Experimenting the addition of a MBean

[Martin Desruisseaux]

Hello all

Yesterday I added I feature that we didn't had before, which can be 
removed from compilation if considered undesirable. We did not had 
previously any monitoring tools about the state of a running instance of 
the library. We found that it was sometime problematic, since it make 
very hard to diagnostic problems on a server. I think there is numerous 
places where JMX MBeans could be used. For example report how many 
objects are cached, provide an option to clear the caches, detect if 
there is a problem in the running SIS instance (e.g. a daemon thread 
which unexpectedly died), etc.

So I added an experimental "Supervisor" MBean in the internal package. 
For now it contains only one method, "warnings()", which report any 
problem detected in the running SIS instance. The only problem reported 
at this time is unexpected death of the daemon thread consuming the weak 
references queue, but future versions could perform more extensive tests 
of SIS state. This MBean can been seen in JConsole (i.e. if an 
application using SIS is currently running, start "jconsole" in another 
window, select the application in the proposed list, click on "MBeans", 
select "org.apache.sis". The "warnings" operation can been seen).

The inconvenient is that SIS has to register its MBean to the 
PlatformMBeanServer (the same instance than the one managing the 
standard ClassLoadingJMXBean, MemoryJMXBean, GarbageCollectionJMXBean, 
etc.), and I have no idea if it put a significant burden. In particular 
I don't know if it trigs JMX class loading that would normally not 
happen. In my few tests, I didn't saw noticeable slowdown at startup time...

Any though?

    Martin