RFR: 8284161: Implementation of Virtual Threads (Preview) [v3]

[Alan Bateman]

On Sun, 17 Apr 2022 12:39:24 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

> I had a look at the referenced [JDK-4006245](https://bugs.openjdk.java.net/browse/JDK-4006245). The previous/existing reference cleaning logic seems to have been added mainly to `null` the reference to the user/application class instance represented by the `Runnable` task. Of course, other references too were being cleared. The description in the linked JBS seems to suggest that the `Thread` instance wasn't being GCed sooner. That was more than 2 decades back. I don't have enough knowledge of this area to know if this is still an issue in recent GC implementations.

The comment on that method dates from early JDK releases with the Classic VM and its conservative GC. It also predates the java.lang.ref API.  The question that I think you are asking is if you create a Thread with a runnable task, let it terminate but keep the reference to the Thread forever then what does it keep alive. We can think about more but so far we haven't seen any issues or tests fail with the reference to the runnable task.

> src/java.base/share/classes/java/lang/Thread.java line 2414:
> 
>> 2412:             sm.checkPermission(new RuntimePermission("setContextClassLoader"));
>> 2413:         }
>> 2414:         if (!isSupportedClassLoader(contextClassLoader)) {
> 
> For a few moments, I thought this was a typo and should instead have been:
> 
> if (!isSupportedClassLoader(cl)) {
> 
> but then thinking a bit more, I realized what's being done here. Essentially, this checks whether the _current_ context classloader  is set to "not allowed to change/set context classloader" (represented by `Constants.NOT_SUPPORTED_CLASSLOADER`).
> 
> Do you think this line deserves some short code comment that it intentionally passes the current contextClassLoader (and not the one passed to the method) to `isSupportedClassLoader`?

I think the comment on isSupportedClassLoader is readable so I think it's okay.

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PR: https://git.openjdk.java.net/jdk/pull/8166