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

Jiangli Zhou jiangli.zhou at oracle.com
Mon Oct 22 17:57:34 UTC 2018

On 10/21/18 6:15 PM, Ioi Lam wrote:

> Re-sending to the correct mailing lists. Please disregard the other 
> email.
> 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.
This is for pre-resolving more constant pool references (to system 
classes only).

> Thanks
> - Ioi

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