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On 30/12/2024 14:42, Aman Sharma wrote:<br>
<blockquote type="cite" cite="mid:4171b12b29df447d882513ff51a13cd7@kth.se">
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<p>Hi all,</p>
<p><br>
</p>
<p>I hope this is the correct email thread to ask a question
related to <span><a href="https://openjdk.org/jeps/451" target="_blank" rel="noopener noreferrer" class="x_OWAAutoLink" moz-do-not-send="true">JEP 451:
Prepare to Disallow the Dynamic Loading of Agents</a>.</span></p>
<p><span><br>
</span></p>
<p><span>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.</span></p>
<p><span><br>
</span></p>
<p><span>While looking at jstack, we noticed that it does not
trigger the warning "<span>WARNING: A {Java,JVM TI} agent
has been loaded dynamically</span>" 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
<a href="https://github.com/openjdk/jdk/blob/24c5ff7ba58cb7cf93df07f81484cd8fae60e31e/src/java.instrument/share/classes/sun/instrument/InstrumentationImpl.java#L107" target="_blank" rel="noopener noreferrer" class="x_OWAAutoLink" moz-do-not-send="true">
here</a> or <a href="https://github.com/openjdk/jdk/blob/24c5ff7ba58cb7cf93df07f81484cd8fae60e31e/src/hotspot/share/prims/jvmtiAgent.cpp#L521" target="_blank" rel="noopener noreferrer" class="x_OWAAutoLink" moz-do-not-send="true">
here</a>. Could anyone please tell us why a warning is not
shown when jstack is dynamically attaching itself?</span></p>
<br>
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</blockquote>
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.<br>
<br>
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.<br>
<br>
-Alan<br>
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