ReaderWriter.java

package spos;
import java.util.concurrent.Semaphore;

class ReaderWriter {
	public static void main(String[] args) throws Exception {
		Read read = new Read();
		Write write = new Write();
		Thread t1 = new Thread(read);
		t1.setName("thread1");
		Thread t2 = new Thread(read);
		t2.setName("thread2");
		Thread t3 = new Thread(write);
		t3.setName("thread3");
		Thread t4 = new Thread(read);
		t4.setName("thread4");
		t1.start();
		t3.start();
		t2.start();
		t4.start();
	}
    static Semaphore readLock = new Semaphore(1);
    static Semaphore writeLock = new Semaphore(1);
    static int readCount = 0;

    static class Read implements Runnable {
        @Override
        public void run() {
            try {
                //Acquire Section
                readLock.acquire();
                readCount++;
                if (readCount == 1) {
                    writeLock.acquire();
                }
                readLock.release();
                //Reading section
                System.out.println("Thread " + Thread.currentThread().getName() + " is READING");
                Thread.sleep(1500);
                System.out.println("Thread " + Thread.currentThread().getName() + " has FINISHED READING");
                //Releasing section
                readLock.acquire();
                readCount--;
                if (readCount == 0) {
                    writeLock.release();
                }
                readLock.release();
            } catch (InterruptedException e) {
                System.out.println(e.getMessage());
            }
        }
    }

    static class Write implements Runnable {
        @Override
        public void run() {
            try {
                writeLock.acquire();
                System.out.println("Thread " + Thread.currentThread().getName() + " is WRITING");
                Thread.sleep(2500);
                System.out.println("Thread " + Thread.currentThread().getName() + " has finished WRITING");
                writeLock.release();
            } catch (InterruptedException e) {
                System.out.println(e.getMessage());
            }
        }
    }
}