Threads - Beginning Java 8 Language Features - page 233

Listing 6-32. Using a ReentrantReadWriteLock to Guard a Read-Mostly Data Structure

// ReadMostlyData.java
package com.jdojo.threads;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadMostlyData {
private int value;
private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
private Lock rLock = rwLock.readLock();
private Lock wLock = rwLock.writeLock();

public ReadMostlyData(int value) {
this.value = value;
}

public int getValue() {
// Use the read lock, so multiple threads may read concurrently
rLock.lock();
try {
return this.value;
}
finally {
rLock.unlock();
}
}

public void setValue(int value) {
// Use the write lock, so only one thread can write at a time
wLock.lock();
try {
this.value = value;
}
finally {
wLock.unlock();
}
}
}

■ the ReadWriteLock allows you have a read and a write version of the same lock. Multiple threads can
own a read lock as long as another thread does not own the write lock. however, only one thread can own the write lock
at a time.

Tip