Can JNI be made to honour wildcard expansion in the classpath?

Question

I have a C binary that calls out to Java via JNI. I set CLASSPATH to somedir/* to pick up all the jars in somedir.

When I run the binary, a required class definition cannot be found. When I run

java that.class's.name 

from the same command line, the class is successfully found. If I explicitly add all the jars in somedir/ to the classpath, everything works great, but that leads to a very long classpath which I'd like to avoid.

Does a JVM executed via JNI honour wildcard expansion of the classpath? Can it be made to do so?

Answer 1

I figured out the answer by reading the hotspot source code.

Only paths passed via either CLASSPATH or -cp / -classpath are subject to wildcard expansion. These are then passed as a system property to the running JVM via -Djava.class.path.

You tell a JNI-invoked JVM about a classpath via a JVMOptions structure, which may include -Djava.class.path but -classpath will not necessarily be honoured (and in practice, isn't by the hotspot implementation). Since java.class.path is directly passed to the JVM as a system property, it doesn't get wildcard expanded and therefore wildcards won't work.

Answer 2

No. No, it cannot. Using JNI doesn't help.

The way you would do this is by implementing your own class loader (in Java), but that class loader would have to be in the wildcard-free CLASSPATH.

You could, of course, set the CLASSPATH to its expanded form before invoking the JVM. That would work and could be done via a shell script (no JNI needed).