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Assume also that there is a Bar application which depends on two libraries, lib1 and lib2. lib1 depends on commons-foo:commons-foo:1 and lib2 depends on commons-foo:commons-foo:2.

Classpath considerations

The Java classpath can contain multiple versions of the same classname (in different jars or directories). However, at most one version of a given class can be loaded into a single class-loader.

So if a class is updated such that its API is no longer backwards-compatible, it's not possible to use both in the same classpath. The only solution is to change the class name, e.g. by changing its package name - or one could change just the classname.
\[In both of these cases the code could be made compatible again by keeping the original code alongside the new. However, at some point the old code will probably need to be deleted, at which time the versions will be incompatible anyway.\]

If there are multiple versions of the same class on the classpath, there's no guarantee which version will be loaded, so in general the classpath should only ever contain a single version of each class.

Maven dependency resolution

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