Why does jstack not trigger a warning in target JVM >= 21?

[Alan Bateman]

On 30/12/2024 14:42, Aman Sharma wrote:
>
> Hi all,
>
>
> I hope this is the correct email thread to ask a question related to 
> JEP 451: Prepare to Disallow the Dynamic Loading of Agents 
> <https://openjdk.org/jeps/451>.
>
>
> For some context, we at KTH are trying to build a serviceability tool 
> that would monitor dependencies that are running in the target JVM. We 
> looked at jstack for inspiration as it is able to return the stack 
> trace of the target JVM, but we want classes instead and then we can 
> get dependencies as we have annotated each class with dependency 
> information.
>
>
> While looking at jstack, we noticed that it does not trigger the 
> warning "WARNING: A {Java,JVM TI} agent has been loaded dynamically" 
> when the target JVM is 21. However, when we try to build our own agent 
> using the attach API, we get the warning exactly how it says in the 
> JEP. I initially thought that an exception was made for jstack in the 
> open JDK code but I could find it either here 
> <https://github.com/openjdk/jdk/blob/24c5ff7ba58cb7cf93df07f81484cd8fae60e31e/src/java.instrument/share/classes/sun/instrument/InstrumentationImpl.java#L107> 
> or here 
> <https://github.com/openjdk/jdk/blob/24c5ff7ba58cb7cf93df07f81484cd8fae60e31e/src/hotspot/share/prims/jvmtiAgent.cpp#L521>. 
> Could anyone please tell us why a warning is not shown when jstack is 
> dynamically attaching itself?
>
>
Attaching to a running VM to run diagnostic commands doesn't trigger a 
warning. The warning is limited to cases where code (in the form of a 
JVMTI agent in a shared library, or a Java agent in an agent JAR file) 
is loaded into a running VM without opt-in on the command line.

It might be that some of of the existing jcmd commands might give you 
inspiration. While most of the diagnostic commands in written in C++, 
there are a number of commands implemented in Java that may be closer to 
what you are looking for.

-Alan
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