RFR: 8282641: Make jdb "threadgroup" command with no args reset the current threadgroup back to the default [v2]

[Kevin Walls]

On Fri, 4 Mar 2022 07:34:35 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> jdb has a probably very little used command called "threadgroup" which is used to set the current TheadGroup. The only purpose of the current ThreadGroup is as the default ThreadGroup to use for the "threads" command when no ThreadGroup argument is passed to it.
>> 
>> "threads" prints out every thread in the ThreadGroup specified as the first argument. If none is specified, it uses the current ThreadGroup. If the current ThreadGroup has not yet been specified, it automatically gets set to the top level ThreadGroup.
>> 
>> Once the current ThreadGroup has been set by using the threadgroup command, it's not that obvious how to reset it back to the default. It turns out the way to do this to set it to the "system" ThreadGroup, which is the top level ThreadGroup (and therefore the initial current ThreadGroup).
>> 
>> With this enhancement I've made it so if you use the "threadgroup" command with no argument, it resets the current ThreadGroup back to the top level ThreadGroup ("system"). Previously with no argument it produces an error for not specifying the ThreadGroup argument.
>
> Chris Plummer has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Use THREADGROUP_NAME instead of "MyThreadGroup#"
>  - fix minor typo in help text

Looks good.
(Choosing the null or top-level thread group means to show all threads in all thread groups, that makes sense though I didn't find if we say it anywhere.  A niche feature like you say, but good to have a way back from choosing a thread group!)

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Marked as reviewed by kevinw (Committer).

PR: https://git.openjdk.java.net/jdk/pull/7687