RFR: 8316417: ObjectMonitorIterator does not return the most recent monitor and is incorrect if no monitors exists [v6]

[Chris Plummer]

On Thu, 21 Sep 2023 06:21:25 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> ObjectMonitorIterator fails to return the most resent monitor added. It start with returning the `nextOM()` ObjectMonitor from the `_head` ObjectMonitor but fails to ever return the `_head` ObjectMonitor.
>> The current implementation can also not handle that the `_head` is nullptr (no monitors in the system) and returns a null ObjectMonitorIterator. Which is interpreted as `monitor list not supported, too old hotspot VM`.
>> 
>> Changed the iterator to keep return the current monitor (starts with `_head`) and decoupled `_head == nullptr` from the question if ObjectMonitorIterator is supported. 
>> 
>> Testing:
>>   * Passes all `serviceability/sa` tests
>>   * Passed tier 1-5
>>   * Passed GHA
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update copyright year

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/ui/MonitorCacheDumpPanel.java line 101:

> 99:       tty.println("You need 1.4.0_04, 1.4.1_01 or later versions");
> 100:       return;
> 101:     }

This code is called when you choose the hsdb "Tools -> Monitor Cache Dump" menu item. Have you tried doing that? I tried running it against a clhsdb process, and currently it doesn't seem to show any monitors. Does it show one after your fix?

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PR Review Comment: https://git.openjdk.org/jdk/pull/15782#discussion_r1335978134