通过volatile的可见性使线程按顺序执行（cas）

volatile static int flag=0;
    //#region
    public static void main(String[] args) throws Exception {
        int num=100;
        for (int i = 0; i < 100; i++) {
            int finalI = i;
            new Thread(()->{
                while(true){
                    if(flag== finalI){
                        System.out.println(Thread.currentThread().getName()+": "+finalI);
                        flag++;
                        break;
                    }
                }
            },"t"+i).start();
        }
    }
    //#endregion
}

直接通过循环不能按顺序是因为线程按顺序启动并不代表线程会按顺序执行，t1.start();t2.start;无法保证t1一定比t2先执行run方法。因此要通过一个线程共享volatile变量保证顺序

for (int i = 0; i < 100; i++) {
            int finalI = i;
            new Thread(()->{
                        System.out.println(Thread.currentThread().getName()+": "+ finalI);
            },"t"+i).start();
        }

 static CountDownLatch cdl =  new CountDownLatch(100);
    //#region
    public static void main(String[] args) throws Exception {
        int num=100;
        for (int i = 0; i < 100; i++) {
            int finalI = i;
            new Thread(()->{
                while(true){
                    if(cdl.getCount()== 100-finalI){
                        System.out.println(Thread.currentThread().getName()+": "+finalI);
                        cdl.countDown();
                        break;
                    }
                }
            },"t"+i).start();
        }
    }

semaphore(1) 类似锁机制。

static AtomicInteger num = new AtomicInteger(100);
    //#region
    public static void main(String[] args) throws Exception {
        Semaphore se = new Semaphore(1);
        for (int i = 0; i < 100; i++) {
            int finalI = i;
            new Thread(()->{
                boolean isAcquire = false;
                try {
                    se.acquire();
                    isAcquire = true;
                    System.out.println(Thread.currentThread().getName()+": "+num.getAndIncrement());

                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally {
                    if(isAcquire){
                    se.release();}
                }

            },"t"+i).start();
        }
    }