How to use Spring WebClient to make multiple calls simultaneously? - spring

Question

I want to execute 3 calls simultaneously and process the results once they're all done.
I know this can be achieved using AsyncRestTemplate as it is mentioned here How to use AsyncRestTemplate to make multiple calls simultaneously?
However, AsyncRestTemplate is deprecated in favor of WebClient. I have to use Spring MVC in the project but interested if I can use a WebClient just to execute simultaneous calls. Can someone advise how this should be done properly with WebClient?

Answer 1

Assuming a WebClient wrapper (like in reference doc):
#Service
public class MyService {
private final WebClient webClient;
public MyService(WebClient.Builder webClientBuilder) {
this.webClient = webClientBuilder.baseUrl("http://example.org").build();
}
public Mono<Details> someRestCall(String name) {
return this.webClient.get().url("/{name}/details", name)
.retrieve().bodyToMono(Details.class);
}
}
..., you could invoke it asynchronously via:
// ...
#Autowired
MyService myService
// ...
Mono<Details> foo = myService.someRestCall("foo");
Mono<Details> bar = myService.someRestCall("bar");
Mono<Details> baz = myService.someRestCall("baz");
// ..and use the results (thx to: [2] & [3]!):
// Subscribes sequentially:
// System.out.println("=== Flux.concat(foo, bar, baz) ===");
// Flux.concat(foo, bar, baz).subscribe(System.out::print);
// System.out.println("\n=== combine the value of foo then bar then baz ===");
// foo.concatWith(bar).concatWith(baz).subscribe(System.out::print);
// ----------------------------------------------------------------------
// Subscribe eagerly (& simultaneously):
System.out.println("\n=== Flux.merge(foo, bar, baz) ===");
Flux.merge(foo, bar, baz).subscribe(System.out::print);
Mono javadoc
Flux javadoc
Spring WebClient reference doc
Spring Boot WebClient reference doc
Projectreactor reference doc
Which (reactive) operator to use!
Thanks, Welcome & Kind Regards,

Answer 2

You can make HTTP calls concurrently using simple RestTemplate and ExecutorService:
RestTemplate restTemplate = new RestTemplate();
ExecutorService executorService = Executors.newCachedThreadPool();
Future<String> firstCallFuture = executorService.submit(() -> restTemplate.getForObject("http://first-call-example.com", String.class));
Future<String> secondCallFuture = executorService.submit(() -> restTemplate.getForObject("http://second-call-example.com", String.class));
String firstResponse = firstCallFuture.get();
String secondResponse = secondCallFuture.get();
executorService.shutdown();
Or
Future<String> firstCallFuture = CompletableFuture.supplyAsync(() -> restTemplate.getForObject("http://first-call-example.com", String.class));
Future<String> secondCallFuture = CompletableFuture.supplyAsync(() -> restTemplate.getForObject("http://second-call-example.com", String.class));
String firstResponse = firstCallFuture.get();
String secondResponse = secondCallFuture.get();

Answer 3

You can use Spring reactive client WebClient to send parallel requests.
In this example,
public Mono<UserInfo> getUserInfo(User user) {
Mono<UserInfo> userInfoMono = getUserInfo(user.getId());
Mono<OrgInfo> organizationInfoMono = getOrgInfo(user.getOrgId());
return Mono.zip(userInfoMono, organizationInfoMono).map(tuple -> {
UserInfo userInfo = tuple.getT1();
userInfo.setOrganization(tuple.getT2());
return userInfo;
});
}
Here:
getUserInfo makes an HTTP call to get user info from another service and returns Mono
getOrgInfo method makes HTTP call to get organization info from another service and returns Mono
Mono.zip() waits all the results from all monos and merges into a new mono and returns it.
Then, call getOrgUserInfo().block() to get the final result.

Answer 4

Another way:
public Mono<Boolean> areVersionsOK(){
final Mono<Boolean> isPCFVersionOK = getPCFInfo2();
final Mono<Boolean> isBlueMixVersionOK = getBluemixInfo2();
return isPCFVersionOK.mergeWith(isBlueMixVersionOK)
.filter(aBoolean -> {
return aBoolean;
})
.collectList().map(booleans -> {
return booleans.size() == 2;
});
}