How to asynchronously call restTemplate using CompletableFuture

Question

I wanna asynchronously call an external api for 3 times and wait them return. My code likes below:

public ResponseClass callApi(xxx) {

    // ... other code
    ResponseClass response = restTemplate.postForEntity(
        requestUrl,
        httpEntity,
        ResponseClass.class
    ).getBody();

    return response;
} 


public void otherMethod() {
    CompletableFuture<Void> completableFuture1 = CompletableFuture.runAsync(() -> {
        callApi(xxx);
    });
    CompletableFuture<Void> completableFuture2 = CompletableFuture.runAsync(() -> {
        callApi(xxx);
    });
    CompletableFuture<Void> completableFuture3 = CompletableFuture.runAsync(() -> {
        callApi(xxx);
    });

    CompletableFuture.allOf(completableFuture).join();

}

I expect process will wait for all 3 call return after CompletableFuture.allOf , But what I got is an exception:

java.util.concurrent.CompletionException: org.springframework.web.client.UnknownContentTypeException: Could not extract response: no suitable HttpMessageConverter found for response type [class ...

I was so confused since it works fine when I call them synchronously ( without CompletableFuture ).

So, could someone help me find a way to do that? Thanks very much.

Answer 1

Create a list of CompletableFuture and add try to add your 3 CompletableFuture like below.

     List<CompletableFuture<Void>> completableFutures = new ArrayList<>(10);
     completableFutures.add(completableFuture1);
     completableFutures.add(completableFuture2);
     completableFutures.add(completableFuture3);

and then finally use CompletableFuture.allOf

CompletableFuture.allOf(completableFutures.toArray(new CompletableFuture[completableFutures.size()])).get();