All JARs in $pkg/share/java (for each $pkg in the build inputs) are
added to $CLASSPATH. Thus, you can say
buildInputs = [ setJavaClassPath someJavaDependency ];
and the JARs in someJavaDependency will be found automatically by
tools like javac or ant.
Note that the manual used to say that JARs should be installed in
lib/java; this is now share/java, following the Debian policy:
http://www.debian.org/doc/packaging-manuals/java-policy/x110.html
The directory share/java makes more sense because JARs are
architecture-independent. (Also, a quick grep shows that we were not
exactly consistent about this in Nixpkgs.)
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