Spring RestTemplate Tutorial

1. Overview

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

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

2. Use GET to Retrieve Resources

2.1. 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-security-rest-full/foos";
ResponseEntity<String> response = 
  restTemplate.getForEntity(fooResourceUrl + "/1", String.class);
assertThat(response.getStatusCode(), is(HttpStatus.OK));

We have full access to the response, so we can do things like checking the status code to make sure the operation was actually successful and then we can start working with the body of the response – which in this case is JSON:

ObjectMapper mapper = new ObjectMapper();
JsonNode root = mapper.readTree(response.getBody());
JsonNode name = root.path("name");
assertThat(name.asText(), is("bar"));

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.

2.1. Retrieving POJO Instead of JSON

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

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);
assertThat(foo.getName(), is("bar"));
assertThat(foo.getId(), is(1L));

3. 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);
assertTrue(httpHeaders.getContentType().includes(MediaType.APPLICATION_JSON));

4. 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.

4.1. The postForObject API

ClientHttpRequestFactory requestFactory = getClientHttpRequestFactory();
RestTemplate restTemplate = new RestTemplate(requestFactory);

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

4.2. 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);
assertThat(location, notNullValue());

4.3. The exchange API

Finally, 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);
assertThat(response.getStatusCode(), is(HttpStatus.CREATED));
Foo foo = response.getBody();
assertThat(foo, notNullValue());
assertThat(foo.getName(), is("bar"));

5. 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};
assertTrue(optionsForAllow.containsAll(Arrays.asList(supportedMethods)));

6. Use PUT to Update a Resource

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

6.1. 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 any body back to the client:

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

6.2. 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);
        clientHttpRequest.getHeaders().add(
          HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);
        clientHttpRequest.getHeaders().add(
          HttpHeaders.AUTHORIZATION, "Basic " + getBase64EncodedLogPass());
    };
}

Next we create the Resource with POST request:

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

And then we update the Resource:

Foo updatedInstance = new Foo("newName");
updatedInstance.setId(response.getBody().getId());
String resourceUrl =fooResourceUrl + '/' + response.getBody().getId();
restTemplate.execute(
  resourceUrl, HttpMethod.PUT, requestCallback(updatedInstance), clientHttpResponse -> null);

7. Use DELETE to Remove a Resource

To remove an existing Resource we’ll make short work of the delete() API:

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

8. Conclusion

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 my write-up on Basic Auth with RestTemplate.

The implementation of all these examples and code snippets can be found in my github project – this is an Eclipse based project, so it should be easy to import and run as it is.