code: static String findJarsDir(String sparkHome, String scalaVersion, boolean failIfNotFound) {
      // TODO: change to the correct directory once the assembly build is changed.
      File libdir = new File(sparkHome, "jars");
      if (!libdir.isDirectory()) {
        libdir = new File(sparkHome, String.format("assembly/target/scala-%s/jars", scalaVersion));
        if (!libdir.isDirectory()) {
          checkState(!failIfNotFound,
            "Library directory '%s' does not exist; make sure Spark is built.",
            libdir.getAbsolutePath());
          return null;
        }
      }
      return libdir.getAbsolutePath();
    }
docs: Find the location of the Spark jars dir, depending on whether we're looking at a build
  or a distribution directory.