synchronized, monitors, wait and notify question 27 of 180 mid

What is reentrancy, and what would break without it?

Reentrancy means two threads can enter the same block
Correct: A thread already holding a monitor can acquire it again; without it, any synchronized method calling another would self-deadlock
It allows recursion only up to the stack depth
It applies to ReentrantLock but not to synchronized

A monitor records its owner and a hold count. The owning thread entering again just increments the count, and the monitor is released when the count reaches zero.

Without reentrancy, an ordinary pattern would hang: a synchronized method calling another synchronized method on the same object, or a synchronized equals on a subclass calling super.equals. Both synchronized and ReentrantLock are reentrant, which is what the name says. ReadWriteLock is the sharp edge here: a read lock cannot be upgraded to a write lock, and trying deadlocks the thread against itself.

synchronized locks

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