A java.util.concurrent.locks.ReadWriteLock is an advanced thread lock mechanism. It allows multiple threads to read a certain resource, but only one to write it, at a time.
The idea is, that multiple threads can read from a shared resource without causing concurrency errors. The concurrency errors first occur when reads and writes to a shared resource occur concurrently, or if multiple writes take place concurrently.
In this text I only cover Java’s built-in ReadWriteLock. If you want to read more about the theory behind the implemenation of a ReadWriteLock, you can read it in my text on Read Write Locks in my Java Concurrency tutorial.
ReadWriteLock Locking Rules
The rules by which a thread is allowed to lock the ReadWriteLock either for reading or writing the guarded resource, are as follows:
| Read Lock | If no threads have locked the ReadWriteLock for writing,and no thread have requested a write lock (but not yet obtained it). Thus, multiple threads can lock the lock for reading. |
| Write Lock | If no threads are reading or writing. Thus, only one thread at a time can lock the lock for writing. |
ReadWriteLock Implementations
ReadWriteLock is an interface. Thus, to use a ReadWriteLock
The java.util.concurrent.locks package contains the following ReadWriteLock implementation:
- ReentrantReadWriteLock
ReadWriteLock Code Example
Here is a simple code example that shows how to create a ReadWriteLock and how to lock it for reading and writing:
ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
readWriteLock.readLock().lock();
// multiple readers can enter this section
// if not locked for writing, and not writers waiting
// to lock for writing.
readWriteLock.readLock().unlock();
readWriteLock.writeLock().lock();
// only one writer can enter this section,
// and only if no threads are currently reading.
readWriteLock.writeLock().unlock();
Notice how the ReadWriteLock actually internally keeps two Lock instances. One guarding read access, and one guarding write access.