See original discussion: http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-January/013429.html

Quote: "When we introduced ThreadLocalRandom, we conservatively implemented it to use an actual ThreadLocal. However, as it becomes more widely used, it is worth improving the implementation by housing ThreadLocalRandom state (and related bookkeeping) in class Thread itself. This would entail three fields (16 total bytes)."