Intro to Spring RestTemplate

We learned to build Spring REST API for XML representation and JSON representation. Now lets learn to build Spring REST client using the Spring RestTemplate to consume the REST APIs which we have written in the linked examples.

Table of Contents

1. Spring RestTemplate class
2. Building RestTemplate Bean
3. HTTP GET Example
4. HTTP POST Example
5. HTTP PUT Example
6. HTTP DELETE Example

1. Spring RestTemplate class

Accessing the REST apis inside a Spring application revolves around the use of the Spring RestTemplate class. The RestTemplate class is designed on the same principles as the many other Spring *Template classes (e.g., JdbcTemplate, JmsTemplate ), providing a simplified approach with default behaviors for performing complex tasks.

Given that the RestTemplate class is a synchronous client and designed to call REST services. It should come as no surprise that its primary methods are closely tied to REST’s underpinnings, which are the HTTP protocol’s methods HEAD, GET, POST, PUT, DELETE, and OPTIONS.

2. How to build RestTemplate

The given below are few examples to create RestTemplate bean in the application. We are only looking at very simple bean definitions.

2.1. Using RestTemplateBuilder

@Bean
public RestTemplate restTemplate(RestTemplateBuilder builder) {

return builder
.setConnectTimeout(Duration.ofMillis(3000))
.setReadTimeout(Duration.ofMillis(3000))
.build();
}

2.2. Using SimpleClientHttpRequestFactory

@Bean
public RestTemplate restTemplate() {

var factory = new SimpleClientHttpRequestFactory();
factory.setConnectTimeout(3000);
factory.setReadTimeout(3000);
return new RestTemplate(factory);
}

2.3. Using Apache HTTPClient (Recommended)

@Autowired
CloseableHttpClient httpClient;

@Value("${api.host.baseurl}")
private String apiHost;

@Bean
public RestTemplate restTemplate() {
RestTemplate restTemplate = new RestTemplate(clientHttpRequestFactory());
restTemplate.setUriTemplateHandler(new DefaultUriBuilderFactory(apiHost));
return restTemplate;
}

@Bean
@ConditionalOnMissingBean
public HttpComponentsClientHttpRequestFactory clientHttpRequestFactory() {
HttpComponentsClientHttpRequestFactory clientHttpRequestFactory
= new HttpComponentsClientHttpRequestFactory();
clientHttpRequestFactory.setHttpClient(httpClient);
return clientHttpRequestFactory;
}

Read More: RestTemplate Configuration with HttpClient.

2.4. Injecting RestTemplate bean

To inject the RestTemplate bean, use the well known @Autowired annotation. If you have multiple beans of type RestTemplate with different configurations, use the @Qualifier annotation as well.

@Autowired
private RestTemplate restTemplate;

3. Spring RestTemplate – HTTP GET Example

Available methods for executing GET APIs are::

• getForObject(url, classType) – retrieve a representation by doing a GET on the URL. The response (if any) is unmarshalled to given class type and returned.
• getForEntity(url, responseType) – retrieve a representation as ResponseEntity by doing a GET on the URL.
• exchange(url, httpMethod, requestEntity, responseType) – execute the specified RequestEntity and return the response as ResponseEntity.
• execute(url, httpMethod, requestCallback, responseExtractor) – execute the httpMethod to the given URI template, preparing the request with the RequestCallback, and reading the response with a ResponseExtractor.

3.1. HTTP GET REST APIs

@Autowired
UserService userService;

@GetMapping("users")
public ResponseEntity<List<User>> getAll() {
return new ResponseEntity<>(userService.getAll(), HttpStatus.OK);
}

@GetMapping("users/{id}")
public ResponseEntity<User> getById(@PathVariable long id) {
Optional<User> user = userService.getById(id);
if (user.isPresent()) {
return new ResponseEntity<>(user.get(), HttpStatus.OK);
} else {
throw new RecordNotFoundException();
}
}

3.2. Consuming REST APIs as String

In the given example, we are fetching the API response as a JSON string. We need to use ObjectMapper to parse it to the POJO before using it in the application.

getForObject() method is pretty useful when we are getting an unparsable response from the server, and we have no control to get it fixed on the server-side. Here, we can get the response as String, and use a custom parser or use a string replacement function to fix the response before handling it to the parser.

private final String URI_USERS_ID = "/users/{id}";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

Map<String, String> params = new HashMap<String, String>();
params.put("id", "1");

//Parse the string after getting the response
String userStr = restTemplate.getForObject(URI_USERS_ID, String.class, params);

Read More: Converting JSON String to Object using Jackson 2

3.3. Consuming REST API response to POJO

In the given example, we are fetching the API response directly into the domain object.

