Any native library calling JNI_FindClass from JNI_OnUnload? (original) (raw)

Mandy Chung mandy.chung at oracle.com
Sun Apr 13 02:48:42 UTC 2014

Does anyone know of a native library whose JNI_OnUnload calls JNI_FindClass?

I'm wondering how much existing code out there depends on the existing behavior that is unsupported.

During the investigation of JDK-4240589, I found that the implementation JNI_FindClass has a special handling that calls ClassLoader.NativeLibrary.getFromClass method to determine if JNI_FindClass is being called by JNI_OnLoad and JNI_OnUnload directly or indirectly. This special handling makes sense for JNI_OnLoad, i.e. when the native library is loaded, that may call JNI_FindClass.

I don't think this should apply to JNI_OnUnload. When a native library is being unloaded, it means that the ClassLoader has become unreachable and GC'ed. When JNI_OnUnloader, there is no guarantee that the class loader still exists for doing any class loading.

The JNI spec of JNI_OnUnload [1] also makes it clear:

The VM calls JNI_OnUnload when the class loader containing the native library is garbage collected. This function can be used to perform cleanup operations. Because this function is called in an unknown context (such as from a finalizer), the programmer should be conservative on using Java VM services, and refrain from arbitrary Java call-backs.

A little more background about this: The current ClassLoader uses finalizer to clean up and unload the native library, if any. I'm considering to replace the finalizer with phantom reference which is generally a good change and discover this VM/JNI/libs dependency.

Any data would be appreciated. Mandy [1] http://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNI_OnUnload

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