Controlling Task execution order with ExecutorService

Question

I have a process which delegates asynch tasks to a pool of threads. I need to ensure that certain tasks are executed in order. So for example

Tasks arrive in order<

Answer 1

When I've done this in the past I've usually had the ordering handled by a component which then submits callables/runnables to an Executor.

Something like.

• Got a list of tasks to run, some with dependencies
• Create an Executor and wrap with an ExecutorCompletionService
• Search all tasks, any with no dependencies, schedule them via the completion service
• Poll the completion service
• As each task completes
  • Add it to a "completed" list
  • Reevaluate any waiting tasks wrt to the "completed list" to see if they are "dependency complete". If so schedule them
  • Rinse repeat until all tasks are submitted/completed

The completion service is a nice way of being able to get the tasks as they complete rather than trying to poll a bunch of Futures. However you will probably want to keep a Map which is populated when a task is schedule via the completion service so that when the completion service gives you a completed Future you can figure out which TaskIdentifier it is.

If you ever find yourself in a state where tasks are still waiting to run, but nothing is running and nothing can be scheduled then your have a circular dependency problem.