3.3.1. Using getForObject() Method

private final String URI_USERS = "/users";
private final String URI_USERS_ID = "/users/{id}";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

// "users"
User[] usersArray = restTemplate.getForObject(URI_USERS, User[].class);

// "users/{id}"
Map<String, String> params = new HashMap<String, String>();
params.put("id", "1");
User user = restTemplate.getForObject(URI_USERS_ID, User.class, params);

3.3.2. Using getForEntity() Method

private final String URI_USERS = "/users";
private final String URI_USERS_ID = "/users/{id}";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

// "users"
ResponseEntity<User[]> responseEntity = restTemplate
.getForEntity(URI_USERS, User[].class);

// "users/{id}"
Map<String, String> params = new HashMap<String, String>();
params.put("id", "1");

ResponseEntity<User> responseEntity = restTemplate
.getForEntity(URI_USERS_ID, User.class, params);

3.4. Sending HTTP Headers using RestTemplate

private final String URI_USERS = "/users";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

HttpHeaders headers = new HttpHeaders();
headers.setAccept(Arrays.asList(MediaType.APPLICATION_JSON));
headers.set("X-COM-PERSIST", "NO");
headers.set("X-COM-LOCATION", "USA");

HttpEntity<String> entity = new HttpEntity<String>(headers);

ResponseEntity<User[]> responseEntity = restTemplate
.exchange(URI_USERS, HttpMethod.GET, entity, User[].class);

3.5. Sending URL Parameters using RestTemplate

Map<String, String> params = new HashMap<String, String>();
params.put("id", "1");

ResponseEntity<User> responseEntity = restTemplate
.getForEntity(URI_USERS_ID, User.class, params);

4. Spring RestTemplate – HTTP POST Example

Available methods for consuming POST APIs are:

• postForObject(url, request, classType) – POSTs the given object to the URL, and returns the representation found in the response as given class type.
• postForEntity(url, request, responseType) – POSTs the given object to the URL, and returns the response as ResponseEntity.
• postForLocation(url, request, responseType) – POSTs the given object to the URL, and returns returns the value of the Location header.
• exchange(url, requestEntity, responseType)
• execute(url, httpMethod, requestCallback, responseExtractor)

4.1. HTTP POST REST API

The POST API, we will consume in this example.

@PostMapping("users")
public ResponseEntity<User> create(@RequestBody User newUser) {
User user = userService.save(newUser);
if (user == null) {
throw new ServerException();
} else {
return new ResponseEntity<>(user, HttpStatus.CREATED);
}
}

4.2. Spring RestTemplate example to consume POST API

Spring REST client using RestTemplate to access HTTP POST api requests.

private final String URI_USERS = "/users";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

User newUser = new User(1, "Alex", "Golan", "a@mail.com");
User createdUser = restTemplate.postForObject(URI_USERS, newUser, User.class);
//Use the object as needed

5. Spring RestTemplate – HTTP PUT Method Example

Available methods are:

• put(url, request) – PUTs the given request object to URL.

5.1. HTTP PUT REST API

@PutMapping("users/{id}")
public ResponseEntity<User> update(@RequestBody User updatedUser) {
User user = userService.save(updatedUser);
if (user == null) {
throw new ServerException();
} else {
return new ResponseEntity<>(user, HttpStatus.OK);
}
}

5.2. Spring RestTemplate example to consume PUT API

private final String URI_USERS_ID = "/users/{id}";

@Autowired
RestTemplate restTemplate;

//Using RestTemplate

Map<String, String> params = new HashMap<String, String>();
params.put("id", "2");

User updatedUser = new User(1, "Alex", "Golan", "a@mail.com");
User user = restTemplate.put(URI_USERS_ID, updatedUser, User.class);
//Use the object as needed

6. Spring RestTemplate – HTTP DELETE Method Example

Available methods are:

• delete(url) – deletes the resource at the specified URL.

6.1. HTTP DELETE REST API

@DeleteMapping("users/{id}")
public HttpStatus delete(@PathVariable long id) {
try {
userService.delete(id);
return HttpStatus.OK;
} catch (Exception e) {
throw new RecordNotFoundException();
}
}

6.2. Spring RestTemplate example to consume DELETE API

private final String URI_USERS_ID = "/users/{id}";

@Autowired
RestTemplate restTemplate;

Map<String, String> params = new HashMap<String, String>();
params.put("id", "2");

//Using RestTemplate

restTemplate.delete ( URI_USERS_ID,  params );

Feel free to copy and modify above Spring RestTemplate examples for building the Spring REST API Consumer in your Spring WebMVC application.

7. RestTemplate Examples

Spring RestTemplate basic authentication example
Spring RestTemplate timeout configuration example
Spring RestTemplateBuilder Example
Spring RestTemplate – HttpClient configuration example
Spring Boot RestTemplate GET Example
Spring Boot RestTemplate POST Example
Spring boot JUnit example with RestTemplate
Spring boot TestRestTemplate POST with headers example
Spring ClientHttpRequestInterceptor with RestTemplate

Happy Learning !!