RFR: 8357828: Add a timestamp to jcmd diagnostic commands

[Thomas Stuefe]

On Thu, 18 Sep 2025 14:57:44 GMT, Ivan Bereziuk <duke at openjdk.org> wrote:

> `jcmd` provides great diagnostics but many commands lack a timestamp in their output.
> Adding a timestamp to the output of some would add value for those debugging JVM data.
> 
> Some diagnostic commands already provide timestamps. For example `Thread.print` already prints one of "yyyy-MM-dd HH:mm:ss" format.
> 
> Adding timestamps to all diagnostic `jcmd` commands with a non-throw-away STDOUT.
> 
> The exceptions are:
> * `VM.uptime` - command run with `-date` argument will also print a timestamp;
> * `VM.system_properties` - already lists timestamp
> * `Thread.dump_to_file` - the content dumped to file already has a timestamp;
> * `VM.version`

Piling on...

I think this feature would be useful. I wished for this more often than not.

I think the printing should be done on the server side, not on the client side (be it a `date` before the invocation of jcmd, or jcmd itself printing the date), since the timestamp on the server side may be different than that of the client jcmd.

I would prefer a timestamp in the UL "time" decoration format: Full ISO-8601, including timezone offset. See https://openjdk.org/jeps/158 . That would also keep in line with gc logs etc.

I am not sure we need a specific option to disable or enable this. Who would want to disable this feature?

-------------

PR Comment: https://git.openjdk.org/jdk/pull/27368#issuecomment-3323234159