并发工具类：CompletionService有什么用？

如何优化任务执行？

开发了一个比价网站，当用户查询某个商品的价格时，从S1和S2两个平台获取到对应的价格，保存在数据库中后，返回给用户

ExecutorService executor = Executors.newFixedThreadPool(2);

Future<Integer> f1 = executor.submit(()-> getPriceByS1());
Future<Integer> f2 = executor.submit(()-> getPriceByS2());

Integer price1 = f1.get();
save(price1);

Integer price2 = f2.get();
save(price2);

如果获取电商S1报价的耗时很长，即使获取电商S2报价的耗时很短，也无法让保存S2报价的操作先执行，因为主线程都阻塞在f1.get()操作上了

解决方案一：

ExecutorService executor = Executors.newFixedThreadPool(2);

Future<Integer> f1 = executor.submit(()-> getPriceByS1());
Future<Integer> f2 = executor.submit(()-> getPriceByS2());

BlockingQueue<Integer> bq = new LinkedBlockingQueue<>();
bq.put(f1.get());
bq.put(f2.get());

for (int i = 0; i < bq.size(); i++) {
    Integer price = bq.take();
    save(price);
}

解决方案二：

ExecutorService executor = Executors.newFixedThreadPool(2);
CompletionService<Integer> cs = new ExecutorCompletionService<>(executor);

cs.submit(() -> getPriceByS1());
cs.submit(() -> getPriceByS2());

for (int i = 0; i < 2; i++) {
    Integer price = cs.take().get();
    save(price);
}