What is a ReadWriteLock?

Locks are used to prevent race conditions. A race condition occurs when two threads are writing and reading the same field: will thread B see the value written by thread A? Synchronization achieves that by blocking everything.

ReadWriteLock works with two locks: one for the read operations, and another one for the write operations. The write lock is exclusive — no other lock can be taken when it is active. The read lock, on the other hand, is not: you can have any number of active read locks.

Of course, this is an optimization only if your number of write operations is much smaller than the number of read operations.