This is a real doozie - when compiling a Spark assembly with JDK7, the produced jar does not work well with JRE6. I confirmed the byte code being produced is JDK 6 compatible (major version 50). What happens is that, silently, the JRE will not load any class files from the assembled jar.
I also noticed that if the jar is unzipped, and the classpath set to the currently directory, it "just works". Finally, if the assembly jar is compiled with JDK6, it also works. The error is seen with any class, not just the UIWorkloadGenerator. Also, this error doesn't exist in branch 0.9, only in master.
The package-time behavior of Java 6 and 7 differ with respect to the format used for jar files:
|Number of entries||JDK 6||JDK 7|
zip* is a workaround for the original zip format that described in JDK-6828461 that allows some versions of Java 6 to support larger assembly jars.
The Scala libraries we depend on have added a large number of classes which bumped us over the limit. This causes the Java 7 packaging to not work with Java 6. We can probably go back under the limit by clearing out some accidental inclusion of FastUtil, but eventually we'll go over again.
The real answer is to force people to build with JDK 6 if they want to run Spark on JRE 6.
I've found that if I just unpack and re-pack the jar (using `jar`) it always works: I also noticed something of note. The Breeze package contains single directories that have huge numbers of files in them (e.g. 2000+ class files in one directory). It's possible we are hitting some weird bugs/corner cases with compatibility of the internal storage format of the jar itself. I narrowed this down specifically to the inclusion of the breeze library. Just adding breeze to an older (unaffected) build triggered the issue. I ran a git bisection and this appeared after the MLLib sparse vector patch was merged: