@Override
    protected void beforeExecute(Thread t, Runnable r) {
      Runnable oldPut = running.put(t, r);
      assert oldPut == null : "inconsistency for thread " + t;
      super.beforeExecute(t, r);
    }
A subclass of ThreadPoolExecutor that keeps track of the Runnables that are executing at any
given point in time.
hbase-server/src/main/java/org/apache/hadoop/hbase/executor/ExecutorService.java