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Showing content from https://github.com/bytedeco/javacpp/issues/737 below:

Potential class loading deadlock when call static methods of Loader or Pointer · Issue #737 · bytedeco/javacpp · GitHub

We observed a problem when calling a utility function like Loader.isLoadLibraries (which calls Loader.load() when this is the first access) in one thread while running e.g. Pointer.maxBytes() in another thread:

// thread 2
public class PrintMaxBytes implements Runnable {
	@Override
	public void run() {
		long l = org.bytedeco.javacpp.Pointer.maxBytes();
		System.out.println(l);
	}
}
var t = new Thread(new PrintMaxBytes ());
t.start();

// thread 1(calls Loader.load())
Loader.isLoadLibraries();

• let thread 2 start class loading / static init block in Pointer up until, BUT excluding Pointer line 521.
• then let thread 1 run to execute the real Loader.load(); triggered by Loader.isLoadLibraries()
• -> deadlock!

The reason seems to be that in the JNI code there is

jclass cls = (jclass)env->CallStaticObjectMethodA(JavaCPP_getClass(env, 0), putMemberOffsetMID, args);

which basically calls the static method Loader.putMemberOffset and by doing so triggers another call to Loader.load() for which the other thread already has a lock.

In our case we solved the issued by doing e.g. new Pointer(0) in an early part of the code where we are sure we still have only one thread.

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