Java 8 parallelStream for concurrent Database / REST call

Question

Here I am using Javaparallel stream to iterate through a List and calling a REST call with each list element as input. I need to add all the results of the REST call to a collection for which I am using an ArrayList. The code given below is working fine except that the non-thread-safety of ArrayList would cause incorrect results, and adding needed synchronization would cause contention, undermining the benefit of parallelism.

Can someone please suggest me a proper way of using parallel stream for my case.

public void myMethod() {
    List<List<String>> partitions = getInputData();
    final List<String> allResult = new ArrayList<String>();
    partitions.parallelStream().forEach(serverList -> callRestAPI(serverList, allResult);
}

private void callRestAPI(List<String> serverList, List<String> allResult) {
    List<String> result = //Do a REST call.
    allResult.addAll(result);
}

Answer 1

You can do the operation with map instead of forEach - that will guarantee thread safety (and is cleaner from a functional programming perspective):

List<String> allResult = partitions.parallelStream()
          .map(this::callRestAPI)
          .flatMap(List::stream) //flattens the lists
          .collect(toList());

And your callRestAPI method:

private List<String> callRestAPI(List<String> serverList) {
    List<String> result = //Do a REST call.
    return result;
}

Answer 2

I wouldn't shy away from synchronising access to your ArrayList. Given that you're accessing a remote service via Rest, I suspect the cost of synchronisation would be negligible. I would measure the effect before you spend time optimising.