Difference between newScheduledThreadPool(1) and newSingleThreadScheduledExecutor()

Question

I am wondering what is the difference between these two methods of Executors class? I have a web application where I'm checking some data every 100 ms so that's why I'm using this scheduler with scheduleWithFixedDelay method. I want to know which method should I use in this case (newScheduledThreadPool or newSingleThreadScheduledExecutor)? I also have one more question - in VisualVM where I monitor my Glassfish server I noticed that I have some threads in PARK state - for example:

java.lang.Thread.State: WAITING
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for <3cb9965d> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1088)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Is it possible that these threads are connected with scheduler because I don't have any idea what else would create them? These threads are never destroyed, so I am afraid that this could cause some troubles. Here is a screenshot (new Thread-35 will be created in 15minutes and so on...):

Answer 1

newSingleThreadScheduledExecuto() is wrapped by a delegate, as you can see in Executors.java:

public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
    return new DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1));
}

Differences (from javadoc):

if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.

Unlike the otherwise equivalent {@code newScheduledThreadPool(1)} the returned executor is guaranteed not to be reconfigurable to use additional threads.

reply to your comment:

do this also apply for newScheduledThreadPool(1) or not?

no, you need to take care of thread failure yourself.

As for Unsafe.park(), see this.

Answer 2

As documentation states:

Unlike the otherwise equivalent newScheduledThreadPool(1) the returned executor is guaranteed not to be reconfigurable to use additional threads.

So when using newScheduledThreadPool(1)you will be able to add more threads later.