当两个或多个竞争的动作在等待其他动作结束，但这种情况不会发生，则称之为死锁。
一旦死锁发生，便会存在以下四个必要条件：
1. 互斥
2. 保持与等待
3. 不可抢占
4. 循环等待
只要可以去除上述条件中的任意一个条件，便可以消除死锁。
Dead-Lock
Dead-lock is situation when two or more competing actions are each waiting for the other to finish, but thus never does.
There are four necessary conditions when a dead-lock occurs: mutual exclusion, hold and wait, no preemption, circular wait. If we can remote one of the conditions, the dead-lock will be avoided. We can use a monitor thread to detect the dead-lock.