(Disclaimer: I am not too familiar with the internals of Mockito; I just
mentioned it as prominent example. Though it seems to be similar to my
usecase, and the use case of other projects.)
How often is appendToBootstrapClassLoaderSearch called in a single run?
My understanding is that it is called once per complete run of the unit
tests. However, that also applies to single test method / class runs
from the IDE during debugging. So while debugging and writing tests for
a project which uses Mockito, that might add up.
Is the thing that execute the VM with -javaagent in a script or POM
configuration?
Generally Mockito is just a test library you declare; it is (to my
understanding) agnostic about what build tool is used. Though I assume
you are asking in the context of JEP 451 ("Prepare to Disallow the
Dynamic Loading of Agents"). For Java 21+ Mockito indeed recommends
using `-javaagent` [1].
If the agent is create a temporary JAR file to put on the boot class
path then I'm wondering why it can't be given its own directory to do
this, and then cleanup after the test run is complete.
I guess that might work, but it would have to be the build tool which
sets up that directory and passes it to the agent through `-javaagent`.
Maybe that would also allow it to be non-temp (respectively not worry
much about concurrency) when using the `target` directory for the
project, defined by the build tool.
That could indeed be a solution, though I am not sure if that would be
desirable for all agent implementations since it would involve
(slightly) more configuration work for users.
[1]
https://javadoc.io/doc/org.mockito/mockito-core/latest/org.mockito/org/mockito/Mockito.html#mockito-instrumentation
