Run multiple threads in sequence using locks

Question

I am trying to run 4 threads in sequence like 4 -> 3 -> 2 -> 1. And I am using int threadId and AtomicInteger to validate if the current thread is eligible to run or not. But the 4th thread starts and the execution gets stuck, I am not getting why the thread 3, 2 and 1 are not getting a chance to run.

Here is what I did:

public class SequenceDemo implements Runnable{
    
    int threadId;
    static AtomicInteger currentThread = new AtomicInteger(4);
    ReentrantLock lock;
    Condition condition;
    
    public SequenceDemo( ReentrantLock lock, Condition condition) {
        this.lock = lock;
        this.condition = condition;
    }
    
    @Override
    public void run() {
        try {
            lock.lock();
            threadId = Integer.parseInt(Thread.currentThread().getName());
            while(currentThread.get()!=threadId) {
                condition.await();
            }
            
            System.out.println("Thread "+Thread.currentThread().getName()+" started executing");
            Thread.sleep(1000);
            System.out.println("Thread "+Thread.currentThread().getName()+" finished executing");
            
            currentThread.decrementAndGet();
            condition.signalAll();
            
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {
        
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        
        SequenceDemo demo1 = new SequenceDemo(lock, condition);
        
        Thread t1 = new Thread(demo1, "1");
        Thread t2 = new Thread(demo1, "2");
        Thread t3 = new Thread(demo1, "3");
        Thread t4 = new Thread(demo1, "4");
        
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        
    }
}

output: Execution gets stuck after execution of thread 4:

Thread 4 started executing
Thread 4 finished executing

Answer 1

Your code is fine, you should just pass a new SequenceDemo instance to each thread, otherwise they share the threadId and it's just stuck on the value 4 for all threads. (Took me a few minutes to see haha)