java - Spring RestTemplate - async vs sync restTemplate

Question

I wrote the following code to test the performance of both the sync RestTemplate and AsyncRestTemplate. I just ran it a few times manually on POSTMAN.

We are just passing 10 references into a GET call so that we can return 10 links:

RestTemplate - synchronous and returns in 2806ms:

ArrayList<String> references = new ArrayList<>();
ArrayList<String> links = new ArrayList<>();
RestTemplate restTemplate = new RestTemplate(); 
restTemplate.getMessageConverters().add(new StringHttpMessageConverter());
for (int i = 0; i < 10; i++) {
    ResponseEntity<String> resource = restTemplate.getForEntity(references.get(i), String.class);
    links.add(resource.getBody().toString());
}

RestTemplate - asynchronous and returns in 2794ms:

//Creating a synchronizedList so that when the async resttemplate returns, there will be no concurrency issues
List<String> links = Collections.synchronizedList(new ArrayList<String>());

//CustomClientHttpRequestFactory just extends SimpleClientHttpRequestFactory but disables automatic redirects in SimpleClientHttpRequestFactory
CustomClientHttpRequestFactory customClientHttpRequestFactory = new CustomClientHttpRequestFactory();
//Setting the ThreadPoolTaskExecutor for the Async calls
org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor pool = new org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor();
pool.setCorePoolSize(5);
pool.setMaxPoolSize(10);
pool.setWaitForTasksToCompleteOnShutdown(true);
pool.initialize();
//Setting the TaskExecutor to the ThreadPoolTaskExecutor
customClientHttpRequestFactory.setTaskExecutor(pool);

ArrayList<String> references = new ArrayList<>();
ArrayList<String> links = new ArrayList<>();
AsyncRestTemplate asyncRestTemplate = new AsyncRestTemplate(customClientHttpRequestFactory); 
restTemplate.getMessageConverters().add(new StringHttpMessageConverter());
for (int i = 0; i < 10; i++) {
    Future<ResponseEntity<String>> resource = asyncRestTemplate.getForEntity(references.get(i), String.class);
    ResponseEntity<String> entity = resource.get(); //this should start up 10 threads to get the links asynchronously
    links.add(entity.getBody().toString());
}

In most cases, both methods actually return back the results with a very similar time, averaging 2800ms in both async and sync calls.

Am I doing something incorrect as I would have expected the async call to be much faster?

Answer 1

Nowadays, AsyncRestTemplate is @Deprecated in favor of WebClient. So nobody should use that class anymore!

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/reactive/function/client/WebClient.html