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readwritelock in java


The ReadWriteLock is an interface which provides an advanced thread lock mechanism. It maintained a pair of locks (one for read only and other for write). The basic idea for ReadWriteLock is to increase the performance by allowing multiple threads to read a shared resource, when no thread is writing.

Note: Only one thread can write the shared resource value at a time.

ReadWriteLock locking rules:

Read Lock: If no thread have or not requested ReadWriteLock for writing then multiple threads can lock the read lock for shared resource.

Write Lock: If no thread have read or write lock then one thread can lock write lock on shared resource.

Note: It performs best when there are more read locks required as compared to write locks.

Example

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
public class LockTest<E> {
	private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();	
	private final Lock readLock = readWriteLock.readLock();
	private final Lock writeLock = readWriteLock.writeLock();
 
	private final List<E> list = new ArrayList<>();
	public void set(E o){
		writeLock.lock();
		try{
			list.add(o);
			System.out.println("Add element: "+Thread.currentThread().getName());
	        } finally {
			writeLock.unlock();
		}
	}
 
	public E get(int i) {
		readLock.lock();
		try	{
                        System.out.println("Print elements: "+Thread.currentThread().getName());
			return list.get(i);
		} finally {
			readLock.unlock();
		}
	}
	public static void main(String[] args) {
		LockTest<String> lockTest = new LockTest<>();
		lockTest.set("1");
		lockTest.set("2");
		lockTest.set("3");
 
                System.out.println("First Element: "+lockTest.get(1));
	}
}

Output

Add element: main
Add element: main
Add element: main
Print elements: main
First Element: 2

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