获得可完成未来的结果

Question

I am using completablefuture to be returned from an async thread in springboot application. My implementation is below. From my understanding a new thread should be started for each item in list and should process parallely. I understand .get will block the execution but as it is parallely running still i dont see any improvement in performance. Any suggestions on below please to improve the performance?

ServiceA.java

@Autowired
ServiceB serviceb;


public List<String> getNames(List<Item> items) {
List<CompletableFuture<String>> list = new ArrayList<>();
List<String> returnList = new ArrayList<>();
for( Item item: items) {
  CompletableFuture<String> getItemName = serviceb.doProcess(item);
  list.add(getItemName):
}

for( CompletableFuture name : list) {
 returnList.add(name.get());
}
return returnList;
}

ServiceB.java

Class ServiceB {
@Async
Public CompletableFuture<String> doProcess(Item item)
{
   //do process
}

Answer 1

You could call allOf to wait for all results. This will wait for all CompletableFuture to complete.

        List<String> returnList = new ArrayList<>(items.size());
        CompletableFuture<String>[] tasks = items.stream()
            .map(value-> serviceb.doProcess(value).thenApply(returnList::add))
            .toArray(CompletableFuture[]::new);
        // wait for all tasks to finish
        CompletableFuture.allOf(tasks).get(50, TimeUnit.SECONDS);
        // return the results
        return returnList;

Second solution would be to use an reactive approach like publisher/subscriber pattern ( Spring WebFlux or JavaRx ). This way your application would have little/no waiting operation. But this would affect your application architecture.

One Advice : In order to create an CompletableFuture use the constructor with ExecutorService in order to keep in check number of threads and have control over running threads or when application shuts down.

Answer 2

The get() will definitely not going to work out as it will block the main thread of execution to fetch the result from the async threads. A better option would be to use callback methods(thenApply() etc) accordingly so as to let the main thread continue its work.

Answer 3

You can use thenAccept to add the items to the list.

List<String> list = new CopyOnWriteArrayList<>();
CompletableFuture.allOf(
    Stream.of(items).map(
        i -> CompletableFuture.supplyAsync(i).thenAccept(list::add)
    ).toArray(CompletableFuture[]::new)
).get(10, SECONDS);

return list;