RFR: 8318707: Remove the Java Management Extension (JMX) Management Applet (m-let) feature [v3]

[Kevin Walls]

On Wed, 10 Jan 2024 17:06:01 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

>> Checking on the original problem noted in the test, looks like it wasn't really about ClassLoaders and MLets:
>> JDK-4909536: Standard MBean introspector keeps reference to last class introspected
>> This is what the test still tests.
>> Do we see value in having the MBean be a ClassLoader and checking if such MBeans are held on to...? 8-)
>
> Yes, that's the whole point of the test. Without using an MBean which is a ClassLoader the fix cannot be tested.
> 1. you need to create an MBean which is an URLClassLoader, and it needs to be a PrivateClassLoader so that it's not added to the ClassLoaderRepository - let's call it  LoaderMBean.
> 2. you need to use that LoaderMBean to load another MBean - let's call it TestMBean. The concrete class of TestMBean needs to be loaded by the LoaderMBean - that is - LoaderMBean needs to be the defining ClassLoader for that class (the TestMBean class must not be loaded by the System/Application ClassLoader).
> 3. In the MBeanServer you then end up with two MBeans, one is the LoaderMBean, the other is the TestMBean whose class has been loaded by the LoaderMBean.
> 4. you then keep a weak reference on the _LoaderMBean_, and unregister both MBeans.
> 5. at that point - if the Introspector still has a strong reference to the class of the TestMBean, then the _LoaderMBean WeakReference_ will never be enqueued, because the class of the TestMBean will have a strong reference to its ClassLoader, which is _LoaderMBean_.
> 
> So you can only test that if you have an MBean which is loaded by a custom ClassLoader, because you want to verify that the Introspector doesn't hold on that ClassLoader (through the TestMBean class).

Hi Daniel -
I am missing see how being a ClassLoader relates to the leak where the Introspector holds on to a reference to the most recent MBean.  I didn't locate the jdk5 change, I only read a short synopsis.

I can make this test create an MBean which is a ClassLoader, and the test can do its check that having the loader MBean load a class, creates a distinct class.  I can add that update here.  But it looks like an MLet test which leaked into the Introspector test. 8-)

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PR Review Comment: https://git.openjdk.org/jdk/pull/16363#discussion_r1448708811