[HDFS-13970] Use MultiMap for CacheManager Directives to simplify the code

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.2.0
• Fix Version/s: 3.3.0
• Component/s: caching, hdfs
• Hadoop Flags: Reviewed
• Assignee: David Mollitor
• Reporter: David Mollitor
• Created: 08/Oct/18 16:05
• Updated: 14/Dec/18 05:54
• Resolved: 14/Dec/18 05:40

Description

1. Use Guava Multimap to simplify code
   1. Currently, code uses a mix of LinkedList and ArrayList - just pick one
   2. Currently, directivesByPath structure is sorted but never used in a sorted way, it only performs remove and add operations, no iteration - use a Set instead of a List for values to support faster remove operation. Use a HashSet instead of a TreeSet for keys since it doesn't appear that order really matters.
2. The CacheDirective class needs a better hashcode implementation since it will be used in a Set.  Do not instantiate a HashBuilder object every time hashcode is called. Ouch.