RFR: 8203629: Produce events in the JDK without a dependency on jdk.jfr

[Erik Gahlin]

Hi,

Could I have a review of an enhancement that will make it possible to 
add JFR events to java.base, and possibly other modules in the JDK, 
without a compile time dependency on jdk.jfr.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8203629

Webrev:
http://cr.openjdk.java.net/~egahlin/8203629.0

Testing:
Tests in test/jdk/jdk/jfr

The functionality is a prerequisite for an upcoming enhancement that 
will add JFR events to the security libraries.

In short, jdk.jfr.Event (located in jdk.jfr) extends 
jdk.internal.event.Event (located in java.base). Metadata for events, 
such as labels and descriptions, are added using a mirror event class 
located in jdk.jfr module. If the fields of the mirror class and the 
event class don't match an InternalError is thrown.

To illustrate how the mechanism can be used, see the following example 
(not meant to be checked in).

http://cr.openjdk.java.net/~egahlin/8203629.example

Since the implementation of jdk.internal.event.Event is empty, the JIT 
will typically eliminate all traces of JFR functionality unless Flight 
Recorder is enabled.

Thanks
Erik