Difference Between ReadWriteLock Interface and ReentrantReadWriteLock Class in Java

ReadWriteLock is an interface in java.  A ReadWriteLock allows us to add a thread-safety feature to a data structure while increasing throughput by allowing multiple threads to read the data concurrently and one thread to update the data exclusively.

ReadWriteLock is described in the java.util.concurrent.locks package, with ReentrantReadWriteLock is an implementation class. ReadWriteLock implementations assure that the memory synchronization effects of writeLock operations (as described in the Lock interface) also hold with respect to the associated readLock. So, we can create a ReadWriteLock like this:

ReadWriteLock rwlock = new ReentrantReadWriteLock();

An assured pair of locks are associated with ReadWriteLocks :

• read-only lock and
• write lock

The read lock may be held at the same time by many reader threads, as long as there are no writers. And the write lock is exclusive. 

ReentrantReadWriteLock Class in java

ReentrantReadWriteLocks can be used to enhance concurrency in some uses of kinds of Collections. ReentrantReadWriteLock class is a useful one and only the collections are expected to be major, accessed by more reader threads than writer threads, and require operations with overhead that outweighs synchronization overhead.

The ReentrantReadWriteLock class has the following properties:

• Acquisition order: As a reader or writer, this class does not impose preference ordering for lock access. Although, an optional fairness policy is being supported.
• Reentrancy: In the style of a ReentrantLock, the readers and writers both are allowed to reacquire the read lock or write locks as well.
• Lock downgrading: Reentrancy also enables downgrading from the write lock to a read lock, by achieving the write lock, then the read lock and then releasing the write lock.
• Interruption of lock acquisition: During lock acquisition, the read lock and write lock both support interruption.
• Condition support: With respect to the write lock, the write lock provides a Condition implementation that behaves in the same way.
• Instrumentation: ReentrantReadWriteLock class supports methods to determine whether locks are held or contended.

ReentrantReadWriteLock is described in the java.util.concurrent.locks package, with implemented interfaces are Serializable, ReadWriteLocks. We can create a ReentrantReadWriteLock like this: 

ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

The ReentrantReadWriteLock has two separate locks:

• The read lock ‘r’ returned by method readLock().
• The write lock ‘w’ returned by method writeLock().

Lock r = rwl.readLock();
Lock w = rwl.writeLock();

Difference Between ReadWriteLock and ReentrantReadWriteLock

S. No.	ReadWriteLock	ReentrantReadWriteLock
1.	ReadWriteLock is an interface.	ReentrantReadWriteLock is a class.
2.	ReadWriteLock implements ReentrantReadWriteLock class.	ReentrantReadWriteLock implements ReadWriteLock interface, supporting similar semantics to ReentrantLock.
3.	The ReadWriteLock interface defines locks similarly, that may be shared among readers but are exclusive to writers.	Here, a single implementation, ReentrantReadWriteLock, is provided, since it covers most standard usage contexts.
4.	It is designed as a high-level locking mechanism that allows you to append thread-safety features to a data structure.	Programmers may develop by their own implementations to cover nonstandard requirements.