What does Future.cancel(true) do?

Question

Hi, I have some (a dozen) Callable object which are submitted to the ExecutorService ( talking about jdk5.0 concurrent package), and I want that the threads executing those Callable task last for not so much time: invoking cancel(true) on a Future object returned, will cause the Callable task to end immediatly (or quite immediatly) ?? Or does it depend on what the Callable do inside its call() method?
I asked that question because I would like to stop tasks if they don't end within a certain amount of time, and up to now I didn't find a solution for my problem since what the Callable object do inside their call() method is launching a 3rd-party" algorithm with some parameters.
thanks
ciao
alessio

Answer 1

I've not used the concurrent package in 1.5, but I've used util.concurrent a fair bit in the past.... However, looking at the 1.5 javadocs, I interpret the description for FutureTask#cancel(boolean) as meaning if you call it with "true", and the FutureTasks Callable is currently being executed, the Thread executing it will be interrupted.
Now, whether or not this actually causes your task to actually finish is another story: It depends on what the task does.
If the task is doing something which will stop when interrupted (e.g, waiting on a monitor, doing a NIO select, whatever), then (unless the task explicitly catches InterruptedExceptions and continues anyway...) your task should stop.
However, if its doing something which isn't affected by interrupts (e.g, good 'ole blocking IO) - then you are out of luck.
This is very similar to a question which comes up quite a bit on these forums - "how do I stop a thread performing some task after a certain amount of time". Have a search around the forums - there are probably some much better answers than mine around.
Dave