JOL object header information

[Rafael Winterhalter]

Hello everybody,

first of all, thank you for this great tool, it helped me a lot to
understand some cache line issues. Keep up the good work!

After using JOL for looking into field layouts I became curious about
HotSpot's mark word which is referred to in the examples on
http://hg.openjdk.java.net/code-tools/jol/file/tip/jol-samples/src/main/java/org/openjdk/jol/samples/

Unfortunately, I stumbled when trying to interpret the mark word's contents
of which I found a description in the commentary to this class:
http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/8389681cd7b1/src/share/vm/oops/markOop.hpp

To be certain, I wrote a minor extension to JOL which allows to print more
detailed header information by something like:

ClassLayout layout = ClassLayout.parseClass(Foo.class).withDetailedHeader();

where each header value is then mapped to its content in JOL's tabular
overview instead of just printing "(object header)". My (minor)
modification can found here:
https://gist.github.com/raphw/7345bae62abd424fb040

However, it seems like the layout JOL retrieves diverges from the layout
that is described in markOop.hpp - As one exemplary divergence, I found
that the identity hash code seems to reach one bit further than the
described 31 bit, e.g. when running

ClassLayout layout =
ClassLayout.parseClass(Object.class).withDetailedHeader();
Object foo = new Object();
System.out.println(Integer.toHexValue(foo.hashCode()));
System.out.println(layout.toPrintable(foo));

the hex code often is one bit longer than the 31 bit that are specified in
markOop.hpp - also, I cannot observe that the biased lock flag is set for
the biased lock example.

I understand that this mailing list is not a support forum for a potential
beginner's problem when trying to learn more about HotSpot, I would still
really appreciate an answer. Furthermore, I was wondering you would
consider adding a similar feature for a detailed view of an instance's
header to JOL. I am sure, others would benefit from the extended output
when going through the examples that concern locking and hash code
computation.

Thank you for your time and best regards,
Rafael