synchronized的获取遵循non-fair锁定策略.也就是说，在阻塞时首先进入的线程可能不是第一个获取的线程.如果您想使用fair锁，请使用new ReentrantLock(true)

synchronized acquisition follows a non-fair lock policy. That is, threads that enter first while blocking may not be the first to acquire. If you want a fair lock use a new ReentrantLock(true)