RFR: 8357828: Add a timestamp to jcmd diagnostic commands

[Kevin Walls]

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`

Hi, that sounds promising.  
I do think we should state the benefit of having this (we don't expect much latency as this is a local attach).

Passing something on to the native impl is interesting - then, if the jcmd launcher prints pid:, then timestamp on a newline, can Thread.print find out a timestamp is already printed, and not duplicate it?

3975026:
2025-09-23 10:31:54
Full thread dump ...

Or, should Thread.print stop printing a timestamp, and rely on the launcher to do it.  That would avoid that communication, but would mean we have to default to timestamps ON, or Thread.print output format will change (which is not completely illegal, but will no doubt surprise some people/scripts).  Or, have the two sides communicate and Thread.print prints the timestamp, if it has not been shown already.  Nothing there is as simple as we'd like. 8-)

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PR Comment: https://git.openjdk.org/jdk/pull/27368#issuecomment-3323188843