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
