This is an example of how to make use of the ReentrantReadWriteLock class of Java. It is an implementation of ReadWriteLock, that also supports ReentrantLock functionality.
The ReadWriteLock is a pair of associated Locks, one for read-only operations and one for writing. Whereas, the ReentrantLock is a reentrant mutual exclusion Lock with the same behavior as the implicit monitor lock accessed using synchronized methods and statements, but with some more extended capabilities.
The ReentrantReadWriteLock is the mix of the above implementations. It does not have ordering preference between read and write, but may offer a fairness policy when needed.
In the example class below, we have created three Runnable implementations. They all use a ReentrantReadWriteLock lock variable. The lock is created using the ReentrantReadWriteLock(boolean fair) constructor, so it is given a fairness policy:
Readgets thelock. It uses thereadLock()API method ofReentrantReadWriteLockto get aReadLock. Then, it acquires the read lock, using thelock()method ofReadLock. While having the lock, it reads the value of a Stringmessagevariable. Then it attempts to release the lock, using theunlock()method ofReadLock.- Both
WriteAandWriteBalso get thelock, usingwriteLock()method, that returns aWriteLock, and then usingunlock()method ofWriteLock. Since having the write lock, they both alter the value of the Stringmessagevariable. Then, they release the write lock, using theunlock()method ofWriteLock.
ReentrantReadWriteLockExample.java
package com.javacodegeeks.snippets.core;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class ReentrantReadWriteLockExample {
private static final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
private static String message = "a";
public static void main(String[] args) throws InterruptedException{
Thread t1 = new Thread(new Read());
Thread t2 = new Thread(new WriteA());
Thread t3 = new Thread(new WriteB());
t1.start();
t2.start();
t3.start();
t1.join();
t2.join();
t3.join();
}
static class Read implements Runnable {
public void run() {
for(int i = 0; i<= 10; i ++) {
if(lock.isWriteLocked()) {
System.out.println("I'll take the lock from Write");
}
lock.readLock().lock();
System.out.println("ReadThread " + Thread.currentThread().getId() + " ---> Message is " + message );
lock.readLock().unlock();
}
}
}
static class WriteA implements Runnable {
public void run() {
for(int i = 0; i<= 10; i ++) {
try {
lock.writeLock().lock();
message = message.concat("a");
} finally {
lock.writeLock().unlock();
}
}
}
}
static class WriteB implements Runnable {
public void run() {
for(int i = 0; i<= 10; i ++) {
try {
lock.writeLock().lock();
message = message.concat("b");
} finally {
lock.writeLock().unlock();
}
}
}
}
}
If you run the example, the result will be something like the one below:
ReadThread 9 ---> Message is a ReadThread 9 ---> Message is aaba ReadThread 9 ---> Message is aababa ReadThread 9 ---> Message is aabababa ReadThread 9 ---> Message is aababababa ReadThread 9 ---> Message is aabababababa ReadThread 9 ---> Message is aababababababa ReadThread 9 ---> Message is aabababababababa ReadThread 9 ---> Message is aababababababababa ReadThread 9 ---> Message is aabababababababababa ReadThread 9 ---> Message is aababababababababababa
Download the Eclipse Project
This was an example of ReentrantReadWriteLockExample class of Java.
You can download the full source code of this example here: JavaReentrantReadWriteLockExample.zip
