This RejectedExecutionHandler implementation runs the rejected Runnable object directly in the calling thread, causing that thread to block until the Runnable completes. If the ThreadPoolExecutor has been shut down, the Runnable is simply discarded instead of being run.

public static class ThreadPoolExecutor.CallerRunsPolicy implements RejectedExecutionHandler {
// Public Constructors
     public CallerRunsPolicy( );  
// Methods Implementing RejectedExecutionHandler
     public void rejectedExecution(Runnable r, ThreadPoolExecutor e);  
}