Table of Contents
1. Overview
This article shows how to set up REST in Spring – the Controller and HTTP response codes, configuration of payload marshalling and content negotiation.
2. Understanding REST in Spring
The Spring framework supports two ways of creating RESTful services:
- using MVC with ModelAndView
- using HTTP message converters
The ModelAndView approach is older and much better documented, but also more verbose and configuration heavy. It tries to shoehorn the REST paradigm into the old model, which is not without problems. The Spring team understood this and provided first-class REST support starting with Spring 3.0.
The new approach, based on HttpMessageConverter and annotations, is much more lightweight and easy to implement. Configuration is minimal, and it provides sensible defaults for what you would expect from a RESTful service.
3. The Java Configuration
@Configuration
@EnableWebMvc
public class WebConfig{
//
}
The new @EnableWebMvc annotation does some useful things – specifically, in the case of REST, it detects the existence of Jackson and JAXB 2 on the classpath and automatically creates and registers default JSON and XML converters. The functionality of the annotation is equivalent to the XML version:
<mvc:annotation-driven />
This is a shortcut, and though it may be useful in many situations, it’s not perfect. When more complex configuration is needed, remove the annotation and extend WebMvcConfigurationSupport directly.
3.1. Using Spring Boot
If we’re using the @SpringBootApplication annotation and the spring-webmvc library is on the classpath, then the @EnableWebMvc annotation is added automatically with a default autoconfiguration.
We can still add MVC functionality to this configuration by implementing the WebMvcConfigurer interface on a @Configuration annotated class. We can also use a WebMvcRegistrationsAdapter instance to provide our own RequestMappingHandlerMapping, RequestMappingHandlerAdapter, or ExceptionHandlerExceptionResolver implementations.
Finally, if we want to discard Spring Boot’s MVC features and declare a custom configuration, we can do so by using the @EnableWebMvc annotation.
4. Testing the Spring Context
Starting with Spring 3.1, we get first-class testing support for @Configuration classes:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(
classes = {WebConfig.class, PersistenceConfig.class},
loader = AnnotationConfigContextLoader.class)
public class SpringContextIntegrationTest {
@Test
public void contextLoads(){
// When
}
}
We’re specifying the Java configuration classes with the @ContextConfiguration annotation. The new AnnotationConfigContextLoader loads the bean definitions from the @Configuration classes.
Notice that the WebConfig configuration class was not included in the test because it needs to run in a Servlet context, which is not provided.
4.1. Using Spring Boot
Spring Boot provides several annotations to set up the Spring ApplicationContext for our tests in a more intuitive way.
We can load only a particular slice of the application configuration, or we can simulate the whole context startup process.
For instance, we can use the @SpringBootTest annotation if we want the entire context to be created without starting the server.
With that in place, we can then add the @AutoConfigureMockMvc to inject a MockMvc instance and send HTTP requests:
@RunWith(SpringRunner.class)
@SpringBootTest
@AutoConfigureMockMvc
public class FooControllerAppIntegrationTest {
@Autowired
private MockMvc mockMvc;
@Test
public void whenTestApp_thenEmptyResponse() throws Exception {
this.mockMvc.perform(get("/foos")
.andExpect(status().isOk())
.andExpect(...);
}
}
To avoid creating the whole context and test only our MVC Controllers, we can use @WebMvcTest:
@RunWith(SpringRunner.class)
@WebMvcTest(FooController.class)
public class FooControllerWebLayerIntegrationTest {
@Autowired
private MockMvc mockMvc;
@MockBean
private IFooService service;
@Test()
public void whenTestMvcController_thenRetrieveExpectedResult() throws Exception {
// ...
this.mockMvc.perform(get("/foos")
.andExpect(...);
}
}
5. The Controller
The @RestController is the central artifact in the entire Web Tier of the RESTful API. For the purpose of this post, the controller is modeling a simple REST resource – Foo:
@RestController
@RequestMapping("/foos")
class FooController {
@Autowired
private IFooService service;
@GetMapping
public List<Foo> findAll() {
return service.findAll();
}
@GetMapping(value = "/{id}")
public Foo findById(@PathVariable("id") Long id) {
return RestPreconditions.checkFound(service.findById(id));
}
@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public Long create(@RequestBody Foo resource) {
Preconditions.checkNotNull(resource);
return service.create(resource);
}
@PutMapping(value = "/{id}")
@ResponseStatus(HttpStatus.OK)
public void update(@PathVariable( "id" ) Long id, @RequestBody Foo resource) {
Preconditions.checkNotNull(resource);
RestPreconditions.checkNotNull(service.getById(resource.getId()));
service.update(resource);
}
@DeleteMapping(value = "/{id}")
@ResponseStatus(HttpStatus.OK)
public void delete(@PathVariable("id") Long id) {
service.deleteById(id);
}
}
You may have noticed I’m using a straightforward, Guava style RestPreconditions utility:
public class RestPreconditions {
public static <T> T checkFound(T resource) {
if (resource == null) {
throw new MyResourceNotFoundException();
}
return resource;
}
}
The Controller implementation is non-public – this is because it doesn’t need to be.
