ReentrantReadWriteLocks

ReentrantReadWriteLocks are synchronization primitives found in many programming languages, notably Java's `java.util.concurrent.locks` package. They provide a mechanism for managing concurrent access to a shared resource where reads are expected to be more frequent than writes. A ReentrantReadWriteLock allows multiple threads to read a resource concurrently, but only one thread to write to it at a time. While a write lock is held, no other thread, whether for reading or writing, can access the resource.

The core concept is the separation of read and write locks. A read lock can be acquired by any number of threads simultaneously, as long as no thread holds the write lock. Conversely, the write lock is exclusive; only one thread can hold it at any given time, and it prevents any other thread from acquiring either a read or a write lock. This design optimizes performance in scenarios where data is read much more often than it is modified.

Reentrancy is a key feature. This means that a thread that has acquired either a read or a write lock can reacquire the same lock without blocking. A thread holding a write lock can also acquire a read lock, and a thread holding a read lock can acquire another read lock. However, a thread holding a read lock cannot acquire the write lock without releasing the read lock first, to prevent potential deadlocks. This reentrant behavior simplifies the logic for threads that need to perform multiple operations on the protected resource.