RFR 8041565: JMX ObjectName could be refactored to save memory

[Jaroslav Bachorik]

Please, review the following change

Issue : https://bugs.openjdk.java.net/browse/JDK-8041565
Webrev: http://cr.openjdk.java.net/~jbachorik/8041565/webrev.00

In situations when there are 10s of thousands ObjectNname instances 
around (enterprise setups etc.) the 3 separate internal boolean fields 
can lead to a noticeable memory waste. Adding insult to the injury, with 
the current field layout it is necessary to align the instances by 4 bytes.

When using JOL (http://openjdk.java.net/projects/code-tools/jol/) to 
inspect the object layout we can see this:

Before optimization (JDK8u40):
---
javax.management.ObjectName object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
       0 12 (object header)| N/A
      12 4 int ObjectName._domain_length N/A
      16 1 boolean ObjectName._domain_pattern N/A
      17 1 boolean ObjectName._property_list_pattern N/A
      18 1 boolean ObjectName._property_value_pattern N/A
      19 1 (alignment/padding gap) N/A
      20 4 String ObjectName._canonicalName N/A
      24 4 Property[] ObjectName._kp_array N/A
      28 4 Property[] ObjectName._ca_array N/A
      32 4 Map ObjectName._propertyList N/A
      36 4 (loss due to the next object alignment)
Instance size: 40 bytes (estimated, the sample instance is not available)
Space losses: 1 bytes internal + 4 bytes external = 5 bytes total
{noformat}

After optimization (JDK9 internal build):
---

javax.management.ObjectName object internals:
  OFFSET SIZE TYPE DESCRIPTION VALUE
       0 12 (object header) N/A
      12 2 short ObjectName._domain_length N/A
      14 1 byte ObjectName._pattern_flag N/A
      15 1 (alignment/padding gap) N/A
      16 4 String ObjectName._canonicalName N/A
      20 4 Property[] ObjectName._kp_array N/A
      24 4 Property[] ObjectName._ca_array N/A
      28 4 Map ObjectName._propertyList N/A
Instance size: 32 bytes (estimated, the sample instance is not available)
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total


After optimization we can save 8 bytes per instance which can translate 
to very interesting numbers on large installations.


To achieve this the domain name length is set to be *short* instead of 
*int* and the three booleans kept for the performance purposes are 
encoded into one byte value (as proposed by the reporter, Jean-Francois 
Denise).

All the regression and JCK tests are passing after this change.


Thanks,

-JB-