Ordered Thread Execution in Java: join, CountDownLatch, Single Thread Pool, and CompletableFuture

When multiple threads need to run in a strict sequence (thread 1 → thread 2 → thread 3), the default scheduler may interleave them, so explicit coordination is required.

One simple way is to use Thread.join(), which makes a thread wait until another finishes. The following example creates three threads where each thread receives the previous thread and calls join() before printing its name.

public class ThreadLoopOne {
    public static void main(String[] args) {
        Thread t1 = new Thread(new Work(null));
        Thread t2 = new Thread(new Work(t1));
        Thread t3 = new Thread(new Work(t2));

        t1.start();
        t2.start();
        t3.start();
    }

    static class Work implements Runnable {
        private Thread beforeThread;
        public Work(Thread beforeThread) {
            this.beforeThread = beforeThread;
        }

        @Override
        public void run() {
            // 如果有线程,就 join 进来,没有的话就直接输出
            if (beforeThread != null) {
                try {
                    beforeThread.join();
                    System.out.println("thread start : " + Thread.currentThread().getName());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                System.out.println("thread start : " + Thread.currentThread().getName());
            }
        }
    }
}

Another approach is to use CountDownLatch as a signaling mechanism. Each thread waits on a latch that is counted down by the preceding thread, ensuring the desired order.

public class ThreadLoopTwo {
    public static void main(String[] args) {
        // 设置线程 1 的信号量为 0
        CountDownLatch cOne = new CountDownLatch(0);
        // 设置线程 2 的信号量为 1
        CountDownLatch cTwo = new CountDownLatch(1);
        // 设置线程 3 的信号量为 1
        CountDownLatch cThree = new CountDownLatch(1);

        // 因为 cOne 为 0 ,故 t1 可以直接执行
        Thread t1 = new Thread(new Work(cOne,cTwo));
        // 线程 t1 执行完毕之后,此时的 cTwo 为 0 , t2 开始执行
        Thread t2 = new Thread(new Work(cTwo,cThree));
        // 线程 t2 执行完毕,此时 cThree 为 0 , t3 开始执行
        Thread t3 = new Thread(new Work(cThree,cThree));

        t1.start();
        t2.start();
        t3.start();
    }

    static class Work implements Runnable {
        CountDownLatch cOne;
        CountDownLatch cTwo;

        public Work(CountDownLatch cOne, CountDownLatch cTwo) {
            super();
            this.cOne = cOne;
            this.cTwo = cTwo;
        }
        @Override
        public void run() {
            try {
                // 当前一个线程信号量为 0 时,才执行
                cOne.await();
                System.out.println("thread start : " + Thread.currentThread().getName());
                // 后一个线程信号量减 1
                cTwo.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

Using a single‑thread executor also guarantees order because tasks are executed one after another in the thread pool.

public class ThreadLoopThree {
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("thread start : " + Thread.currentThread().getName() + " run one");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("thread start : " + Thread.currentThread().getName() + " run two");
            }
        });

        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("thread start : " + Thread.currentThread().getName() + " run three");
            }
        });

        ExecutorService executor = Executors.newSingleThreadExecutor();
        // 将线程依次加入到线程池中
        executor.submit(t1);
        executor.submit(t2);
        executor.submit(t3);
        // 及时将线程池关闭
        executor.shutdown();
    }
}

Finally, CompletableFuture can chain asynchronous actions, starting each thread after the previous one has been launched.

public class ThreadLoopFour {
    public static void main(String[] args)  {
        Thread t1 = new Thread(new Work());
        Thread t2 = new Thread(new Work());
        Thread t3 = new Thread(new Work());

        CompletableFuture.runAsync(()-> t1.start())
                .thenRun(()->t2.start())
                .thenRun(()->t3.start());
    }

    static class Work implements Runnable {
        @Override
        public void run() {
            System.out.println("thread start : " + Thread.currentThread().getName());
        }
    }
}