Question

If I submit some tasks to an Executor using invokeAll, am I guaranteed that the submitted thread sees all the side effects of the task executions, even if I don’t call get() on each of the returned Futures?

From a practical point of view, it would seem that this would be a useful guarantee, but I don’t see anything in the javadoc.

More precisely, do all actions in the body of a Callable submitted to an executor happen-before the return from the invokeAll() call?

It’s annoying to uselessly call get() on each future, when in fact the return type is Void and no exceptions are thrown – all the work in the happens as side-effects.

Answer 1

If invokeAny() promises that no tasks are still in execution when invokeAny() returns, this will be the case: all side effects are visible.

In order for invokeAny() to know that all tasks are done, it needs to have synchronized with those threads, meaning that the returning of the functions happens after the tasks completing (and everything that happens in the task). However the API of ‘ExecutorSerive’ and ‘Future.cancel()’ does not explicitly say what happens when you cancel a running task (in particular: will cancel() wait with returning until the tasks has stopped running. The fact that after calling cancel(), isDone() must return true, does imply that cancel() will not return until the task has actually finished executing.

One more thing to watch out for is that you will not know if a task ever started execution, when using invokeAny() without inspecting the Future objects.