When a thread created by a threaded runnable starts a synchronous runnable, the thread eventually enters the run() member of that runnable. At that point, the thread is considered to be executing inside of the synchronous runnable as well.

In this situation, the synchronous runnable is nested inside of the threaded runnable. A nested runnable registers itself with the enclosing parent runnable for the purpose of supporting cancellation and other operations.

Starting a threaded runnable from within another runnable does not produce a nesting relationship because the threaded runnable launches its own thread of execution. No nesting relationship is formed when a synchronous runnable is started by a thread that does not originate from a Threading package runnable.

A handle to the nested runnable can be retrieved from an enclosing runnable by calling the getNestedRunnable() member. If a runnable does not have a nested runnable executing within it, an empty handle is return. This operation allows nested runnables to be traversed to find the innermost runnable.