I know there already is a issue #3799, and I thought it should be solved in 2.4.0.
But recently I was trying to upgrade my project to play 2.4.0,
it appears to one thread named "ForkJoinPool-n-worker-n" was leaked after every

1. run
2. refresh http://127.0.0.1:9000 in browser
3. ctrl d

cycle.

By following these steps, you could reproduce it easily.

1. use activator new project-name play-scala to create a new sample project.
2. run "activator" command in terminal
3. start "Visual VM", connect to the jvm you just started
4. then open threads tab
5. run "run" in terminal
6. access "http://127.0.0.1:9000" in browser
7. press "ctrl d" in terminal
8. then you could see a thread named "ForkJoinPool-n-worker-n" increased in Visual VM's threads list.