Multithreading on Java

[Joe Ross]

Originally Posted by Ether

That's a sort of state machine, with time being the state variable.

A command is a state machine. The scheduler handles transitioning between states. The states are listed below:

initialize
execute
end
interrupted

The user provides the code that goes in those methods. There is a fifth method (isFinished()) that allows the scheduler to know when to transition from execute to end.

The normal flow is from initialize to execute to end. The scheduler handles interrupting, if a new command is scheduled that requires the same subsystem.

A CommandGroup allows multiple commands to be scheduled sequentially or in parallel (being in parallel requires that they don't require the same subsystem).

The only times we have had to add an explicit state machine inside a command, is when there is decision logic that needs to go backwards in states. In our case, that's been relatively rare.

The scheduler is entirely cooperative. It depends on the commands methods executing quickly and returning. If the user puts an explicit Timer.wait() in a command, it breaks the execution model. The user can either use a WaitCommand, or check a timer in the isFinished method, or implement their own state machine. We've done all 3.