ReadersWritersDemo.java

// 6. READERS-WRITERS PROBLEM - Coordinate multiple readers and exclusive writers
// Problem: Multiple readers can read simultaneously, but writer needs exclusive access
public class ReadersWritersDemo {
    static class ReadWriteResource {
        private int data = 0;
        private int readers = 0;
        private int writers = 0;
        private int waitingWriters = 0;
        
        // Readers can acquire simultaneously
        public synchronized void acquireRead() throws InterruptedException {
            while (writers > 0 || waitingWriters > 0) {
                wait(); // Wait if writers are active or waiting
            }
            readers++;
            System.out.println("Reader acquired (Active readers: " + readers + ")");
        }
        
        public synchronized void releaseRead() {
            readers--;
            System.out.println("Reader released (Active readers: " + readers + ")");
            if (readers == 0) {
                notifyAll(); // Notify waiting writers
            }
        }
        
        // Writers need exclusive access
        public synchronized void acquireWrite() throws InterruptedException {
            waitingWriters++;
            while (readers > 0 || writers > 0) {
                wait(); // Wait if any readers or writers are active
            }
            waitingWriters--;
            writers++;
            System.out.println("Writer acquired (Writing...)");
        }
        
        public synchronized void releaseWrite() {
            writers--;
            data++;
            System.out.println("Writer released (Data updated to: " + data + ")");
            notifyAll(); // Notify waiting readers/writers
        }
        
        public int readData() {
            return data;
        }
    }

    public static void main(String[] args) {
        ReadWriteResource resource = new ReadWriteResource();
        
        // Create multiple reader threads
        for (int i = 0; i < 3; i++) {
            new Thread(() -> {
                try {
                    resource.acquireRead();
                    System.out.println("  Reader reading data: " + resource.readData());
                    Thread.sleep(500);
                    resource.releaseRead();
                } catch (InterruptedException e) {}
            }).start();
        }
        
        // Create writer thread
        new Thread(() -> {
            try {
                Thread.sleep(100);
                resource.acquireWrite();
                System.out.println("  Writer updating data...");
                Thread.sleep(500);
                resource.releaseWrite();
            } catch (InterruptedException e) {}
        }).start();
    }
}