I have a virtual machine where I do some testing with Errai at home and today I wanted to use the MonitorExtension. To my surprise the MonitorExtension fails to load with an IllegalArgumentException.
So far I have narrowed it down to ActivityProcessor line 34 where a new ThreadPoolExecutor is created. The instantiation requires a minimum of 2 threads (corePoolSize) and provides the result of Runtime.getRuntime().availableProcessors() as the argument to maximumPoolSize.
In my case I only have a single CPU configured for my virtual machine making the ThreadPoolExecutor throw the exception as corePoolSize is now larger than maximumPoolSize.
I can work around this by reconfiguring my VM to use 2 CPU's but I figured that it would probably be better to change the code to handle 1 CPU.
(Even if it is just by doing new ThreadPoolExecutor(2, (Runtime.getRuntime().availableProcessors() >= 2 ? Runtime.getRuntime().availableProcessors() : 2), ...);)
Hmmm. Okay. I will open a JIRA to make this change. Good catch!