Complete Guide to Spring RestTemplate

REST-styled APIs are all around us. Many applications need to invoke REST APIs for some or all of their functions. Hence for applications to function gracefully, they need to consume APIs elegantly and consistently.

RestTemplate is a class within the Spring framework that helps us to do just that. In this tutorial, we will understand how to use RestTemplate for invoking REST APIs of different shapes.

Example Code

This article is accompanied by a working code example on GitHub.

What is Spring `RestTemplate` ?

According to the official documentation, RestTemplate is a synchronous client to perform HTTP requests.

It is a higher-order API since it performs HTTP requests by using an HTTP client library like the JDK HttpURLConnection, Apache HttpClient, and others.

The HTTP client library takes care of all the low-level details of communication over HTTP while the RestTemplate adds the capability of transforming the request and response in JSON or XML to Java objects.

By default, RestTemplate uses the class java.net.HttpURLConnection as the HTTP client. However, we can switch to another HTTP client library which we will see in a later section.

Some Useful Methods of `RestTemplate`

Before looking at the examples, it will be helpful to take a look at the important methods of the RestTemplate class.

RestTemplate provides higher-level methods for each of the HTTP methods which make it easy to invoke RESTful services.

The names of most of the methods are based on a naming convention:

the first part in the name indicates the HTTP method being invoked
the second part in the name indicates returned element.

For example, the method getForObject() will perform a GET and return an object.

getForEntity() : executes a GET request and returns an object of ResponseEntity class that contains both the status code and the resource as an object.
getForObject() : similar to getForEntity() , but returns the resource directly.
exchange() : executes a specified HTTP method, such as GET, POST, PUT, etc, and returns a ResponseEntity containing both the HTTP status code and the resource as an object.
execute() : similar to the exchange() method, but takes additional parameters: RequestCallback and ResultSetExtractor .
headForHeaders() : executes a HEAD request and returns all HTTP headers for the specified URL.
optionsForAllow() : executes an OPTIONS request and uses the Allow header to return the HTTP methods that are allowed under the specified URL.
delete() : deletes the resources at the given URL using the HTTP DELETE method.
put() : updates a resource for a given URL using the HTTP PUT method.
postForObject() : creates a new resource using HTTP POST method and returns an entity.
postForLocation() : creates a new resource using the HTTP POST method and returns the location of the newly created resource.

For additional information on the methods of RestTemplate , please refer to the Javadoc.

We will see how to use the above methods of RestTemplate with the help of some examples in subsequent sections.

Project Setup for Running the Examples

To work with the examples of using RestTemplate , let us first create a Spring Boot project with the help of the Spring boot Initializr, and then open the project in our favorite IDE. We have added the web dependency to the Maven pom.xml. .

The dependency spring-boot-starter-web is a starter for building web applications. This dependency contains a dependency to the RestTemplate class.

We will use this POJO class Product in most of the examples:

public class Product {
    public Product(String name, String brand, Double price, String sku) {
        super();
        id = UUID.randomUUID().toString();
        this.name = name;
        this.brand = brand;
        this.price = price;
        this.sku = sku;
    }
    private String id;
    private String name;
    private String brand;
    private Double price;
    private String sku;

    ...
}

We also have built a minimal REST web service with the following @RestController :

@RestController
public class ProductController {
    
    private List<Product> products = List.of(
               new Product("Television", "Samsung",1145.67,"S001"),
               new Product("Washing Machine", "LG",114.67,"L001"),
               new Product("Laptop", "Apple",11453.67,"A001"));
    
    @GetMapping(value="/products/{id}", 
        produces=MediaType.APPLICATION_XML_VALUE)
    public @ResponseBody Product fetchProducts(
        @PathParam("id") String productId){
        
        return products.get(1);
    }

    @GetMapping("/products")
    public List<Product> fetchProducts(){
        
        return products;
    }
    
    @PostMapping("/products")
    public ResponseEntity<String> createProduct(
        @RequestBody Product product){
        
        // Create product with ID;
        String productID = UUID.randomUUID().toString();
        product.setId(productID);
        products.add(product);
        
        return ResponseEntity.ok().body(
            "{\"productID\":\""+productID+"\"}");
    }

    @PutMapping("/products")
    public ResponseEntity<String> updateProduct(
        @RequestBody Product product){
        
        products.set(1, product);
        // Update product. Return success or failure without response body
        return ResponseEntity.ok().build();
    }
    
    @DeleteMapping("/products")
    public ResponseEntity<String> deleteProduct(
        @RequestBody Product product){
        
        products.remove(1);
        // Update product. Return success or failure without response body
        return ResponseEntity.ok().build();
    }

}

The REST web service contains the methods to create, read, update, and delete product resources and supports the HTTP verbs GET, POST, PUT, and DELETE.

