How does JVM deal with duplicate JARs of different versions

Question

When there were duplicate JARs with different versions, the behavior was very inconsistent. Does anyone know how the JVM deals with duplicates?

Answer 1

The classloader searches the classpath in order, and uses the first matching class it finds.

Answer 2

FWIW, this is an example of a larger topic (modularity) that is addressed by OSGi and Project Jigsaw/JSR 294 in JDK 7 Java 9.

Your question is good motivation for the topic: the venerable, simple classpath may have been a good idea at its inception, but it is certainly a pain point in today's age of high dependence on 3rd-party-libraries.