Hello Ioi,
On 21/10/21 9:59 pm, Ioi Lam wrote:
On 10/21/21 5:25 AM, Alan Bateman wrote:
On 21/10/2021 10:49, Jaikiran Pai wrote:
:
Digging into it, it appears that since the ModuleDescriptor#equals()
calls equals() on enum types (in this specific case on
ModuleDescriptor.Requires.Modifier) and since enum type equality is
implemented as identity checks, those identity checks are
surprisingly failing. More specifically
ModuleDescriptor.Requires.Modifier.MANDATED ==
ModuleDescriptor.Requires.Modifier.MANDATED is equating to false
because at runtime I see that two different instances of
ModuleDescriptor.Requires.Modifier.MANDATED have been loaded (by the
same boot module classloader). Although I use
ModuleDescriptor.Requires.Modifier.MANDATED as an example, the same
is reproducible with other enum values like
ModuleDescriptor.Requires.Modifier.TRANSITIVE.
This appears to be specific to CDS since running the above program
with:
java -Xshare:off EnumEquality
succeeds and the ModuleDescriptor equality check passes.
In short, it looks like there is some general issue with CDS and
equality checks with enums and perhaps deserves a separate JBS issue?
I've asked Ioi Lam to comment on this, off-hand I'm not aware of any
issues with CDS here but it may be related to the archiving of object
graphs.
-Alan
Hi Jaikiran and Alan,
Thanks for reporting this issue. It's a bug in CDS. I have filed
https://bugs.openjdk.java.net/browse/JDK-8275731 and am working on a fix.
Thank you for looking into this.
This is my initial analysis of the problem.
====>>> Can anyone think of similar problems that may happen elsewhere?
The static constructors of enum classes are executed at both CDS dump
time and run time. E.g.,
public enum Modifier {
OPEN
}
The <clinit> method essentially does this:
public static final Modifier OPEN = new Modifier("OPEN");
If a reference of Modifier.OPEN is stored inside the CDS archived heap
during dump time, it will be different than the value of Modifier.OPEN
that is re-created at runtime by the execution of Modifier.<clinit>
I have almost next to nothing knowledge about CDS internals. My only
understanding of it is based on some documentation that I have read. One
of them being this one
https://docs.oracle.com/en/java/javase/17/vm/class-data-sharing.html#GUID-7EAA3411-8CF0-4D19-BD05-DF5E1780AA91.
Based on that documentation (and other similar ones), it was my
understanding that CDS was meant to store/share class "metadata" like it
states in that doc:
"When the JVM starts, the shared archive is memory-mapped to allow
sharing of read-only JVM metadata for these classes among multiple JVM
processes."
But from what you explain in that enum example, it looks like it also
stores class instance data that is computed at build time on the host
where the JDK image was generated? Did I understand it correctly? Is
this only for enums or does it also store the static initialization data
of "class" types too? If it does store the static init data of class
types too, then wouldn't such data be host/build time specific and as
such the classes that need to be enrolled into the default CDS archive
of the JDK should be very selective (by reviewing what they do in their
static init)? Like I said, I haven't looked into this in detail so
perhaps it already is selective in the JDK build?
-Jaikiran
