Review Request JDK-8164512: Replace ClassLoader use of finalizer with phantom reference to unload native library

[mandy chung]

This patch proposes to replace the ClassLoader use of finalizer with 
phantom reference, specifically Cleaner, for unloading native 
libraries.  It registers the class loader for cleanup only if it's not a 
builtin class loader which will never be unloaded.

The spec for JNI_OnUnload [1] specifies that this function is called in 
an unknown context whereas the hotspot implementation uses the class 
loader being unloaded as the context, which I consider a bug.   It 
should not load a class defined to that class loader.  The proposed spec 
change for JNI_FindClass if called from JNI_OnUnload to use system class 
loader (consistent with no caller context case).

Webrev:
http://cr.openjdk.java.net/~mchung/jdk10/webrevs/8164512/webrev.00/

CSR:
    https://bugs.openjdk.java.net/browse/JDK-8187882

Please see the spec change an behavioral change in this CSR.  A native 
library may fail to be reloaded if it is loaded immediately or soon 
after a class loader is GC'ed but the cleaner thread doesn't yet have 
the chance to handle the unloading of the native library.  Likewise, 
there is no guarantee regarding the timing of finalization and a native 
library may fail to reload.  It's believed that the compatibility risk 
should be low.

I'm looking into adding a native jtreg test that will be added in the 
next revision.

Mandy
[1] 
http://docs.oracle.com/javase/9/docs/specs/jni/invocation.html#jni_onunload