Impossible to make a cached thread pool with a size limit?

Question

It seems to be impossible to make a cached thread pool with a limit to the number of threads that it can create.

Here is how static Executors.newCachedThreadPool is implemented in the standard Java library:

 public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}

So, using that template to go on to create a fixed sized cached thread pool:

new ThreadPoolExecutor(0, 3, 60L, TimeUnit.SECONDS, new SynchronusQueue<Runable>());

Now if you use this and submit 3 tasks, everything will be fine. Submitting any further tasks will result in rejected execution exceptions.

Trying this:

new ThreadPoolExecutor(0, 3, 60L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runable>());

Will result in all threads executing sequentially. I.e., the thread pool will never make more than one thread to handle your tasks.

This is a bug in the execute method of ThreadPoolExecutor? Or maybe this is intentional? Or there is some other way?

Edit: I want something exactly like the cached thread pool (it creates threads on demand and then kills them after some timeout) but with a limit on the number of threads that it can create and the ability to continue to queue additional tasks once it has hit its thread limit. According to sjlee's response this is impossible. Looking at the execute() method of ThreadPoolExecutor it is indeed impossible. I would need to subclass ThreadPoolExecutor and override execute() somewhat like SwingWorker does, but what SwingWorker does in its execute() is a complete hack.

Answer 1

This is what you want (atleast I guess so). For an explanation check Jonathan Feinberg answer

Executors.newFixedThreadPool(int n)

Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue. At any point, at most nThreads threads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.

Answer 2

Per the Javadoc for ThreadPoolExecutor:

If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full. By setting corePoolSize and maximumPoolSize the same, you create a fixed-size thread pool.

(Emphasis mine.)

jitter's answer is what you want, although mine answers your other question. :)

Answer 3

In your first example, subsequent tasks are rejected because the AbortPolicy is the default RejectedExecutionHandler. The ThreadPoolExecutor contains the following policies, which you can change via the setRejectedExecutionHandler method:

CallerRunsPolicy
AbortPolicy
DiscardPolicy
DiscardOldestPolicy

It sounds like you want cached thread pool with a CallerRunsPolicy.

Answer 4

The ThreadPoolExecutor has the following several key behaviors, and your problems can be explained by these behaviors.

When tasks are submitted,

1. If the thread pool has not reached the core size, it creates new threads.
2. If the core size has been reached and there is no idle threads, it queues tasks.
3. If the core size has been reached, there is no idle threads, and the queue becomes full, it creates new threads (until it reaches the max size).
4. If the max size has been reached, there is no idle threads, and the queue becomes full, the rejection policy kicks in.

In the first example, note that the SynchronousQueue has essentially size of 0. Therefore, the moment you reach the max size (3), the rejection policy kicks in (#4).

In the second example, the queue of choice is a LinkedBlockingQueue which has an unlimited size. Therefore, you get stuck with behavior #2.

You cannot really tinker much with the cached type or the fixed type, as their behavior is almost completely determined.

If you want to have a bounded and dynamic thread pool, you need to use a positive core size and max size combined with a queue of a finite size. For example,

new ThreadPoolExecutor(10, // core size
    50, // max size
    10*60, // idle timeout
    TimeUnit.SECONDS,
    new ArrayBlockingQueue<Runnable>(20)); // queue with a size