That seems an ideal solution. I think there are some potential code
consolidation work further. With this change, some existing C++ JFR Jcmd
structure definitions(and other Jcmd commands) in VM level can also be lifted
to Java level because they simply forward request to Java level by
JavaCalls::call_static.
------------------------------------------------------------------
From:Ioi Lam <ioi....@oracle.com>
Send Time:2021 Oct. 8 (Fri.) 15:22
To:David Holmes <david.hol...@oracle.com>; dong denghui
<denghui....@alibaba-inc.com>; serviceability-dev
<serviceability-dev@openjdk.java.net>; hotspot-runtime-...@openjdk.java.net
<hotspot-runtime-...@openjdk.java.net>
Subject:Re: Extend jcmd to java application level
On 10/7/21 6:25 PM, David Holmes wrote:
> Hi Denghui,
>
> On 7/10/2021 11:58 pm, Denghui Dong wrote:
>> Hi team,
>>
>> The `jcmd` command can be used to call some built-in diagnostic commands in
>> vm.
>>
>> Can we consider extending it to the java layer like perf data, so that Java
>> developers can
>>
>> customize their diagnostic commands and then call them through `jcmd`?
>>
>> One application scenario I can think of for this extension is that some
>> statistical information
>>
>> may be collected in a java application. Triggering the output of this
>> statistical information through
>>
>> the `jcmd` command seems to me relative to other mechanisms that trigger
>> output (such as through
>>
>> an HTTP service, or periodic Printing) is more convenient.
>>
>> Any input is appreciated.
>
> If the intent is that you could issue a jcmd:
>
> jcmd <vmid> MyClass.foo
>
> to have it run/use a Java thread to execute arbitrary code then I
> think a lot of careful consideration would need to be given to making
> this facility safe and secure. I can easily imagine that the thread
> used, and the timing, could cause failures. Executing arbitrary code
> may be far too general, so it might mean we need a new "service"
> interface defined that the target class has to implement.
>
> It might well be useful but will need some deep thought IMO.
>
If I understood correctly, the app would need to call an API like:
JcmdProvider.register("mycmd1", new JcmdHandler() {
public void handleCommand(String args[], PrintStream out) {
out.print("my response is ");
... }
});
and then the user can issue:
jcmd <pid> mycmd1 args .....
which will reach the handleCommand() method provided by the app.
Thanks
- Ioi
> Cheers,
> David
>
>> Thanks,
>> Denghui Dong
