Test a REST API with Java

1. Overview

This tutorial focuses on the basic principles and mechanics of testing a REST API with live Integration Tests (with a JSON payload).

The main goal is to provide an introduction to testing the basic correctness of the API – and we’re going to be using the latest version of the GitHub REST API for the examples.

For an internal application, this kind of testing will usually run as a late step in a Continuous Integration process, consuming the REST API after it has already been deployed.

When testing a REST resource, there are usually a few orthogonal responsibilities the tests should focus on:

• the HTTP response code
• other HTTP headers in the response
• the payload (JSON, XML)

Each test should only focus on a single responsibility and include a single assertion. Focusing on a clear separation always has benefits, but when doing this kind of black box testing is even more important, as the general tendency is to write complex test scenarios in the very beginning.

Another important aspect of the integration tests is adherence to the Single Level of Abstraction Principle – the logic within a test should be written at a high level. Details such as creating the request, sending the HTTP request to the server, dealing with IO, etc should not be done inline but via utility methods.

2. Testing the Status Code

@Test
public void givenUserDoesNotExists_whenUserInfoIsRetrieved_then404IsReceived()
  throws ClientProtocolException, IOException {
 
    // Given
    String name = RandomStringUtils.randomAlphabetic( 8 );
    HttpUriRequest request = new HttpGet( "https://api.github.com/users/" + name );

    // When
    HttpResponse httpResponse = HttpClientBuilder.create().build().execute( request );

    // Then
    assertThat(
      httpResponse.getStatusLine().getStatusCode(),
      equalTo(HttpStatus.SC_NOT_FOUND));
}

This is a rather simple test – it verifies that a basic happy path is working, without adding too much complexity to the test suite.

If for whatever reason, it fails, then there is no need to look at any other test for this URL until this is fixed.

3. Testing the Media Type

@Test
public void 
givenRequestWithNoAcceptHeader_whenRequestIsExecuted_thenDefaultResponseContentTypeIsJson()
  throws ClientProtocolException, IOException {
 
   // Given
   String jsonMimeType = "application/json";
   HttpUriRequest request = new HttpGet( "https://api.github.com/users/eugenp" );

   // When
   HttpResponse response = HttpClientBuilder.create().build().execute( request );

   // Then
   String mimeType = ContentType.getOrDefault(response.getEntity()).getMimeType();
   assertEquals( jsonMimeType, mimeType );
}

This ensures that the Response actually contains JSON data.

As you might have noticed, we’re following a logical progression of tests – first the Response Status Code (to ensure that the request was OK), then the Media Type of the Response, and only in the next test will we look at the actual JSON payload.

4. Testing the JSON Payload

@Test
public void 
  givenUserExists_whenUserInformationIsRetrieved_thenRetrievedResourceIsCorrect()
  throws ClientProtocolException, IOException {
 
    // Given
    HttpUriRequest request = new HttpGet( "https://api.github.com/users/eugenp" );

    // When
    HttpResponse response = HttpClientBuilder.create().build().execute( request );

    // Then
    GitHubUser resource = RetrieveUtil.retrieveResourceFromResponse(
      response, GitHubUser.class);
    assertThat( "eugenp", Matchers.is( resource.getLogin() ) );
}

In this case, I know the default representation of GitHub resources is JSON, but usually, the <code>[/code]Content-Type header of the response should be tested alongside the Accept header of the request – the client asks for a particular type of representation via Accept, which the server should honor.

5. Utilities for Testing

We’re going to use Jackson 2 to unmarshall the raw JSON String into a type-safe Java Entity:

public class GitHubUser {

    private String login;

    // standard getters and setters
}

We’re only using a simple utility to keep the tests clean, readable and at a high level of abstraction:

public static <T> T retrieveResourceFromResponse(HttpResponse response, Class<T> clazz) 
  throws IOException {
 
    String jsonFromResponse = EntityUtils.toString(response.getEntity());
    ObjectMapper mapper = new ObjectMapper()
      .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    return mapper.readValue(jsonFromResponse, clazz);
}

Notice that Jackson is ignoring unknown properties that the GitHub API is sending our way – that’s simply because the Representation of a User Resource on GitHub gets pretty complex – and we don’t need any of that information here.

6. Dependencies

The utilities and tests make use of the following libraries, all available in Maven central:

• HttpClient
• Jackson 2
• Hamcrest (optional)

7. Conclusion

This is only one part of what the complete integration testing suite should be. The tests focus on ensuring basic correctness for the REST API, without going into more complex scenarios,

For example, the following are not covered: Discoverability of the API, consumption of different representations for the same Resource, etc.

The implementation of all these examples and code snippets can be found over on Github – this is a Maven-based project, so it should be easy to import and run as it is.