thread.hpp is 2133 lines long and is included by about 800 out of 1000 HotSpot .o files. It also pulls in many other header files.

Many of the Thread subtypes are infrequently used:

Number of HotSpot cpp/hpp files with the word:
Thread 1027
JavaThread 451
CompilerThread 38
WorkerThread 3
WatcherThread 14
NonJavaThread 15

We should move some of these subtypes into their own headers like compilerThread.hpp, nonJavaThread.hpp, etc.

This RFE will leave JavaThread inside thread.hpp for now. Refactoring JavaThread is more involved, amd may be done in a separate RFE if it's found to be doable/beneficial (see some comments below).