RFR: 8275259: Add support for Java level DCmd

[Chris Plummer]

On 10/14/21 12:38 AM, David Holmes wrote:
> On 14/10/2021 5:14 pm, Denghui Dong wrote:
>> On Thu, 14 Oct 2021 06:43:20 GMT, David Holmes 
>> <david.holmes at oracle.com> wrote:
>>
>>> I'm not sure exactly where this API would need to go. IIUC jcmd doesn't
>> exist at the Java level it is just a tool, so introducing an API to
>> interact with it seems problematic to me.
>>
>> IMO, `jcmd` is a client of `dcmd`, the user also can use JMX to 
>> execute `dcmd`, this means there already are Java APIs interact with 
>> `dcmd`.
>
> Ah right - so you're really looking at extending the capabilities of 
> the DiagnosticCommandMBean to add a way to register a Java diagnostic 
> command.
>
> David

Just reviewing some previous emails and noticed this one since I 
recently looked into the JMX support. JMX supports a rather generic 
concept of executing diagnostics commands (there is no actual reference 
to "dcmds"). See com.sun.management.DiagnosticCommandMBean and its 
superinterface javax.management.DynamicMBean:

https://docs.oracle.com/en/java/javase/15/docs/api/jdk.management/com/sun/management/DiagnosticCommandMBean.html
https://docs.oracle.com/en/java/javase/15/docs/api/java.management/javax/management/DynamicMBean.html

DynamicMBean provides the invoke() interface, which when used on a 
DiagnosticCommandMBean will invoke the specified diagnostic command. It 
says nothing about how diagnostic commands are implemented or registered 
with the jvm. That I assume would be JVM implementation dependent.

> Ah right - so you're really looking at extending the capabilities of 
> the DiagnosticCommandMBean to add a way to register a Java diagnostic 
> command. 
Yes to "add a way to register a Java diagnostic command", but the new 
API will not involve DiagnosticCommandMBean in any way. The new API will 
interface with the existing hotspot native support for dcmds to allow 
dcmds to be implemented by and registered from java.

In any case, I think having an API that allows you to invoke a 
diagnostic command from java is very different in scope from an API that 
allows you to implement one in java, so I'm not so sure this JMX 
precedence serves us much purpose here.

Chris
>
>> Denghui
>>
>> -------------
>>
>> PR: https://git.openjdk.java.net/jdk/pull/5938
>>