Loading Issues with Multiple Class Loaders

[michaelahlers]

Particularly when working in hosted environments (like IDEs), where different class loaders may be used between one run and the next, developers will encounter errors like the following:

java.lang.UnsatisfiedLinkError: Native Library /private/var/folders/d1/3323g01s4bn3ntx1g_yzm_dc0000gn/T/1621396069368823549/nu/pattern/opencv/osx/x86_64/libopencv_java249.dylib already loaded in another classloader
    at java.lang.ClassLoader.loadLibrary0(test.sc5011658132463706269.tmp:1891)
    at java.lang.ClassLoader.loadLibrary(test.sc5011658132463706269.tmp:1843)
    at java.lang.Runtime.loadLibrary0(test.sc5011658132463706269.tmp:866)
    at java.lang.System.loadLibrary(test.sc5011658132463706269.tmp:1115)
    at nu.pattern.OpenCV$Loader.loadLibrary(test.sc5011658132463706269.tmp:221)
    at nu.pattern.OpenCV$Loader.access$100(test.sc5011658132463706269.tmp:156)
    at nu.pattern.OpenCV.loadLibrary(test.sc5011658132463706269.tmp:153)
    at #worksheet#.get$$instance$$res1(test.sc5011658132463706269.tmp:5)
    at #worksheet#.#worksheet#(test.sc5011658132463706269.tmp:27)

[michaelahlers]

Turns out this is a common problem, especially when using SBT.

[michaelahlers]

See the JNI 1.2 specification.

[michaelahlers]

If sys_paths is not set to null, and the library is loaded by absolute path, this problem goes away. Comments by Holger Hoffstätte on a similar topic suggest stale class loaders are not garbage collected—possibly due to the reflection hack.

[michaelahlers]

User Gili offers a useful class for debugging these issues on Stack Overflow.

[michaelahlers]

Ultimately, there are no good solutions—only partial workarounds. There are now two loader methods on nu.pattern.OpenCV. Both are now safe to call spuriously, and give developers a choice between adding the native binary to java.library.path at runtime and giving “shared” access, or forcibly loading a different copy of the library (which is not blocked by guards against the static ClassLoader.loadedLibraryNames collection).