Is there a default thread pool in java

Question

I can create a new threadpool in java and execute tasks on it using the ExecutorService.newFixedThreadPool and ExecutorService.submit methods.

Is there a 'default' threadpool that I can reuse for all executor services in my java program? Or do I just have to create a singleton that contains a default threadpool? C# has a default threadpool that runs tasks when the Task.Factory.StartNew method is called.

Answer 1

Since Java-8 there's ForkJoinPool.commonPool() which is used by default by many methods involving parallel or asyncronous execution. For example, Arrays.parallelSort() or parallel Stream API operation use this pool. You can submit your own tasks to this pool using many methods of CompletableFuture class like CompletableFuture.supplyAsync().

Answer 2

Using separate threadpools is good, default practice, and sharing threadpools is a (possibly premature) optimization.

Through Java 7 the answer is no, there is not a default threadpool, and the recommendation is to have many threadpools. It's good separation and will prevent blocking behavior on one collection of tasks from interfering with another.

If you share threadpools you should ask questions like:

• will the logging framework be able to distinguish tasks? (Threads is one way to distinguish.)
• If task pool A accidentally requests way too many threads and gets cut off, should task pool B starve? When you notice task pool B is failing will you be able to diagnose the problem in task pool A?
• If pool A blocks should B starve?

Maybe you create something like a LightweightThreadpool. And the first 5 tasks you write use it in a lightweight fashion. And the 6th task... does, except it also writes errors to disk, and those errors are surprisingly big, and sometimes there's many of them, and they're not throttled. Suddenly the first 5 tasks are starved and have no idea what hit them, and furthermore, when you wrote those tasks, you really believed they were secure and might not have prepared for this type of incident.

So sharing threadpools is about as okay as having two different processes run on the same server is okay. You should think about resource management very carefully first and understand that the tasks are resource-coupled now. The lack of a default threadpool is trying to force you to use separate ones by default, and think about these questions carefully before sharing one.

As of Java 8 the answer is "yes" (per Tagir's answer on this question). But you will notice everything will start horribly failing if you submit blocking tasks to that threadpool.