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ReadWriteLock

Jakob Jenkov
Last update: 2014-06-23

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.

Jakob Jenkov




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