Java ReentrantReadWriteLock

ReentrantReadWriteLock Introduction

A read-write lock provides greater level of concurrency than a mutual exclusion lock when working with shared data. Read-write locks allows simultaneous read only operations by multiple threads whereas a write operation can be performed by only one thread. Read-write locks provide increased level of concurrency and typically results in better performance if the frequency of read operations is more than write operations.

Java ReentrantReadWriteLock

Java supports ReadWriteLock interface in java.utils.concurrent.locks package. ReentrantReadWriteLock is an implementation of the ReadWriteLock interface. ReadWriteLock maintains a pair of locks one for read only operations and one for write operations. The read lock can be held by multiple threads simultaneously whereas a write lock can held only by one thread at a time. Some of the key APIs of ReentrantReadWriteLock are listed below.

• ReentrantReadWriteLock() - Creates a new instance of  ReentrantReadWriteLock.
• ReentrantReadWriteLock(boolean fair) - Creates a new instance of  ReentrantReadWriteLock with fairness policy.
• ReentrantReadWriteLock.ReadLock readLock() - Returns lock used for reading.
• ReentrantReadWriteLock.WriteLock writeLock() - Returns lock used for writing.

Java ReentrantReadWriteLock Example

In this example we have a producer task requesting for write lock and two reader tasks requesting for read lock. When write lock is provided both the reader threads are blocked. However both the reader threads work simultaneously since they request only for read locks.

package com.sourcetricks.readwritelock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockDemo {
  
  // Shared resources to be guarded
  private static int count = 0;

  // Producer thread. Counts up the resource.
  private static class ProducerTask implements Runnable {

    private ReentrantReadWriteLock lock = null;

    ProducerTask(ReentrantReadWriteLock lock) {
      this.lock = lock;
    }

    @Override
    public void run() {
      System.out.println("Producer requesting lock");
      lock.writeLock().lock();
      System.out.println("Producer gets lock");
      for (int i = 0; i < 5; i++) {
        count = count + 1;
        System.out.println("Counting up");
      }
      try {
        Thread.sleep(5000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println("Producer releases lock");
      lock.writeLock().unlock();
    }

  }

  // Reader task
  private static class AnotherReadTask implements Runnable {

    private ReentrantReadWriteLock lock = null;

    AnotherReadTask(ReentrantReadWriteLock lock) {
      this.lock = lock;
    }

    @Override
    public void run() {
      System.out.println("Another Reader requesting lock");
      lock.readLock().lock();
      System.out.println("Another Reader gets lock");
      System.out.println("Current count = " + count);
      try {
        Thread.sleep(5000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println("Another Reader releases lock");
      lock.readLock().unlock();
    }
  }
  
  // Reader task
  private static class ReadTask implements Runnable {

    private ReentrantReadWriteLock lock = null;

    ReadTask(ReentrantReadWriteLock lock) {
      this.lock = lock;
    }
    
    @Override
    public void run() {
      System.out.println("Reader requesting lock");
      lock.readLock().lock();
      System.out.println("Reader gets lock");
      System.out.println("Current count = " + count);
      try {
        Thread.sleep(5000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println("Reader releases lock");
      lock.readLock().unlock();     
    }   
  }

  public static void main(String[] args) {
    ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    Thread producer = new Thread(new ProducerTask(lock));
    Thread anotherReader = new Thread(new AnotherReadTask(lock));
    Thread reader = new Thread(new ReadTask(lock));
    producer.start();
    reader.start();
    anotherReader.start();
  }
}

This program produces the following output.

Producer requesting lock
Reader requesting lock
Producer gets lock
Counting up
Counting up
Counting up
Counting up
Counting up
Another Reader requesting lock
Producer releases lock
Reader gets lock
Current count = 5
Another Reader gets lock
Current count = 5
Reader releases lock
Another Reader releases lock

Refer to this link for more Java Tutorials.