private boolean addIfUnderCorePoolSize(Runnable firstTask) { Thread t = null; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (poolSize < corePoolSize && runState == RUNNING) t = addThread(firstTask); } finally { mainLock.unlock(); } if (t == null) return false; t.start(); return true; }

ThreadPoolExecutor源码实现中大量使用了ReentrantLock 锁，请问为什么使用的是ReentrantLock 锁而不是别的锁机制呢？