通过ThreadLocal的线程隔离特性，查看程序是否真的时同一个时间点触发的。以此来解决每个线程都有存储独立信息的需求

class ThreadSafeFormatter {
    public static ThreadLocal dateFormatThreadLocal = new ThreadLocal() {
        // 每个线程会调用本方法一次，用于初始化
        @Override
        protected Object initialValue() {
            // 只查看分秒即可
            return new SimpleDateFormat("mm:ss");
        }

        // 首次调用本方法时，会调用initialValue；后面的调用会返回第一次创建的值
        @Override
        public Object get() {
            return super.get();
        }
    };
}

public class TestCache10 {

    static TestCache7 expensiveComputer = new TestCache7<>(new ExpensiveFunction());
    static CountDownLatch countDownLatch = new CountDownLatch(1);

    public static void main(String[] args) throws InterruptedException {

        ExecutorService service = Executors.newFixedThreadPool(100);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 100; i++) {
            service.submit(() -> {
                Integer result = null;
                try {
                    System.out.println(Thread.currentThread().getName() + "开始等待");
                    countDownLatch.await();
                    SimpleDateFormat simpleDateFormat = ThreadSafeFormatter.dateFormatThreadLocal.get();
                    String time = simpleDateFormat.format(new Date());
                    System.out.println(time+Thread.currentThread().getName() + "被放行");
                    result = expensiveComputer.computer("666");
                    System.out.println(result);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (ExecutionException e) {
                    e.printStackTrace();
                }
            });

        }
        Thread.sleep(3000l);
        countDownLatch.countDown();

    }
}

结果

pool-2-thread-3开始等待
pool-2-thread-4开始等待
pool-2-thread-1开始等待
...
27:54pool-2-thread-14被放行
27:54pool-2-thread-30被放行
从FutureTask调用了计算逻辑
27:54pool-2-thread-44被放行
27:54pool-2-thread-39被放行
27:54pool-2-thread-98被放行
...
666
666
...
666
666

从打印结果可以看出，所有的线程都是统一行动，所有时间不会超过1秒钟