What are the implications of having duplicate classes in java jar?

Question

I am building java jar file using ant. I need to include additional jars using "zipfileset src="xxx.jar" "zipfileset src="yyy.jar" and both xxx.jar and yyy.jar have the classes with the SAME fully-qualified class names. So the resulting jar file has duplicate class names. What are the possible implications of having duplicates?

Thank you.

Answer 1

If they're duplicate implementations, nothing–it wouldn't matter which is loaded.

If not, you're at the mercy of class load order, and may get a different version than you want.

It is specified that classpath entries will be searched in the order listed (as per this classpath doc). but that's only relevant if you're in complete control of classpath creation (unlike in a web app, for example).

(With the caveat that classpath wildcarding makes the order non-deterministic.)

Answer 2

In general this situatioin is highly non recommended and should be avoided.

Jars in java are just containers for your class files. java uses classloaders that look at the classpath and load class files from there. so if you have 2 jars A.jar and B.jar that have the same class x.y.Foo inside, the class from the jar that comes first in the classpath will be loaded. So, if your classpath is A.jar,B.jar (in this order) the class Foo from A.jar will be used in runtime. This inconsistency can lead to very hard-to-fix bugs from my experience