I use the processCommandApdu() method in a subclass of HostApduService to react on NFC commands. This method gets called on the Android main thread. These commands should be processed and sent over the NFC channel sequentially. Therefore, I want to process them synchronously.
Is there any good way to enforce sequential order without blocking inside of the processCommandApdu() method?
There is no need for you to synchronize command processing. Sequencing is inherent to the ISO/IEC 14443 protocol: A command exchange always consists of a command sent by the reader to the card (HCE device) and a response sent by the card (HCE) device to the reader. A reader won't issue a new command before the response to the previous command was received. If a command times out before a response is received by the reader, the reader will interpret this as if the connection to the card was lost.
Therefore, unless the reader resets/drops the connection, you won't receive a new command before you sent a response to the previous command (either through returning no-null from processCommandApdu()) or by calling sendResponseApdu()).