When building an API using Java Spring, it is crucial to handle errors and exceptions gracefully. This not only improves the overall user experience but also makes debugging and troubleshooting easier. In this blog post, we will explore how to document API error handling and exception mapping using Java Spring REST Docs.

What is Java Spring REST Docs?

Java Spring REST Docs is a powerful tool that allows you to generate documentation for your RESTful APIs. It helps you describe the expected request and response formats, as well as document various aspects of your API, including error handling and exception mapping.

Error Handling in Java Spring

Java Spring provides various mechanisms to handle errors and exceptions in your APIs. One common approach is to use the @ControllerAdvice annotation to define a global exception handler. This allows you to centralize error handling logic and provide consistent error responses across all your endpoints.

To document error handling using Java Spring REST Docs, you need to follow these steps:

1. Add the spring-restdocs-mockmvc dependency to your project.

<dependency>
  <groupId>org.springframework.restdocs</groupId>
  <artifactId>spring-restdocs-mockmvc</artifactId>
  <scope>test</scope>
</dependency>

1. Create a test class and configure REST Docs.

@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class)
@AutoConfigureRestDocs(outputDir = "target/snippets")
public class UserControllerTest {

  @Autowired
  private MockMvc mockMvc;

  @Autowired
  private ObjectMapper objectMapper;

  @Autowired
  private WebApplicationContext context;

  @Before
  public void setUp() {
    mockMvc = MockMvcBuilders.webAppContextSetup(context)
        .apply(documentationConfiguration(this.restDocumentation))
        .build();
  }

  // Test methods and documentation snippets
}

1. Write a test method to simulate an error scenario and document it.

@Test
public void testInvalidUserCreation() throws Exception {
  UserRequest userRequest = new UserRequest("John Doe", "invalid_email");

  mockMvc.perform(post("/users")
    .contentType(MediaType.APPLICATION_JSON)
    .content(objectMapper.writeValueAsString(userRequest)))
    .andExpect(status().isBadRequest())
    .andDo(document("user-create-invalid-email",
        requestFields(
            fieldWithPath("name").description("The name of the user"),
            fieldWithPath("email").description("The email address of the user")
        ),
        responseFields(
            fieldWithPath("status").description("The HTTP status code"),
            fieldWithPath("error").description("The error message")
        )
    ));
}

In the above example, we simulate an invalid user creation request by providing an invalid email address. We expect a 400 Bad Request response from the API. The andDo(document(...)) method call generates documentation snippets for the request and response fields.

1. Run the test and generate API documentation.

Once you’ve written test methods for all your error scenarios, run the tests to generate the API documentation. The documentation will be stored in the specified outputDir directory.

Exception Mapping in Java Spring

Exception mapping is the process of converting application-specific exceptions into meaningful error responses. Java Spring provides various ways to map exceptions to HTTP status codes and error messages.

To document exception mapping using Java Spring REST Docs, you can follow a similar approach as error handling:

1. Create a custom exception class.

public class UserNotFoundException extends RuntimeException {

  public UserNotFoundException(String message) {
    super(message);
  }
}

1. Create a global exception handler using @ControllerAdvice and annotate the appropriate methods with @ExceptionHandler.

@ControllerAdvice
public class GlobalExceptionHandler {

  @ExceptionHandler(UserNotFoundException.class)
  @ResponseStatus(HttpStatus.NOT_FOUND)
  @ResponseBody
  public ErrorResponse handleUserNotFoundException(UserNotFoundException ex) {
    return new ErrorResponse(HttpStatus.NOT_FOUND.value(), ex.getMessage());
  }
}

1. Write a test method to simulate the exception scenario and document it.

@Test
public void testUserNotFound() throws Exception {
  String nonExistingUserId = "non_existing_user";

  mockMvc.perform(get("/users/{id}", nonExistingUserId))
    .andExpect(status().isNotFound())
    .andDo(document("user-not-found",
        pathParameters(
            parameterWithName("id").description("The ID of the user")
        ),
        responseFields(
            fieldWithPath("status").description("The HTTP status code"),
            fieldWithPath("error").description("The error message")
        )
    ));
}

In the above example, we simulate a scenario where a user is not found by providing a non-existing user ID. We expect a 404 Not Found response from the API. The andDo(document(...)) method call generates documentation snippets for the path parameters and response fields.

1. Run the test and generate API documentation.

Similar to error handling, run the tests to generate the API documentation.

Conclusion

Documenting API error handling and exception mapping is essential for building robust and developer-friendly APIs. Java Spring REST Docs provides an elegant way to generate API documentation that includes comprehensive coverage of error handling and exception mapping. By following the steps outlined in this blog post, you can effectively document these aspects of your API and ensure a smooth and error-free experience for your API consumers.

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