How can we make asynchronous REST api call in Java?

Question

I am using Spring RestTemplate and want to make a call to another service that doesn't return any response body. So, I don't want to wait for the response. So, it's just fire and forget, and continue with the remaining code. I am thinking of creating a new Thread to do this but really not sure what's the correct approach.

Solution 1

The correct approach is to execute the async with a callback (using DeferredResult, like this (assuming we have a class someClass that we want to retrieve from the API:

@GetMapping(path = "/testingAsync")
public DeferredResult<String> value() throws ExecutionException, InterruptedException, TimeoutException {
   AsyncRestTemplate restTemplate = new AsyncRestTemplate();
   String baseUrl = "http://someUrl/blabla";
   HttpHeaders requestHeaders = new HttpHeaders();
   requestHeaders.setAccept(Arrays.asList(MediaType.APPLICATION_JSON));
   String value = "";

   HttpEntity entity = new HttpEntity("parameters", requestHeaders);
   final DeferredResult<String> result = new DeferredResult<>();
   ListenableFuture<ResponseEntity<someClass>> futureEntity = restTemplate.getForEntity(baseUrl, someClass.class);

   futureEntity.addCallback(new ListenableFutureCallback<ResponseEntity<someClass>>() {
      @Override
      public void onSuccess(ResponseEntity<someClass> result) {
         System.out.println(result.getBody().getName());
         result.setResult(result.getBody().getName());
      }

      @Override
      public void onFailure(Throwable ex) {
         result.setErrorResult(ex.getMessage());
      }
  });

  return result;
}

Solution 2

If you use Java 11, java support asynchronous HTTP Client. Asynchronous client using CompletableFuture in the back. You can see javadoc.

HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("http://openjdk.java.net/"))
            .timeout(Duration.ofMinutes(1))
            .header("Content-Type", "application/json")
            .POST(BodyPublishers.ofFile(Paths.get("file.json")))
            .build();

    client.sendAsync(request, BodyHandlers.ofString())
            .thenApply(response -> { System.out.println(response.statusCode());
                return response; } )
            .thenApply(HttpResponse::body)
            .thenAccept(System.out::println);

Comments

[viswa]

It's throwing error as 'org.springframework.http.ResponseEntity cannot be cast to org.springframework.util.concurrent.ListenableFuture' for this line "ListenableFuture<ResponseEntity<InitiateEventResponseVO>> response = restTemplate.postForEntity(url, request, InitiateEventResponseVO.class);

[Pravin]

AsyncRestTemplate is Deprecated

[Dhana]

AsyncRestTemplate is deprecated.

[Soham]

As Pravin mentioned, AsyncRestTemplate is deprecated.