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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();


    // multiple readers can enter this section
    // if not locked for writing, and not writers waiting
    // to lock for writing.



    // only one writer can enter this section,
    // and only if no threads are currently reading.


Notice how the ReadWriteLock actually internally keeps two Lock instances. One guarding read access, and one guarding write access.

Jakob Jenkov

Copyright  Jenkov Aps
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