Null ContextClassLoader in new threads

[audetto]

I am facing an issue whereby the ClassLoader seems unable to load classes in separate threads.

This is the error I get
SEVERE: MASM0010: Unable to unmarshall metro config file from location [ jar:file:/E:/dev/code/java-lib/someapi.java8-12.0.7773.jar!/META-INF/jaxws-tubes-default.xml ]

It is not linked to a particular java version, happens in 8, 11 and 14. It started happening when I upgraded a 3rd party lib, which I have little control over.
I am using jnius 1.3 in python 3.6.6 in Windows 7 64 bit.

After a lot of debugging I noticed that

jnius.autoclass('java.lang.Thread').currentThread().getContextClassLoader()

returns None in all new threads. And this explains why it cannot find the file which is indeed present in the jar. Maybe the 3rd party lib has changed the exact dynamic of class loading and now does it in each new thread.

One way to fix this is to mirror the Android wrapper, https://github.com/kivy/pyjnius/blob/master/jnius/__init__.py#L76:

import threading
import jnius
 
classloader = jnius.autoclass('java.lang.ClassLoader').getSystemClassLoader()
orig_thread_run = threading.Thread.run
def jnius_thread_hook(*args, **kwargs):
    jnius.autoclass('java.lang.Thread').currentThread().setContextClassLoader(classloader)
    return orig_thread_run(*args, **kwargs)

threading.Thread.run = jnius_thread_hook

but I really think I am missing something here. Does jnius require any particular setup to work in multiple threads?

[audetto]

These are probably relevant:
https://stackoverflow.com/questions/18354535/why-do-subsequent-jni-threads-return-null-on-currentthread-getclassloader/29194820

and

https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/invocation.html

around Attaching to the VM, it says

Should another thread need to access the Java VM, it must first call AttachCurrentThread() to attach itself to the VM and obtain a JNI interface pointer. Once attached to the VM, a native thread works just like an ordinary Java thread running inside a native method. The native thread remains attached to the VM until it calls DetachCurrentThread() to detach itself.

and below

When a thread is attached to the VM, the context class loader is the bootstrap loader.

I can see that jnus uses AttachCurrentThread:

pyjnius/jnius/jnius_env.pxi

Line 17 in 88212e2

# XXX it threads are created from C (not java), we'll leak here.

and the comment is a bit worrying.

What is the exact status of jnius with python-created threads?

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