RFR: 8357828: Add a timestamp to jcmd diagnostic commands

[Alan Bateman]

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 timestamp in a "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`

src/hotspot/share/services/diagnosticCommand.cpp line 1093:

> 1091: 
> 1092: void ThreadDumpToFileDCmd::execute(DCmdSource source, TRAPS) {
> 1093:   print_local_time(output());

I assume you didn't mean to print a time stamp here as the output already has a timestamp. Also for -format=json and sending the output to stdout, then it will be unparsable if a timestamp appears before the JSON object.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27368#discussion_r2361986465