The stream documentation discusses stateful lambdas but does not explicitly discuss a lambda that performs blocking operations such as using say j.u.c.Semaphore or Thread.sleep.

Such documentation needs to warn about using such blocking operations and indicate what to do if they are required to be used, namely by wrapping in a Fork/Join ManagedBlocker.

(It may also be appropriate to add ManagedBlocker to relevant j.u.c classes, if so a new issue will be created)