Sun, in their infinite wisdom, circa Java 1.3, extended the jar file specification to allow dependencies to be expressed inside jar files by adding a Class-Path attribute to the MANIFEST.MF file. This causes the JVM (and javac, and various app servers) to try to magically add dependent jar files to the classpath. This is half-assed and wrong in too many ways to enumerate here.

Some other enthusiastic Sun engineer then helpfully added activation.jar to mail.jar's Class-Path attribute, under the multiple misguided assumptions that no one would ever possibly need to use mail.jar without also having activation.jar in their classpath, with precisely that name, and located in precisely the same directory. Great.

Maven then upped the ante on this little fiasco by deciding that any time the Java compiler generates a warning that they can't parse, they should fail the build. Clearly it's critical that your build system be conversant in every possible warning that might be emitted by your compiler. As a result, when I fix their boneheaded decision to suppress warnings by default, my build now fails with this demonstration of awesomeness:

could not parse error message: warning: [path] bad path element
"/home/mdb/.m2/repository/javax/mail/mail/1.4.1/activation.jar": no such file or directory

Thank you Sun, and thank you Maven.

For those of you arriving from the Googles and looking for a solution to this problem, it is as follows: pass -Xlint:-path to javac to prevent it from issuing a warning when it cannot find activation.jar. If you will be passing this compiler argument via Maven, be sure to read Why Maven Sucks: Act I before doing so, to avoid the various pitfalls that await.

I should further mention the irony that activation.jar is listed as a dependency of mail.jar, but it's being placed in .m2/repository/javax/activation/activation/1.1/activation-1.1.jar which is where Maven wants it and not where the utility deficient Class-Path mechanism expects it to be.