When we run our example, this web service will be available at the endpoint http://localhost:8080/products .

We will consume all these APIs using RestTemplate in the following sections.

Making an HTTP GET Request to Obtain the JSON Response

The simplest form of using RestTemplate is to invoke an HTTP GET request to fetch the response body as a raw JSON string as shown in this example:

public class RestConsumer {
    
    public void getProductAsJson() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Fetch JSON response as String wrapped in ResponseEntity
        ResponseEntity<String> response
          = restTemplate.getForEntity(resourceUrl, String.class);
        
        String productsJson = response.getBody();
        
        System.out.println(productsJson);
    }
    
}

Here we are using the getForEntity() method of the RestTemplate class to invoke the API and get the response as a JSON string. We need to further work with the JSON response to extract the individual fields with the help of JSON parsing libraries like Jackson.

We prefer to work with raw JSON responses when we are interested only in a small subset of an HTTP response composed of many fields.

Making an HTTP GET Request to Obtain the Response as a POJO

A variation of the earlier method is to get the response as a POJO class. In this case, we need to create a POJO class to map with the API response.

public class RestConsumer {
    
    public void getProducts() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Fetch response as List wrapped in ResponseEntity
        ResponseEntity<List> response
          = restTemplate.getForEntity(resourceUrl, List.class);
        
        List<Product> products = response.getBody();
        System.out.println(products);
    }
}

Here also we are calling the getForEntity() method for receiving the response as a List of Product objects.

Instead of using getForEntity() method, we could have used the getForObject() method as shown below:

public class RestConsumer {
    
    public void getProductObjects() {
       
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Fetching response as Object  
        List<?> products
          = restTemplate.getForObject(resourceUrl, List.class);
        
        System.out.println(products);
    }

Instead of the ResponseEntity object, we are directly getting back the response object.

While getForObject() looks better at first glance, getForEntity() returns additional important metadata like the response headers and the HTTP status code in the ResponseEntity object.

Making an HTTP POST Request

After the GET methods, let us look at an example of making a POST request with the RestTemplate .

We are invoking an HTTP POST method on a REST API with the postForObject() method:

public class RestConsumer {
        
    public void createProduct() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Create the request body by wrapping
        // the object in HttpEntity 
        HttpEntity<Product> request = new HttpEntity<Product>(
            new Product("Television", "Samsung",1145.67,"S001"));

        // Send the request body in HttpEntity for HTTP POST request
        String productCreateResponse = restTemplate
               .postForObject(resourceUrl, request, String.class);
        
        System.out.println(productCreateResponse);
    }
}

Here the postForObject() method takes the request body in the form of an HttpEntity class. The HttpEntity is constructed with the Product class which is the POJO class representing the HTTP request.

Using `exchange()` for POST

In the earlier examples, we saw separate methods for making API calls like postForObject() for HTTP POST and getForEntity() for GET. RestTemplate class has similar methods for other HTTP verbs like PUT, DELETE, and PATCH.

The exchange() method in contrast is more generalized and can be used for different HTTP verbs. The HTTP verb is sent as a parameter as shown in this example:

public class RestConsumer {
    
    public void createProductWithExchange() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Create the request body by wrapping
        // the object in HttpEntity   
        HttpEntity<Product> request = 
          new HttpEntity<Product>(
            new Product("Television", "Samsung",1145.67,"S001"));

        ResponseEntity<String> productCreateResponse = 
               restTemplate
                .exchange(resourceUrl, 
                    HttpMethod.POST, 
                    request, 
                    String.class);
            
        System.out.println(productCreateResponse);
    }
}

Here we are making the POST request by sending HttpMethod.POST as a parameter in addition to the request body and the response type POJO.

Using `exchange()` for PUT with an Empty Response Body

Here is another example of using the exchange() for making a PUT request which returns an empty response body:

public class RestConsumer {
    
    public void updateProductWithExchange() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";

        // Create the request body by wrapping
        // the object in HttpEntity 
        HttpEntity<Product> request = new HttpEntity<Product>(
            new Product("Television", "Samsung",1145.67,"S001"));

        // Send the PUT method as a method parameter
        restTemplate.exchange(
            resourceUrl, 
            HttpMethod.PUT, 
            request, 
            Void.class);
        
        
    }
}

Here we are sending HttpMethod.PUT as a parameter to the exchange() method. Since the REST API returns an empty body, we are using the Void class to represent the same.

