Course – LS (cat=HTTP Client-Side)
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1. Overview

In this tutorial, we’re going to illustrate the broad range of operations where the Spring REST Client — RestTemplate — can be used, and used well.

For the API side of all examples, we’ll be running the RESTful service from here.

Further reading:

Basic Authentication with the RestTemplate

How to do Basic Authentication with the Spring RestTemplate.

RestTemplate with Digest Authentication

How to set up Digest Authentication for the Spring RestTemplate using HttpClient 4.

Exploring the Spring Boot TestRestTemplate

Learn how to use the new TestRestTemplate in Spring Boot to test a simple API.

2. Deprecation Notice

As of Spring Framework 5, alongside the WebFlux stack, Spring introduced a new HTTP client called WebClient.

WebClient is a modern, alternative HTTP client to RestTemplate. Not only does it provide a traditional synchronous API, but it also supports an efficient nonblocking and asynchronous approach.

That said, if we’re developing new applications or migrating an old one, it’s a good idea to use WebClient. Moving forward, RestTemplate will be deprecated in future versions.

3. Use GET to Retrieve Resources

3.1. Get Plain JSON

Let’s start simple and talk about GET requests, with a quick example using the getForEntity() API:

RestTemplate restTemplate = new RestTemplate();
String fooResourceUrl
  = "http://localhost:8080/spring-rest/foos";
ResponseEntity<String> response
  = restTemplate.getForEntity(fooResourceUrl + "/1", String.class);
Assertions.assertEquals(response.getStatusCode(), HttpStatus.OK);

Notice that we have full access to the HTTP response, so we can do things like check the status code to make sure the operation was successful or work with the actual body of the response:

ObjectMapper mapper = new ObjectMapper();
JsonNode root = mapper.readTree(response.getBody());
JsonNode name = root.path("name");

We’re working with the response body as a standard String here and using Jackson (and the JSON node structure that Jackson provides) to verify some details.

3.2. Retrieving POJO Instead of JSON

We can also map the response directly to a Resource DTO:

public class Foo implements Serializable {
    private long id;

    private String name;
    // standard getters and setters

Now we can simply use the getForObject API in the template:

Foo foo = restTemplate
  .getForObject(fooResourceUrl + "/1", Foo.class);
Assertions.assertEquals(foo.getId(), 1L);

4. Use HEAD to Retrieve Headers

Let’s now have a quick look at using HEAD before moving on to the more common methods.

We’re going to be using the headForHeaders() API here:

HttpHeaders httpHeaders = restTemplate.headForHeaders(fooResourceUrl);

5. Use POST to Create a Resource

In order to create a new Resource in the API, we can make good use of the postForLocation(), postForObject() or postForEntity() APIs.

The first returns the URI of the newly created Resource, while the second returns the Resource itself.

5.1. The postForObject() API

RestTemplate restTemplate = new RestTemplate();

HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
Foo foo = restTemplate.postForObject(fooResourceUrl, request, Foo.class);
Assertions.assertEquals(foo.getName(), "bar");

5.2. The postForLocation() API

Similarly, let’s have a look at the operation that instead of returning the full Resource, just returns the Location of that newly created Resource:

HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
URI location = restTemplate
  .postForLocation(fooResourceUrl, request);

5.3. The exchange() API

Let’s have a look at how to do a POST with the more generic exchange API:

RestTemplate restTemplate = new RestTemplate();
HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
ResponseEntity<Foo> response = restTemplate
  .exchange(fooResourceUrl, HttpMethod.POST, request, Foo.class);
Assertions.assertEquals(response.getStatusCode(), HttpStatus.CREATED);
Foo foo = response.getBody();
Assertions.assertEquals(foo.getName(), "bar");

5.4. Submit Form Data

Next, let’s look at how to submit a form using the POST method.

First, we need to set the Content-Type header to application/x-www-form-urlencoded.

