When an active runnable starts a second, synchronous runnable, the second runnable is executing or nested inside the first. Each runnable that is executing a nested runnable maintains an internal reference or link to that runnable. These links are traversed to find the innermost runnable currently executing under a thread.

Execution nesting is implemented by the following functions:

The rwThread() function can be used to get a handle to the threaded runnable that originated the calling thread. If the calling thread was not started by a Threading package runnable class, as is the case with the main thread, then the handle returned by this function is empty. Whenever the source of a thread is in question, you should use the isValid() function to test the handle returned by rwThread() before using it, as shown Example 19.