Using `execute()` for Downloading Large Files

The execute() in contrast to the exchange() method is the most generalized way to perform a request, with full control over request preparation and response extraction via callback interfaces.

We will use the execute() method for downloading large files.

The execute() method takes a callback parameter for creating the request and a response extractor callback for processing the response as shown in this example:

public class RestConsumer {
    
    public void getProductasStream() {
        final Product fetchProductRequest = 
        new Product("Television", "Samsung",1145.67,"S001");

        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";
    
        // Set HTTP headers in the request callback
        RequestCallback requestCallback = request -> {
            ObjectMapper mapper = new ObjectMapper();
                mapper.writeValue(request.getBody(), 
                        fetchProductRequest);

                request.getHeaders()
                 .setAccept(Arrays.asList(
                         MediaType.APPLICATION_OCTET_STREAM, 
                         MediaType.ALL));
                };

        // Processing the response. Here we are extracting the 
        // response and copying the file to a folder in the server.
        ResponseExtractor<Void> responseExtractor = response -> {
                 Path path = Paths.get("some/path");
                 Files.copy(response.getBody(), path);
                 return null;
             };

        restTemplate.execute(resourceUrl, 
            HttpMethod.GET, 
            requestCallback, 
            responseExtractor );    
        
    }
}

Here we are sending a request callback and a response callback to the execute() method. The request callback is used to prepare the HTTP request by setting different HTTP headers like Content-Type and Authorization .

The responseExtractor used here extracts the response and creates a file in a folder in the server.

Invoking APIs with `application/form` Type Input

Another class of APIs takes HTTP form as an input. To call these APIs, we need to set the Content-Type header to application/x-www-form-urlencoded in addition to setting the request body. This allows us to send a large query string containing name and value pairs separated by & to the server.

We send the request in form variables by wrapping them in a LinkedMultiValueMap object and use this to create the HttpEntity class as shown in this example:

public class RestConsumer {
    public void submitProductForm() {
        RestTemplate restTemplate = new RestTemplate();

        String resourceUrl
          = "http://localhost:8080/products";
        
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
        
        // Set the form inputs in a multivaluemap
        MultiValueMap<String, String> map= new LinkedMultiValueMap<>();
        map.add("sku", "S34455");
        map.add("name", "Television");
        map.add("brand", "Samsung");
        
        // Create the request body by wrapping
        // the MultiValueMap in HttpEntity  
        HttpEntity<MultiValueMap<String, String>> request = 
            new HttpEntity<>(map, headers);
        
        ResponseEntity<String> response = restTemplate.postForEntity(
                  resourceUrl+"/form", request , String.class); 

        System.out.println(response.getBody());
    }
}

Here we have sent three form variables sku , name , and brand in the request by first adding them to a MultiValueMap and then wrapping the map in HttpEntity . After that, we are invoking the postForEntity() method to get the response in a ResponseEntity object.

Configuring the HTTP Client in `RestTemplate`

The simplest form of RestTemplate is created as a new instance of the class with an empty constructor as seen in the examples so far.

As explained earlier, RestTemplate uses the class java.net.HttpURLConnection as the HTTP client by default. However, we can switch to a different HTTP client library like Apache HttpComponents, Netty, OkHttp, etc. We do this by calling the setRequestFactory() method on the class.

In the example below , we are configuring the RestTemplate to use Apache HttpClient library. For this, we first need to add the client library as a dependency.

Let us add a dependency on the httpclient module from the Apache HttpComponents project:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
        <groupId>org.apache.httpcomponents</groupId>
        <artifactId>httpclient</artifactId>
    </dependency>
</dependencies>

Here we can see the dependency on httpclient added in Our Maven pom.xml .

