Spring - RestTemplate

Due to high traffic and quick access to services, REST APIs are getting more popular and have become the backbone of modern web development. It provides quick access to services and also provides fast data exchange between applications. REST is not a protocol or a standard, rather, it is a set of architectural constraints. It is also called a RESTful API or web API. When a client request is made, it just transfers a representation of the state of the resource to the requester or at the endpoint via HTTP. This information delivered to the client can be in several formats as follows:

• JSON (JavaScript Object Notation )
• XML
• HTML
• Plain text

Key Features of RestTemplate:

The key features of RestTemplate are listed below:

• Synchronous operations: It simplifies the programming model for many use cases.
• Method variety: It provides 41 methods covering all the HTTP verbs.
• Response conversion: It provides automatic mapping to domain objects.
• Exception handling: It also provides built-in error handling capabilities.
• Integration: Seamless with Spring ecosystem.

Why use Spring RestTemplate?

RestTemplate is a tool that makes it easier to talk to other web services from the Spring application. RestTemplate is useful in various ways which are listed below:

• Creating client instances and request objects
• Executing HTTP requests
• Interpreting responses
• Mapping responses to domain objects
• Handling exceptions

Note: RestTemplate is deprecated as of Spring 5. It is recommended to use WebClient from Spring WebFlux, it supports non-blockin and reactive communication.

Pre-requisites: The Client can be of any front-end framework like Angular, React, for reasons like developing a Single Page Application (SPA ), etc. or it can be a back-end internal/external Spring application itself. 

Interacting with REST API

To interact with REST, the client needs to create a client instance and request object, execute the request, interpret the response, map the response to domain objects, and also handle the exceptions. It is common for the Spring framework to both create an API and consume internal or external application APIs. This advantage also helps us in the development of microservices. To avoid such boilerplate code, Spring provides a convenient way to consume REST APIs through RestTemplate.

Consuming REST API

The image below demonstrates the flow of requesting and getting a resource using Spring Framework, with RestTemplate for requesting and RestAPI for retrieving the resource.

RestTemplate is a synchronous REST client provided by the core Spring Framework.

Path:

org.springframework.web.client.RestTemplate

Constructors:

RestTemplate()

RestTemplate(ClientHttpRequestFactory requestFactory)

RestTemplate(List<HttpMessageConverter<?>> messageConverters)

It provides a total of 41 methods for interacting with REST resources. But there are only a dozen of unique methods each overloaded to form a complete set of 41 methods.

HTTP Operations Using RestTemplate

The below table demonstrates the main HTTP operations using RestTemplate

Common Method Forms

Except for TRACE, RestTemplate has at least one method for each of the standard HTTP methods.  execute() and exchange() provide lower-level, general-purpose methods for sending requests with any HTTP method. Most of the above methods overload in these 3 forms:

• One accepts a String URL specification with URL parameters specified in a variable argument list.
• One accepts a String URL specification with URL parameters specified in a Map<String, String>.
• One accepts a java.net.URI as the URL specification, with no support for parameterized URLs.

Initializing RestTemplate

In order to use RestTemplate, we can create an instance via as shown below: 

RestTemplate rest = new RestTemplate();

Also, you can declare it as a bean and inject it as shown below as follows:

Project Structure

The Project structure is as follow:

Step 1: Setting up the Project

To use RestTemplate, include the necessary dependencies in the pom.xml file:

pom.xml( Maven Configurations):

Step 2: Bootstrapping the Application

Create a Spring Boot application class to bootstrap the application.

GfgRestTemplateApplication.java (Bootstrapping of application):

Step 3: Creating the Domain Class

This class uses the Lombok library to automatically generate Getter/Setter methods with @Data annotation. Lombok's dependency is as depicted below as follows:

Maven Dependency:

<dependency>

<groupId>org.projectlombok</groupId>

<artifactId>lombok</artifactId>

<optional>true</optional>

</dependency>

UserData.java (Domain class):

Step 4: Implementing the REST Controller

Create a REST controller to expose endpoints for GET and POST requests.

• GET: Returns domain data in JSON form.
• POST: Returns domain data wrapped in ResponseEntity along with headers.

RestApiController.java (Rest Controller):

Step 5: Consuming the REST API with RestTemplate

Create a class to implement RestTemplate for consuming the REST API.

• GET: Consumes REST API's GET mapping response and returns domain object.
• POST: Consumes REST API's POST mapping response and return ResponseEntity object.

File: RestTemplateProvider.java (RestTemplate implementation):

Step 6: Creating a Controller the API

Create a regular controller to interact with the REST API and display the results.

ConsumeApiController.java (Regular Controller - Consume REST API):

Step 7: Displaying Results with Thymeleaf

Create a Thymeleaf template (GetData.html) to display the results.

GetData.html:

Outputs: They are as follows sequentially

Note:

• RestTemplate is synchronous and deprecated in Spring 5. for new projects, consider using WebClient.
• When working with self-signed certificates, use HTTP for development purposes to avoid SSL errors.