Executor service fixes

[pepone]

Fix #3300

[externl]

what do these magic numbers refer to?

[pepone]

I would add a comment, but see https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/ThreadPoolExecutor.html#ThreadPoolExecutor-int-int-long-java.util.concurrent.TimeUnit-java.util.concurrent.BlockingQueue-

[externl]

Ok, so the 1000 for keepAliveTime will have no effect here since we're setting core and max to 10.

[bernardnormier]

I am concerned the original QueueExecutorService has a single thread by design, to ensure the jobs are serialized. Hard to say without comment. If it was by design, this PR is changing the design.

[bernardnormier]

The comment is not clear.

Presumably, maximumPoolSize has not effect because the executor uses an unbounded queue? As written, it sounds like a consequence of the wait?

It's also not clear to me what this means / add:

The tasks would wait in the queue until a core pool thread is available to run it

This is not obvious?

[pepone]

If you look at when we use this executor there are two situations:

• We want to allow interrupt threads
• We are running on Android and need to avoid IO calls from UI thread

ice/java/src/com.zeroc.ice/src/main/java/com/zeroc/Ice/Instance.java

Lines 920 to 936 in e99d2e5

	//
	// If Ice.ThreadInterruptSafe is set or we're running on Android all
	// IO is done on the background thread. For Android we use the queue
	// executor as Android doesn't allow any network invocations on the main
	// thread even if the call is non-blocking.
	//
	if (properties.getIcePropertyAsInt("Ice.ThreadInterruptSafe") > 0 || Util.isAndroid()) {
	_queueExecutor =
	new QueueExecutor(
	properties, Util.createThreadName(properties, "Ice.BackgroundIO"));
	_queueExecutorService = new QueueExecutorService(_queueExecutor);
	// Caching message buffers is not supported with background IO.
	_cacheMessageBuffers = 0;
	} else {
	_cacheMessageBuffers = properties.getIcePropertyAsInt("Ice.CacheMessageBuffers");
	}

The interrupt case is about about ensuring that Java interrupt works correctly. See https://doc.zeroc.com/ice/latest/property-reference/miscellaneous-ice-properties#id-.MiscellaneousIce.*Propertiesv3.7-Ice.ThreadInterruptSafe

We have Ice/interrupt test in Java with seems to work fine with this PR.

In the android case this is about not calling IO APIs from the main thread, and unrelated to serialization.

I guess it would be safer to just change the behavior for Android, as this doesn't seems necessary for the other use case.