Usually, the controller is the last in the chain of dependencies. It receives HTTP requests from the Spring front controller (the DispatcherServlet) and simply delegates them forward to a service layer. If there’s no use case where the controller has to be injected or manipulated through a direct reference, then I prefer not to declare it as public.
The request mappings are straightforward. As with any controller, the actual value of the mapping, as well as the HTTP method, determine the target method for the request. @RequestBody will bind the parameters of the method to the body of the HTTP request, whereas @ResponseBody does the same for the response and return type.
The @RestController is a shorthand to include both the @ResponseBody and the @Controller annotations in our class.
They also ensure that the resource will be marshalled and unmarshalled using the correct HTTP converter. Content negotiation will take place to choose which one of the active converters will be used, based mostly on the Accept header, although other HTTP headers may be used to determine the representation as well.
6. Mapping the HTTP Response Codes
The status codes of the HTTP response are one of the most important parts of the REST service, and the subject can quickly become very complicated. Getting these right can be what makes or breaks the service.
6.1. Unmapped Requests
If Spring MVC receives a request which doesn’t have a mapping, it considers the request not to be allowed and returns a 405 METHOD NOT ALLOWED back to the client.
It’s also a good practice to include the Allow HTTP header when returning a 405 to the client, to specify which operations are allowed. This is the standard behavior of Spring MVC and doesn’t require any additional configuration.
6.2. Valid Mapped Requests
For any request that does have a mapping, Spring MVC considers the request valid and responds with 200 OK if no other status code is specified otherwise.
It’s because of this that the controller declares different @ResponseStatus for the create, update and delete actions but not for get, which should indeed return the default 200 OK.
6.3. Client Error
In the case of a client error, custom exceptions are defined and mapped to the appropriate error codes.
Simply throwing these exceptions from any of the layers of the web tier will ensure Spring maps the corresponding status code on the HTTP response:
@ResponseStatus(HttpStatus.BAD_REQUEST)
public class BadRequestException extends RuntimeException {
//
}
@ResponseStatus(HttpStatus.NOT_FOUND)
public class ResourceNotFoundException extends RuntimeException {
//
}
These exceptions are part of the REST API and, as such, should only be used in the appropriate layers corresponding to REST; if for instance, a DAO/DAL layer exists, it should not use the exceptions directly.
Note also that these are not checked exceptions but runtime exceptions – in line with Spring practices and idioms.
6.4. Using @ExceptionHandler
Another option to map custom exceptions on specific status codes is to use the @ExceptionHandler annotation in the controller. The problem with that approach is that the annotation only applies to the controller in which it’s defined. This means that we need to declares in each controller individually.
Of course, there are more ways to handle errors in both Spring and Spring Boot that offer more flexibility.
7. Additional Maven Dependencies
In addition to the spring-webmvc dependency required for the standard web application, we’ll need to set up content marshalling and unmarshalling for the REST API:
<dependencies>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.8</version>
</dependency>
<dependency>
<groupId>javax.xml.bind</groupId>
<artifactId>jaxb-api</artifactId>
<version>2.3.1</version>
<scope>runtime</scope>
</dependency>
</dependencies>
These are the libraries used to convert the representation of the REST resource to either JSON or XML.
7.1. Using Spring Boot
If we want to retrieve JSON-formatted resources, Spring Boot provides support for different libraries, namely Jackson, Gson and JSON-B.
Auto-configuration is carried out by just including any of the mapping libraries in the classpath.
Usually, if we’re developing a web application, we’ll just add the spring-boot-starter-web dependency and rely on it to include all the necessary artifacts to our project:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>2.4.0</version>
</dependency>
Spring Boot uses Jackson by default.
If we want to serialize our resources in an XML format, we’ll have to add the Jackson XML extension (jackson-dataformat-xml) to our dependencies, or fallback to the JAXB implementation (provided by default in the JDK) by using the @XmlRootElement annotation on our resource.
8. Conclusion
This tutorial illustrated how to implement and configure a REST Service using Spring and Java-based configuration.
In the next articles of the series, I will focus on Discoverability of the API, advanced content negotiation and working with additional representations of a Resource.
All the code of this article is available over on Github. This is a Maven-based project, so it should be easy to import and run as it is.