This makes sure that a large query string can be sent to the server, containing name/value pairs separated by &:

HttpHeaders headers = new HttpHeaders();

We can wrap the form variables into a LinkedMultiValueMap:

MultiValueMap<String, String> map= new LinkedMultiValueMap<>();
map.add("id", "1");

Next, we build the Request using an HttpEntity instance:

HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);

Finally, we can connect to the REST service by calling restTemplate.postForEntity() on the Endpoint: /foos/form

ResponseEntity<String> response = restTemplate.postForEntity(
  fooResourceUrl + "/form", request , String.class);
Assertions.assertEquals(response.getStatusCode(), HttpStatus.CREATED);

6. Use OPTIONS to Get Allowed Operations

Next, we’re going to have a quick look at using an OPTIONS request and exploring the allowed operations on a specific URI using this kind of request; the API is optionsForAllow:

Set<HttpMethod> optionsForAllow = restTemplate.optionsForAllow(fooResourceUrl);
HttpMethod[] supportedMethods
  = {HttpMethod.GET, HttpMethod.POST, HttpMethod.PUT, HttpMethod.DELETE};

7. Use PUT to Update a Resource

Next, we’ll start looking at PUT and more specifically the exchange() API for this operation, since the template.put API is pretty straightforward.

7.1. Simple PUT With exchange()

We’ll start with a simple PUT operation against the API — and keep in mind that the operation isn’t returning a body back to the client:

Foo updatedInstance = new Foo("newName");
String resourceUrl = 
  fooResourceUrl + '/' + createResponse.getBody().getId();
HttpEntity<Foo> requestUpdate = new HttpEntity<>(updatedInstance, headers);, HttpMethod.PUT, requestUpdate, Void.class);

7.2. PUT With exchange() and a Request Callback

Next, we’re going to be using a request callback to issue a PUT.

Let’s make sure we prepare the callback, where we can set all the headers we need as well as a request body:

RequestCallback requestCallback(final Foo updatedInstance) {
    return clientHttpRequest -> {
        ObjectMapper mapper = new ObjectMapper();
        mapper.writeValue(clientHttpRequest.getBody(), updatedInstance);
          HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);
          HttpHeaders.AUTHORIZATION, "Basic " + getBase64EncodedLogPass());

Next, we create the Resource with a POST request:

ResponseEntity<Foo> response = restTemplate
  .exchange(fooResourceUrl, HttpMethod.POST, request, Foo.class);
Assertions.assertEquals(response.getStatusCode(), HttpStatus.CREATED);

And then we update the Resource:

Foo updatedInstance = new Foo("newName");
String resourceUrl = fooResourceUrl + '/' + response.getBody().getId();
  clientHttpResponse -> null);

8. Use DELETE to Remove a Resource

To remove an existing Resource, we’ll make quick use of the delete() API:

String entityUrl = fooResourceUrl + "/" + existingResource.getId();

9. Configure Timeout

We can configure RestTemplate to time out by simply using ClientHttpRequestFactory:

RestTemplate restTemplate = new RestTemplate(getClientHttpRequestFactory());

private ClientHttpRequestFactory getClientHttpRequestFactory() {
    int timeout = 5000;
    HttpComponentsClientHttpRequestFactory clientHttpRequestFactory
      = new HttpComponentsClientHttpRequestFactory();
    return clientHttpRequestFactory;

And we can use HttpClient for further configuration options:

private ClientHttpRequestFactory getClientHttpRequestFactory() {
    int timeout = 5000;
    RequestConfig config = RequestConfig.custom()
    CloseableHttpClient client = HttpClientBuilder
    return new HttpComponentsClientHttpRequestFactory(client);

10. Conclusion

In this article, we went over the main HTTP Verbs, using RestTemplate to orchestrate requests using all of these.

If you want to dig into how to do authentication with the template, check out our article on Basic Auth with RestTemplate.

The implementation of all these examples and code snippets can be found over on GitHub.

Course – LS (cat=HTTP Client-Side)
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Get started with Spring Boot and with core Spring, through the Learn Spring course:


res – HTTP Client (eBook) (cat=Http Client-Side)
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