Creating many different threads in a for loop only producing one thread (java)

Question

I'm trying to spin up many threads in a Producer-Consumer model in Java and for some reason it's not doing what it's supposed to.

I have code that does

List<Thread> consumers = new ArrayList<>();
            for (int i = 0; i < noOfThreads; i++) {

                Thread consumer = new Thread(new Runnable() {

                    String name = "Thread-" + UUID.randomUUID();
                    @Override
                    public void run() {
                        try {
                            pc.consume(name);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                });
                consumers.add(consumer);
                consumer.start();
                consumer.join();
            }

where pc is a ProducerConsumer whose consume method is

 public void consume(String name) throws InterruptedException {
            while (true) {
                synchronized (this) {
                    while (subQueue.size() == 0) {
                        wait(2000);
                    }
                    Subscriber sub = subQueue.poll();

                     System.out.println(name);

                    uploadEndpointToPinpoint(sub);
                    notify();
                }
            }
        }

So, when I run this I expect to see it print out 10 different UUIDs, indicating that there are 10 threads running - but that's not happening. Instead I just get:

Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab
Thread-c42ac697-d4ad-4944-a20e-907952f5d5ab

which means that it only started one of the threads. Why is the for loop not doing what I think it should?

Any and all help would be appreciated.

Answer 1

Calling Thread#join() blocks the current thread until the target thread is not running anymore. In your loop you're essentially starting a single thread and then waiting for it to complete, which is why you only see one thread. If you want to wait for all the threads to complete you need to move the join() call outside the loop that creates the thread.

    List<Thread> consumers = new ArrayList<>();
    for (int i = 0; i < noOfThreads; i++) {

        Thread consumer = new Thread(new Runnable() {

            String name = "Thread-" + UUID.randomUUID();

            @Override
            public void run() {
                try {
                    pc.consume(name);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        consumers.add(consumer);
        consumer.start();
    }
    for (Thread thread : consumers) {
        thread.join();
    }