RFR(S) 8212200 assert when shared java.lang.Object is redefined by JVMTI agent

[Ioi Lam]

http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v03/
https://bugs.openjdk.java.net/browse/JDK-8212200

Hi,

CDS has various built-in assumptions that classes loaded by
SystemDictionary::resolve_well_known_classes must not be replaced
by JVMTI ClassFileLoadHook during run time, including

- field offsets computed in JavaClasses::compute_offsets
- the layout of the strings objects in the shared strings table

The "well-known" classes can be replaced by ClassFileLoadHook only
when JvmtiExport::early_class_hook_env() is true. Therefore, the
fix is to disable CDS under this condition.

I have added a few test cases to try to replace shared classes,
including well-known classes and other classes. See
comments in ReplaceCriticalClasses.java for details.

As a clean up, I also renamed all use of "preloaded" in
the source code to "well-known". They refer to the same thing
in HotSpot, so there's no need to use 2 terms. Also, The word
"preloaded" is ambiguous -- it's unclear when "preloading" happens,
and could be confused with what CDS does during archive dump time.


 > In early e-mails Jiangli wrote:
 >
 > We should consider including more classes from the default classlist
 > in the test. Archived classes loaded during both 'early' phase and after
 > should be tested.

Done.


 > For future optimizations, we might want to prevent loading additional
 > shared classes if any of the archived system classes is changed.

What's the benefit of doing this? Today we already stop loading a shared
class if its super class was not loaded from the archive.


Thanks
- Ioi