code: public List<ExchangeRateProvider> getExchangeRateProviders(String... providers) {
          List<ExchangeRateProvider> provInstances = new ArrayList<>();
          Collection<String> providerNames = Arrays.asList(providers);
          if (providerNames.isEmpty()) {
              providerNames = getProviderNames();
          }
          for (String provName : providerNames) {
              ExchangeRateProvider provider = getExchangeRateProvider(provName);
              if(provider==null){
                  throw new MonetaryException("Unsupported conversion/rate provider: " + provName);
              }
              provInstances.add(provider);
          }
          return provInstances;
      }
docs: Access the current registered {@link javax.money.convert.ExchangeRateProvider} instances. If no provider
  names are passed ALL current registered providers are returned in undefined order.
  
  @param providers the provider names of hte providers to be accessed
  @return the list of providers, in the same order as requested.
  @throws javax.money.MonetaryException if a provider could not be resolved.