RFR: 8357650: ThreadSnapshot to take snapshot of thread for thread dumps

[Alan Bateman]

On Sun, 25 May 2025 13:11:41 GMT, Chen Liang <liach at openjdk.org> wrote:

>> This is first (hotspot) part of the update for `HotSpotDiagnosticMXBean.dumpThreads` and `jcmd Thread.dump_to_file` to include lock information in thread dumps (JDK-8356870).
>> The update has been split into parts to simplify reviewing.
>> The fix contains an implementation of `jdk.internal.vm.ThreadSnapshot` class to gather required information about a thread.
>> Second (dependent) part includes changes in `HotSpotDiagnosticMXBean.dumpThreads`/`jcmd Thread.dump_to_file`, spec updates and tests for the functionality.
>> 
>> Testing: new `HotSpotDiagnosticMXBean.dumpThreads`/`jcmd Thread.dump_to_file` functionality was tested in loom repo;
>>   sanity tier1 (this fix only)
>
> src/hotspot/share/classfile/javaClasses.cpp line 5519:
> 
>> 5517: oop java_util_concurrent_locks_AbstractOwnableSynchronizer::get_owner_threadObj(oop obj) {
>> 5518:   assert(_owner_offset != 0, "Must be initialized");
>> 5519:   return obj->obj_field_acquire(_owner_offset);
> 
> We might split this into a separate patch if this affects existing usages in threadService.cpp.

The pre-existing usage is from a VM operation so I don't think there is an existing issue. The new usage is in handshake operation and this field is on the ownable synchronizer rather than the thread so need to change it.

> src/java.base/share/classes/jdk/internal/vm/ThreadSnapshot.java line 33:
> 
>> 31:  * Represents a snapshot of information about a Thread.
>> 32:  */
>> 33: class ThreadSnapshot {
> 
> All class declarations here should be made final (except enum and record which are already implicitly final)

This is a JDK internal class, it doesn't really need to be final but it should have a private constructor to at least avoid something in this internal package from creating one directly.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25425#discussion_r2107753356
PR Review Comment: https://git.openjdk.org/jdk/pull/25425#discussion_r2107754207