Next we will configure the HTTP client with settings like connect timeout, socket read timeout, pooled connection limit, idle connection timeout, etc as shown below:

import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.web.client.RestTemplate;

public class RestConsumer {

    private ClientHttpRequestFactory getClientHttpRequestFactory() {

        // Create an instance of Apache HttpClient
        HttpComponentsClientHttpRequestFactory clientHttpRequestFactory
          = new HttpComponentsClientHttpRequestFactory();

        int connectTimeout = 5000;
        int readTimeout = 5000;
          
        clientHttpRequestFactory.setConnectTimeout(connectTimeout);
        clientHttpRequestFactory.setReadTimeout(readTimeout);

        return clientHttpRequestFactory;
    }

    public void fetchProducts() {
        RestTemplate restTemplate = new RestTemplate(
            getClientHttpRequestFactory());
        ...
        ...
    }
}

In this example, we have specified the HTTP connection timeout and socket read timeout intervals to 5 seconds. This allows us to fine-tune the behavior of the HTTP connection.

Other than the default HttpURLConnection and Apache HttpClient, Spring also supports Netty and OkHttp client libraries through the ClientHttpRequestFactory abstraction.

Attaching an ErrorHandler to `RestTemplate`

RestTemplate is associated with a default error handler which throws the following exceptions:

HTTP status 4xx : HttpClientErrorException
HTTP status 5xx : HttpServerErrorException
unknown HTTP status : UnknownHttpStatusCodeException

These exceptions are subclasses of RestClientResponseException which is a subclass of RuntimeException . So if we do not catch them they will bubble up to the top layer.

The following is a sample of an error produced by the default error handler when the service responds with an HTTP status of 404:

Default error handler::org.springframework.web.client.DefaultResponseErrorHandler@30b7c004
...
...
...org.springframework.web.client.RestTemplate - Response 404 NOT_FOUND
Exception in thread "main" org.springframework.web.client
.HttpClientErrorException$NotFound: 404 : 
"{"timestamp":"2021-12-20T07:20:34.865+00:00","status":404,
"error":"Not Found","path":"/product/error"}" 
    at org.springframework.web.client.HttpClientErrorException
    .create(HttpClientErrorException.java:113)
    ... 
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:122) 
    at org.springframework.web.client.ResponseErrorHandler
    .handleError(ResponseErrorHandler.java:63)

RestTemplate allows us to attach a custom error handler. Our custom error handler looks like this:

// Custom runtime exception
public class RestServiceException extends RuntimeException {

    private String serviceName;
    private HttpStatus statusCode;
    private String error;

    public RestServiceException(
        String serviceName, 
        HttpStatus statusCode, 
        String error) {

        super();
        this.serviceName = serviceName;
        this.statusCode = statusCode;
        this.error = error;
    }
}

// Error POJO
public class RestTemplateError {
    private String timestamp;
    private String status;
    private String error;
    private String path;
    ...
    ...
}

// Custom error handler
public class CustomErrorHandler implements ResponseErrorHandler{

    @Override
    public boolean hasError(ClientHttpResponse response) 
            throws IOException {
        return (
                  response.getStatusCode().series() ==
                      HttpStatus.Series.CLIENT_ERROR 
                      
                  || response.getStatusCode().series() == 
                      HttpStatus.Series.SERVER_ERROR
               );
            
    }

    @Override
    public void handleError(ClientHttpResponse response) 
            throws IOException {

        if (response.getStatusCode().is4xxClientError() 
                || response.getStatusCode().is5xxServerError()) {


            try (BufferedReader reader = new BufferedReader(
                new InputStreamReader(response.getBody()))) {
              String httpBodyResponse = reader.lines()
                        .collect(Collectors.joining(""));
              
              ObjectMapper mapper = new ObjectMapper();

              RestTemplateError restTemplateError = mapper
               .readValue(httpBodyResponse, 
                RestTemplateError.class);

              
              throw new RestServiceException(
                            restTemplateError.getPath(), 
                            response.getStatusCode(), 
                            restTemplateError.getError());
            }   
        
        }
   
    }
}

The CustomErrorHandler class implements the ResponseErrorHandler interface. It also uses an error POJO: RestTemplateError and a runtime exception class RestServiceException .

We override two methods of the ResponseErrorHandler interface: hasError() and handleError() . The error handling logic is in the handleError() method. In this method, we are extracting the service path and error message from the error response body returned as a JSON with the Jackson ObjectMapper.

The response with our custom error handler looks like this:

`1`	`error occured: [Not Found] in service:: /product/error`

The output is more elegant and can be produced in a format compatible with our logging systems for further diagnosis.

