countdownLatch

当某项工作需要由若干项子任务并行地完成，并且只有在所有的子任务结束之后，当前主任务才能进入 下一阶段，这时候我们可以使用CountDownLatch。 CountDownLatch，直译为倒计数门阀，它的作用就与其名字所表达的意思一样，是指有一个门阀在等 待倒计数，直到计数器为0的时候才打开。

public class Test6 {
    public static void main(String[] args) {
        int[] productIds = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        List priceList = Arrays.stream(productIds).mapToObj(ProductPrice::new).collect(Collectors.toList());
        CyclicBarrier barrier = new CyclicBarrier(priceList.size());
        List threadList = new ArrayList<>();
        priceList.forEach(p -> {
            Thread thread = new Thread(() -> {
                try {
                    System.out.println(p.getProductId() + "开始计算价格");
                    TimeUnit.SECONDS.sleep(new Random().nextInt(10));
                    p.setPrice(p.getProductId() * 0.9D);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        barrier.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } catch (BrokenBarrierException e) {
                        e.printStackTrace();
                    }
                }
            });
            threadList.add(thread);
            thread.start();
        });
        threadList.forEach(t -> {
            try {
                t.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println("所有商品价格计算完毕");
        priceList.forEach(System.out::println);
    }

    private static class ProductPrice {
        private int productId;
        private double price;

        @Override
        public String toString() {
            final StringBuilder sb = new StringBuilder("{");
            sb.append(""productId":")
                    .append(productId);
            sb.append(","price":")
                    .append(price);
            sb.append('}');
            return sb.toString();
        }

        public ProductPrice(int productId) {
            this.productId = productId;
        }

        public ProductPrice(int productId, double price) {
            this.productId = productId;
            this.price = price;
        }

        public int getProductId() {
            return productId;
        }

        public void setProductId(int productId) {
            this.productId = productId;
        }

        public double getPrice() {
            return price;
        }

        public void setPrice(double price) {
            this.price = price;
        }
    }
}