Many concurrency issues (
HIVE-12768, HIVE-16175, HIVE-16060) have been found with respect to class static variable usages. With fact that HS2 supports concurrent compilation and task execution as well as some backend engines (such as Spark) running multiple tasks in a single JVM, traditional assumption (or mindset) of single threaded execution needs to be abandoned.
This purpose of this JIRA is to do a global scan of static variables in Hive code base, and correct potential thread-safety issues. However, it's not meant to be exhaustive.