facing Java multithread concurrency issue

Question

I have created three thread like client1, client2, client3 with corresponding read request "read 2", "read 1", "read 3". i wanted to process a read request in following manner:

Client2 read 1

Client1 read 2

Client3 read 3

i don't have idea to run a thread (client2) first and then thread (client1) and so on based on read request sequence.There is a one condition that i can't use sleep in my program.

Please provide help in above context of problem if anyone know about the solution.

Answer 1

Depending on the complexity of your program, it could be done using 2 CountDownLatch to synchronize your threads, one to release Client1 once Client2 has done reading and another one to release Client3 once Client1 has done reading.

// Used to release client1 
CountDownLatch startThread1 = new CountDownLatch(1);
Thread client2 = new Thread(
    () -> {
        System.out.println("Read 1");
        // Release client1
        startThread1.countDown();
    }
);
// Used to release client3
CountDownLatch startThread3 = new CountDownLatch(1);
Thread client1 = new Thread(
    () -> {
        try {
            // Waiting to be released
            startThread1.await();
            System.out.println("Read 2");
            // Release client3
            startThread3.countDown();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
);
Thread client3 = new Thread(
    () -> {
        try {
            // Waiting to be released
            startThread3.await();
            System.out.println("Read 3");
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
);
// Start the threads in reverse order intentionally to show that 
// we still have the expected order
client3.start();
client1.start();
client2.start();

Output:

Read 1
Read 2
Read 3

This approach guarantees to get the correct read sequence whatever the order of the start sequence of your threads.