When using RestTemplate in Spring Boot applications, we can use an auto-configured RestTemplateBuilder to create RestTemplate instances as shown in this code snippet:

@Service
public class InventoryServiceClient {
    
    private RestTemplate restTemplate;
    
    public InventoryServiceClient(RestTemplateBuilder builder) {
        restTemplate = builder.errorHandler(
                new CustomErrorHandler())
                .build();
        
        ...
        ...
    }
}

Here the RestTemplateBuilder autoconfigured by Spring is injected in the class and used to attach the CustomErrorHandler class we created earlier.

Attaching `MessageConverters` to the `RestTemplate`

REST APIs can serve resources in multiple formats (XML, JSON, etc) to the same URI following a principle called content negotiation. REST clients request for the format they can support by sending the accept header in the request. Similarly, the Content-Type header is used to specify the format of the request.

The conversion of objects passed to the methods of RestTemplate is converted to HTTP requests by instances of HttpMessageConverter interface. This converter also converts HTTP responses to Java objects.

We can write our converter and register it with RestTemplate to request specific representations of a resource. In this example, we are requesting the XML representation of the Product resource:

public class RestConsumer {
    public void getProductAsXML() {
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.setMessageConverters(getXmlMessageConverter());
        
        HttpHeaders headers = new HttpHeaders();
        headers.setAccept(
            Collections.singletonList(MediaType.APPLICATION_XML));
        HttpEntity<String> entity = new HttpEntity<>(headers);
        
        String productID = "P123445";

        String resourceUrl
          = "http://localhost:8080/products/"+productID;

        ResponseEntity<Product> response = 
          restTemplate.exchange(
            resourceUrl, 
            HttpMethod.GET, 
            entity, Product.class, "1");
        Product resource = response.getBody();
    }
    
    private List<HttpMessageConverter<?>> getXmlMessageConverter() {
        XStreamMarshaller marshaller = new XStreamMarshaller();
        marshaller.setAnnotatedClasses(Product.class);
        MarshallingHttpMessageConverter marshallingConverter = 
          new MarshallingHttpMessageConverter(marshaller);

        List<HttpMessageConverter<?>> converters = new ArrayList<>();
        converters.add(marshallingConverter);
        return converters;
    }
}

Here we have set up the RestTemplate with a message converter XStreamMarshaller since we are consuming XML representation of the Product resource.

Comparison with Other HTTP Clients

As briefly mentioned in the beginning RestTemplate is a higher-level construct which makes use of a lower-level HTTP client.

Starting with Spring 5, the RestTemplate class is in maintenance mode. The non-blocking WebClient is provided by the Spring framework as a modern alternative to the RestTemplate.

WebClient offers support for both synchronous and asynchronous HTTP requests and streaming scenarios. Therefore, RestTemplate will be marked as deprecated in a future version of the Spring Framework and will not contain any new functionalities.

RestTemplate is based on a thread-per-request model. Every request to RestTemplate blocks until the response is received. As a result, applications using RestTemplate will not scale well with an increasing number of concurrent users.

The official Spring documentation also advocates using WebClient instead of RestTemplate .

However, RestTemplate is still the preferred choice for applications stuck with an older version(< 5.0) of Spring or those evolving from a substantial legacy codebase.

Conclusion

Here is a list of the major points for a quick reference:

RestTemplate is a synchronous client for making REST API calls over HTTP
RestTemplate has generalized methods like execute() and exchange() which take the HTTP method as a parameter. execute() method is most generalized since it takes request and response callbacks which can be used to add more customizations to the request and response processing.
RestTemplate also has separate methods for making different HTTP methods like getForObject() and getForEntity() .
We have the option of getting the response body in raw JSON format which needs to be further processed with a JSON parser or a structured POJO that can be directly used in the application.
Request body is sent by wrapping the POJOs in a HttpEntity class.
RestTemplate can be customized with an HTTP client library, error handler, and message converter.
Lastly, calling RestTemplate methods results in blocking the request thread till the response is received. Reactive WebClient is advised to be used for new applications.

You can refer to all the source code used in the article on Github.

Reference https://reflectoring.io/spring-resttemplate/

Table of Contents

Example Code

What is Spring RestTemplate ?

Some Useful Methods of RestTemplate

Project Setup for Running the Examples

Making an HTTP GET Request to Obtain the JSON Response

Making an HTTP GET Request to Obtain the Response as a POJO

Making an HTTP POST Request

Using exchange() for POST

Using exchange() for PUT with an Empty Response Body

Using execute() for Downloading Large Files

Invoking APIs with application/form Type Input

Configuring the HTTP Client in RestTemplate

Attaching an ErrorHandler to RestTemplate

Attaching MessageConverters to the RestTemplate