread"), unless you requested it to be scheduled on a different thread using the registerReceiver() variant:

Context.registerReceiver(BroadcastReceiver receiver,
                         IntentFilter filter,
                         String broadcastPermission,
                         Handler scheduler)

Are Android's BroadcastReceivers started in a new thread?

Usually but not always, it all depends on how you register it.

If you register your BroadcastReceiver using:

registerReceiver(BroadcastReceiver receiver, IntentFilter filter)

It will run in the main activity thread(aka UI thread).

If you register your BroadcastReceiver using a valid Handler running on a different thread:

registerReceiver (BroadcastReceiver receiver, IntentFilter filter, String broadcastPermission, Handler scheduler)

It will run in the context of your Handler

For example:

HandlerThread handlerThread = new HandlerThread("ht");
handlerThread.start();
Looper looper = handlerThread.getLooper();
Handler handler = new Handler(looper);
context.registerReceiver(receiver, filter, null, handler); // Will not run on main thread

Details here & here.

The onReceive() method is called on the main thread. So, in case all your access to the service class is done from within the main thread, you don't need any synchronization.