I don't understand what a bluetooth UUID denotes. Do UUIDs denote protocols (e.g. RFCOMM)? If so, why do the createRfcommSocketToServiceRecord() methods require UUIDs, when they specify rfcomm right in their names? Why does the BluetoothChat sample code have a seemingly arbitrary, hardcoded UUID?
My question arises because, as per this question, I'm getting a null pointer exception when devices running 4.0.4 try to connect (to an external, non-android device) using reflection. However, the solution to that question doesn't work for me. UUID muuid = device.getUuids()[0].getUuid(); raises an exception.
Edit: I've solved that problem by hardcoding the UUID for Serial port service as per this answer (using UUID.fromString("00001101-0000-1000-8000-00805f9b34fb");).
I'm further puzzled by why I need to supply a UUID to create an unsecured rfcomm socket using createInsecureRfcommSocketToServiceRecord(), but not using the reflection method.
Can anyone straighten me out?
In Bluetooth, all objects are identified by UUIDs. These include services, characteristics and many other things. Bluetooth maintains a database of assigned numbers for standard objects, and assigns sub-ranges for vendors (that have paid enough for a reservation). You can view this list here:
https://www.bluetooth.com/specifications/assigned-numbers/
If you are implementing a standard service (e.g. a serial port, keyboard, headset, etc.) then you should use that service's standard UUID - that will allow you to be interoperable with devices that you didn't develop.
If you are implementing a custom service, then you should generate unique UUIDs, in order to make sure incompatible third-party devices don't try to use your service thinking it is something else. The easiest way is to generate random ones and then hard-code the result in your application (and use the same UUIDs in the devices that will connect to your service, of course).