Method that may be invoked by a ThreadPoolExecutor when execute cannot accept a task. This may occur when no more threads or queue slots are available because their bounds would be exceeded, or upon shutdown of the Executor.

In the absence of other alternatives, the method may throw an unchecked RejectedExecutionException, which will be propagated to the caller of execute.

r	the runnable task requested to be executed
executor	the executor attempting to execute this task
Throws	RejectedExecutionException: if there is no remedy