Nested Monitor Lockout

释放了里面的锁，但没有释放外面的锁。

避免嵌套

Nested Monitor Lockout vs. Deadlock

The result of nested monitor lockout and deadlock are pretty much the same: The threads involved end up blocked forever waiting for each other.

The two situations are not equal though. As explained in the text onDeadlocka deadlock occurs when two threads obtain locks in different order. Thread 1 locks A, waits for B. Thread 2 has locked B, and now waits for A. As explained in the text onDeadlock Preventiondeadlocks can be avoided by always locking the locks in the same order (Lock Ordering). However, a nested monitor lockout occurs exactly by two threads taking the locksin the same order. Thread 1 locks A and B, then releases B and waits for a signal from Thread 2. Thread 2 needs both A and B to send Thread 1 the signal. So, one thread is waiting for a signal, and another for a lock to be released.

The difference is summed up here:

In deadlock, two threads are waiting for each other to release locks.

In nested monitor lockout, Thread 1 is holding a lock A, and waits
for a signal from Thread 2. Thread 2 needs the lock A to send the
signal to Thread 1.