void ensurePrestart(){
  int wc=workerCountOf(ctl.get());
  if (wc < corePoolSize)   addWorker(null,true);
 else   if (wc == 0)   addWorker(null,false);
}

void ensurePrestart ( ) { int wc = workerCountOf ( ctl . get ( ) ) ; if ( wc < corePoolSize ) addWorker ( null , true ) ; else if ( wc == 0 ) addWorker ( null , false ) ; }
same as prestartcorethread except arranges that at least one thread is started even if corepoolsize is 0 .
