Spring RestTemplate

 

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 that is 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. Building RestTemplate Bean

The given below are few examples to create RestTemplate bean in the application. We are only looking at very simple bean definitions. For extensive configuration options, please refer to RestTemplate Configuration with HttpClient.

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

@Autowired CloseableHttpClient httpClient;   @Bean public RestTemplate restTemplate() {       RestTemplate restTemplate = new RestTemplate(clientHttpRequestFactory());     return restTemplate; }   @Bean public HttpComponentsClientHttpRequestFactory clientHttpRequestFactory() {       HttpComponentsClientHttpRequestFactory clientHttpRequestFactory                             = new HttpComponentsClientHttpRequestFactory();     clientHttpRequestFactory.setHttpClient(httpClient);     return clientHttpRequestFactory; }

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 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(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

@GetMapping(value = "/employees",         produces = {MediaType.APPLICATION_XML_VALUE, MediaType.APPLICATION_JSON_VALUE}) public EmployeeListVO getAllEmployees(         @RequestHeader(name = "X-COM-PERSIST", required = true) String headerPersist,         @RequestHeader(name = "X-COM-LOCATION", defaultValue = "ASIA") String headerLocation) {     LOGGER.info("Header X-COM-PERSIST :: " + headerPersist);     LOGGER.info("Header X-COM-LOCATION :: " + headerLocation);           EmployeeListVO employees = getEmployeeList();     return employees; }   @GetMapping(value = "/employees/{id}",         produces = {MediaType.APPLICATION_XML_VALUE, MediaType.APPLICATION_JSON_VALUE}) public ResponseEntity<EmployeeVO> getEmployeeById (@PathVariable("id") Integer id) {     LOGGER.info("Requested employee id :: " + id);           if (id != null && id > 0) {         //TODO: Fetch the employee and return from here         EmployeeVO employee = new EmployeeVO(id, "Lokesh","Gupta", "howtodoinjava@gmail.com");         return new ResponseEntity<EmployeeVO>(employee, HttpStatus.OK);     }     return new ResponseEntity<EmployeeVO>(HttpStatus.NOT_FOUND); }

3.2. Spring RestTemplate example to consume REST API

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

This method is 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 string replacement functions before using the response.

private static void getEmployees() {     final String uri = "http://localhost:8080/springrestexample/employees";       //TODO: Autowire the RestTemplate in all the examples     RestTemplate restTemplate = new RestTemplate();       String result = restTemplate.getForObject(uri, String.class);     System.out.println(result); }

3.3. Spring RestTemplate example to consume API response into POJO

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

Using getForObject() Method

private static void getEmployees() {     final String uri = "http://localhost:8080/springrestexample/employees";     RestTemplate restTemplate = new RestTemplate();           EmployeeListVO result = restTemplate.getForObject(uri, EmployeeListVO.class);           //Use the response }

Using getForEntity() Method

private static void getEmployees() {     final String uri = "http://localhost:8080/springrestexample/employees";     RestTemplate restTemplate = new RestTemplate();       ResponseEntity<EmployeeListVO> response = restTemplate.getForEntity(uri, EmployeeListVO.class);       //Use the response.getBody() }

3.4. Sending HTTP Headers using RestTemplate

private static void getEmployees() {     final String uri = "http://localhost:8080/springrestexample/employees";     RestTemplate restTemplate = new 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<String> response = restTemplate.exchange(uri, HttpMethod.GET, entity, String.class);           //Use the response.getBody() }

3.5. Sending URL Parameters using RestTemplate

private static void getEmployeeById() {     final String uri = "http://localhost:8080/springrestexample/employees/{id}";     RestTemplate restTemplate = new RestTemplate();           Map<String, String> params = new HashMap<String, String>();     params.put("id", "1");           EmployeeVO result = restTemplate.getForObject(uri, EmployeeVO.class, params);           //Use the result }

4. Spring RestTemplate – HTTP POST Example

Available methods 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(value = "/employees") public ResponseEntity<String> createEmployee(EmployeeVO employee) {     //TODO: Save employee details which will generate the employee id     employee.setId(111);           //Build URI      URI location = ServletUriComponentsBuilder.fromCurrentRequest()                          .path("/{id}")                          .buildAndExpand(employee.getId())                          .toUri();     return ResponseEntity.created(location).build(); }

4.2. Spring RestTemplate example to consume POST API

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

private static void createEmployee() {     final String uri = "http://localhost:8080/springrestexample/employees";     RestTemplate restTemplate = new RestTemplate();       EmployeeVO newEmployee = new EmployeeVO(-1, "Adam", "Gilly", "test@email.com");           EmployeeVO result = restTemplate.postForObject( uri, newEmployee, EmployeeVO.class);       System.out.println(result); }

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(value = "/employees/{id}") public ResponseEntity<EmployeeVO> updateEmployee(@PathVariable("id") int id                 ,EmployeeVO employee) {     //TODO: Save employee details     return new ResponseEntity<EmployeeVO>(employee, HttpStatus.OK); }

5.2. Spring RestTemplate example to consume PUT API

private static void updateEmployee() {     final String uri = "http://localhost:8080/springrestexample/employees/{id}";     RestTemplate restTemplate = new RestTemplate();           Map<String, String> params = new HashMap<String, String>();     params.put("id", "2");           EmployeeVO updatedEmployee = new EmployeeVO(2, "New Name", "Gilly", "test@email.com");           restTemplate.put ( uri, updatedEmployee, params ); }

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(value = "/employees/{id}") public ResponseEntity<String> deleteEmployee(@PathVariable("id") int id) {     //TODO: Delete the employee record     return new ResponseEntity<String>(HttpStatus.OK); }

6.2. Spring RestTemplate example to consume DELETE API

private static void deleteEmployee() {     final String uri = "http://localhost:8080/springrestexample/employees/{id}";     RestTemplate restTemplate = new RestTemplate();           Map<String, String> params = new HashMap<String, String>();     params.put("id", "2");           restTemplate.delete ( uri,  params ); }

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