How can I Java webstart multiple, dependent, native libraries?

Question

Example: I have two shared objects (same should apply to .dlls). The first shared object is from a third-party library, we'll call it libA.so. I have wrapped some of this with JNI and created my own library, libB.so. Now libB depends on libA.

When webstarting, both libraries are places in some webstart working area. My java code attempts to load libB. At this point the system loader will attempt to load libA which is not in the system library path (java.library.path won't help this). The end result is that libB has an unsatisfied link and cannot be used.

I have tried loading libA before libB, but that still does not work. Seems the OS wants to do that loading for me. Is there any way I can make this work other than statically compiling?

Answer 1

I'm not sure if this would be handled exactly the same way for webstart, but we ran into this situation in a desktop application when dealing with a set of native libraries (dlls in our case).

Loading libA before libB should work, unless one of those libraries has a dependency that is unaccounted for and not in the path. My understanding is that once it gets to a system loadLibrary call (i.e. Java has found the library in its java.library.path and is now telling the OS to load it) - it is completely dependent on the operating system to find any dependent libraries, because at that point it is the operating system that is loading the library for the process, and the OS only knows how to look in the system path. That seems hard to set in the case of a Webstart app, but there is a way around this that does not involve static compiling. You may be able to shuffle where your libraries are - I am unsure

If you use a custom classloader, you can override loadLibrary and findLibrary so that it can locate your libraries from within a jar in your classpath, and if you also make it aware of your native library dependencies (i.e. libB depends on libA depends on libX, then when loading libB you can catch yourself and ensure you load libA first, and in checking that notice and load libX first. Then the OS doesn't try to find a library that isn't in your path. It's klunky and a bit painful, but ensuring Java finds them and loads them all in the correct order can work.

Answer 2

Static compilation proved to be the only way to webstart multiple dependent native libraries.