How to call multiple rest api parallel in Java?

Question

What is the best way to call multiple api parallel in Java? I want to do multiple rest calls, combine the results and return a jsonArray. I am using ExecutorService and get the correct output when i access the url from only one client pc. But when i access the url from multiple clients, each time i am getting a jsonArray of different size even though i am calling same url.

What i have done is given below:

ExecutorService executor = Executors.newFixedThreadPool(5); 
resultArray = new JSONArray();
for (Branch branch : Branches) {
    Future<Response> response = executor.submit(new Request(branch.getUrl(), 
      branch.getUserName(), branch.getPassword()));
    responseBody = response.get().getResponseBody();
    resultArray.put(responseBody);
}
executor.shutdown();
while(!executor.isTerminated()) {

}
return resultArray.toString();


public class Request implements Callable<Response> {

private HttpURLConnection con;
private URL obj;
private String response;

private String url;
private String username;
private String password;

public Request(String url, String username, String password) {
    this.url = url;
    this.username = username;
    this.password = password;
}

@Override
public Response call() {
    try {
        obj = new URL(url);
        con = (HttpURLConnection) obj.openConnection();
        String userCredentials = username + ":" + password;
        String basicAuth = "Basic " + java.util.Base64.getEncoder().encodeToString(userCredentials.getBytes());
        con.setRequestProperty ("Authorization", basicAuth);
        con.setRequestMethod("GET");

        int responseCode = con.getResponseCode();
        if(responseCode == 200) {
            BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
            String inputLine;
            StringBuffer stringBuffer = new StringBuffer();

            while ((inputLine = in.readLine()) != null) {
                stringBuffer.append(inputLine);
            }
            in.close();
            response = stringBuffer.toString();
            return new Response(responseCode, response);
        }
        else {
            response = "{\"response\":\"some error occurred\"}";
            return new Response(responseCode, response);
        }

    } catch (IOException e) {
        response = "{\"output\":\"some error occurred\"}";
        return new Response(404, response);
    }
}
}

public class Response {

private String responseBody;
private int responseCode;

public Response(int responseCode, String responseBody) {
    this.responseBody = responseBody;
    this.responseCode = responseCode;
}

public int getResponseCode() {
    return responseCode;
}

public String getResponseBody() {
    return responseBody;
}
}

Answer 1

Please check this,

Collection<Callable<Response>> tasks = new ArrayList<>();
for (Branch branch : Branches) {
    tasks.add(new Request(branch.getUrl(), branch.getUserName(), branch.getPassword()));
}

int numThreads = Branches.size() > 4 ? 4 : Branches.size();
ExecutorService executor = Executors.newFixedThreadPool(numThreads);
List<Future<Response>> results = executor.invokeAll(tasks);
for(Future<Response> response : results){
    responseBody = response.get().getResponseBody();
    resultArray.put(responseBody);
}

Java 7 onwards we you can try replace ExecutorService with ForkJoin Pool,

Collection<Callable<Response>> tasks = new ArrayList<>();
for (Branch branch : Branches) {
    tasks.add(new Request(branch.getUrl(), branch.getUserName(), branch.getPassword()));
}

int numThreads = Branches.size() > 4 ? 4 : Branches.size();
ForkJoinPool pool = new ForkJoinPool(Runtime.getRuntime().availableProcessors());
List<Future<Response>> results = pool.invokeAll(tasks);
for(Future<Response> response : results){
    responseBody = response.get().getResponseBody();
    resultArray.put(responseBody);
}

Note:- The code is not tested, i have used something like this few months back.