RestTemplate Post Request with JSON

Table of Contents

1. Introduction
2. Setting Up the Example
3. REST API Setup
4. Using RestTemplate
5. Conclusion

1. Introduction

In this quick tutorial, we illustrate how to use Spring’s RestTemplate to make POST requests sending JSON content.

2. Setting Up the Example

Let’s start by adding a simple Person model class to represent the data to be posted:

public class Person {
    private Integer id;
    private String name;

    // standard constructor, getters, setters
}

To work with Person objects, we’ll add a PersonService interface and implementation with two methods:

public interface PersonService {

    public Person saveUpdatePerson(Person person);
    public Person findPersonById(Integer id);
}

The implementation of these methods will simply return an object. We’re using a dummy implementation of this layer here so we can focus on the web layer.

3. REST API Setup

Let’s define a simple REST API for our Person class:

@PostMapping(
  value = "/createPerson", consumes = "application/json", produces = "application/json")
public Person createPerson(@RequestBody Person person) {
    return personService.saveUpdatePerson(person);
}

@PostMapping(
  value = "/updatePerson", consumes = "application/json", produces = "application/json")
public Person updatePerson(@RequestBody Person person, HttpServletResponse response) {
    response.setHeader("Location", ServletUriComponentsBuilder.fromCurrentContextPath()
      .path("/findPerson/" + person.getId()).toUriString());
    
    return personService.saveUpdatePerson(person);
}

Remember, we want to post the data in JSON format. In order to that, we added the consumes attribute in the @PostMapping annotation with the value of “application/json” for both methods.

Similarly, we set the produces attribute to “application/json” to tell Spring that we want the response body in JSON format.

We annotated the person parameter with the @RequestBody annotation for both methods. This will tell Spring that the person object will be bound to the body of the HTTP request.

Lastly, both methods return a Person object that will be bound to the response body. Let’s note that we’ll annotate our API class with @RestController to annotate all API methods with a hidden @ResponseBody annotation.

4. Using RestTemplate

Now we can write a few unit tests to test our Person REST API. Here, we’ll try to send POST requests to the Person API by using the POST methods provided by the RestTemplate: postForObject, postForEntity, and postForLocation.

Before we start to implement our unit tests, let’s define a setup method to initialize the objects that we’ll use in all our unit test methods:

@BeforeClass
public static void runBeforeAllTestMethods() {
    createPersonUrl = "http://localhost:8082/spring-rest/createPerson";
    updatePersonUrl = "http://localhost:8082/spring-rest/updatePerson";

    restTemplate = new RestTemplate();
    headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_JSON);
    personJsonObject = new JSONObject();
    personJsonObject.put("id", 1);
    personJsonObject.put("name", "John");
}

Besides this setup method, note that we’ll refer to the following mapper to convert the JSON String to a JSONNode object in our unit tests:

private final ObjectMapper objectMapper = new ObjectMapper();

As previously mentioned, we want to post the data in JSON format. To achieve this, we’ll add a Content-Type header to our request with the APPLICATION_JSON media type.

Spring’s HttpHeaders class provides different methods to access the headers. Here, we set the Content-Type header to application/json by calling the setContentType method. We’ll attach the headers object to our requests.

4.1. Posting JSON With postForObject

RestTemplate‘s postForObject method creates a new resource by posting an object to the given URI template. It returns the result as automatically converted to the type specified in the responseType parameter.

Let’s say that we want to make a POST request to our Person API to create a new Person object and return this newly created object in the response.

First, we’ll build the request object of type HttpEntity based on the personJsonObject and the headers containing the Content-Type. This allows the postForObject method to send a JSON request body:

@Test
public void givenDataIsJson_whenDataIsPostedByPostForObject_thenResponseBodyIsNotNull()
  throws IOException {
    HttpEntity<String> request = 
      new HttpEntity<String>(personJsonObject.toString(), headers);
    
    String personResultAsJsonStr = 
      restTemplate.postForObject(createPersonUrl, request, String.class);
    JsonNode root = objectMapper.readTree(personResultAsJsonStr);
    
    assertNotNull(personResultAsJsonStr);
    assertNotNull(root);
    assertNotNull(root.path("name").asText());
}

The postForObject() method returns the response body as a String type.

We can also return the response as a Person object by setting the responseType parameter:

Person person = restTemplate.postForObject(createPersonUrl, request, Person.class);

assertNotNull(person);
assertNotNull(person.getName());

Actually, our request handler method matching with the createPersonUrl URI produces the response body in JSON format.

But this is not a limitation for us — postForObject is able to automatically convert the response body into the requested Java type (e.g. String, Person) specified in the responseType para

4.2. Posting JSON With postForEntity

Compared to postForObject(), postForEntity() returns the response as a ResponseEntity object. Other than that, both methods do the same job.

Let’s say that we want to make a POST request to our Person API to create a new Person object and return the response as a ResponseEntity.

We can make use of the postForEntity method to implement this:

@Test
public void givenDataIsJson_whenDataIsPostedByPostForEntity_thenResponseBodyIsNotNull()
  throws IOException {
    HttpEntity<String> request = 
      new HttpEntity<String>(personJsonObject.toString(), headers);
    
    ResponseEntity<String> responseEntityStr = restTemplate.
      postForEntity(createPersonUrl, request, String.class);
    JsonNode root = objectMapper.readTree(responseEntityStr.getBody());
 
    assertNotNull(responseEntityStr.getBody());
    assertNotNull(root.path("name").asText());
}

Similar to the postForObject, postForEntity has the responseType parameter to convert the response body to the requested Java type.

Here, we were able to return the response body as a ResponseEntity<String>.

We can also return the response as a ResponseEntity<Person> object by setting the responseType parameter to Person.class:

ResponseEntity<Person> responseEntityPerson = restTemplate.
  postForEntity(createPersonUrl, request, Person.class);
 
assertNotNull(responseEntityPerson.getBody());
assertNotNull(responseEntityPerson.getBody().getName());

4.3. Posting JSON With postForLocation

Similar to the postForObject and postForEntity methods, postForLocation also creates a new resource by posting the given object to the given URI. The only difference is that it returns the value of the Location header.

Remember, we already saw how to set the Location header of a response in our updatePerson REST API method above:

response.setHeader("Location", ServletUriComponentsBuilder.fromCurrentContextPath()
  .path("/findPerson/" + person.getId()).toUriString());

Now let’s imagine that we want to return the Location header of the response after updating the person object we posted.

We can implement this by using the postForLocation method:

@Test
public void givenDataIsJson_whenDataIsPostedByPostForLocation_thenResponseBodyIsTheLocationHeader() 
  throws JsonProcessingException {
    HttpEntity<String> request = new HttpEntity<String>(personJsonObject.toString(), headers);
    URI locationHeader = restTemplate.postForLocation(updatePersonUrl, request);
    
    assertNotNull(locationHeader);
}