如何在Spring Boot中使用CompletableFuture并行调用方法多次

Question

I have a web service in Spring Boot which needs to make requests to an external web service to retrieve live rates for multiple currency pairs.

The method that makes the external API call currently looks like this;

@Async
    public CompletableFuture<Double> getBaseRateForCurrencyPair(String sourceCurrency, String targetCurrency) {
        String key = sourceCurrency.toUpperCase() + targetCurrency.toUpperCase();

        Double baseRate = 0.00;

            String baseRateProviderUrl = "https://apilayer.net/api/live?access_key=" + currencyLayerAccessKey + "&source=" + sourceCurrency + "&currencies=" + targetCurrency;
            HttpClientRequest clientRequest = new HttpClientRequest(baseRateProviderUrl);

            try {
                Response response = clientRequest.get();
                String responseStr = response.body().string();

                JSONObject jsonObject = new JSONObject(responseStr);
                baseRate = AppHelper.round(jsonObject.getJSONObject("quotes").getDouble(key), 2);
            } catch (Exception e) {
                e.printStackTrace();
            }

        return CompletableFuture.completedFuture(baseRate);
    }

What I'm trying to do is call this method an unknown number of times depending on how many currency pairs we need to get rates for, wait for all of them to complete and store the results ready to be returned in an API response.

I have a method that receives a source currency code, and a list of destination currency rates.

public RateResponse getLiveRates(String sourceCurrency, String[] targetCurrencies) {
        RateResponse rateResponse = new RateResponse();

        rateResponse.setSourceCurrency(sourceCurrency);

        /* Loop through target currencies and start a thread for each */
        Map<String, CompletableFuture<Double>> ratesMap = new HashMap<>();
        for (String targetCurrency : targetCurrencies) {
            ratesMap.put(targetCurrency, matrixHelper.getBaseRateForCurrencyPair(sourceCurrency, targetCurrency));
        }

        /* At this point, I would loop through the Map and grab the results, but only when all are finished */

        return rateResponse;
    }

My issue is, I can't figure out how to wait until all async calls are finished before I start looping through the Map and retrieving the results.

Answer 1

Have you tried CompletableFuture method allOf(CompletableFuture<?>... cfs) ? Look for docs here

Answer 2

A simple solution might look like this:

• store the created Future objects in some global set/list
• to then loop and wait on the entries of that set, before doing anything else

So, your first snippet changes to:

CompletableFuture rv = CompletableFuture.completedFuture(baseRate);
thatGlobalList.append(rv);
return rv;

And then:

availableFutures = 0;
while (availableFutures < thatGlobalList.size()) {
  availableFutures=0;
  for (CompletableFuture future : thatGlobalList) { 
    if (future.isDone()) {
      availableFutures++;
    }

Of course, when your map is already built for you, and contains all the future objects, then simply iterate over the values of the map, and sure, all of them are giving you isDone() !

Answer 3

嗯，也许我在这里错过了一些东西，但是如果您迭代所有可完成对象并调用其CompletableFuture＃get，那么根据定义，您将在所有期货完成后完成迭代。

Answer 4

Even though you're using CompletableFuture , your getBaseRateForCurrencyPair method returns only after all the processing is done. To do all that in parallel, you'll have to use something like this:

public CompletableFuture<Double> getBaseRateForCurrencyPair(String sourceCurrency, String targetCurrency) {
    CompletableFuture<String> completableFuture 
  = new CompletableFuture<>();
    Executors.newCachedThreadPool().submit(() -> {
        String key = sourceCurrency.toUpperCase() + targetCurrency.toUpperCase();

        Double baseRate = 0.00;

        String baseRateProviderUrl = "https://apilayer.net/api/live?access_key=" + currencyLayerAccessKey + "&source=" + sourceCurrency + "&currencies=" + targetCurrency;
        HttpClientRequest clientRequest = new HttpClientRequest(baseRateProviderUrl);

        try {
            Response response = clientRequest.get();
            String responseStr = response.body().string();

            JSONObject jsonObject = new JSONObject(responseStr);
            baseRate = AppHelper.round(jsonObject.getJSONObject("quotes").getDouble(key), 2);
        } catch (Exception e) {
            e.printStackTrace();
        }
        completableFuture.complete(baseRate);
        return null;
     });


    return completableFuture;
}

And to wait for all the tasks to be done, you can use something like this:

CompletableFuture.allOf(ratesMap.values().stream().toArray(CompletableFuture[]::new))

The you can just iterate the map and use mapValue.get() to get the Double value.