Client-Server Synchronous Communication via REST

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In the world of distributed systems, the client-server model forms the foundation of communication between applications. Whether you are building a web application, a mobile app, or a microservice, you are likely interacting with a server that exposes resources over the network. REST (Representational State Transfer) is one of the most popular architectural styles for designing such networked applications.

In a synchronous communication scenario, a client sends a request to a server and waits for the server’s response before proceeding. This pattern is widely used when immediate feedback is necessary—such as retrieving user data, processing an order, or validating a transaction.

This article explores how to implement client-side synchronous communication in a Java-based application using two approaches:

  1. RestTemplate (a well-established client utility in Spring)

  2. RestClient (a newer, more modern alternative introduced in recent Spring versions)

Why Focus on the Client Side?

While the server is responsible for exposing RESTful APIs, the client is equally important because it:

  • Initiates the communication by sending HTTP requests (GET, POST, PUT, DELETE, etc.)

  • Handles authentication and authorization headers

  • Processes responses and converts them into usable objects

  • Manages exceptions such as timeouts or network failures

Understanding the client side ensures that the application can effectively consume services and remain robust against unexpected errors.

Key Components of Synchronous REST Communication

Before diving into the specific implementations, let’s break down the essential steps involved in client-server synchronous communication via REST:

  1. Request Preparation
    The client constructs an HTTP request that includes the URL, HTTP method (GET, POST, etc.), headers (like Content-Type or Authorization), and an optional body (for methods like POST or PUT).

  2. Request Execution
    The client sends the request and blocks execution until a response is received from the server.

  3. Response Handling
    Upon receiving the response, the client processes the HTTP status code, headers, and body. If the response contains JSON or XML, the client typically deserializes it into Java objects.

  4. Error Handling
    The client must handle scenarios such as network timeouts, 4xx client errors, or 5xx server errors gracefully.

Implementation 1: Using RestTemplate

RestTemplate is a core part of the Spring Framework, designed to simplify synchronous HTTP communication. Although it is now in maintenance mode and no longer actively developed, it remains widely used due to its simplicity and reliability.

Step-by-Step Implementation

Project Setup

Ensure that your project uses Spring Boot or has the following Maven dependency:

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

Create a Model Class

Suppose we are communicating with a REST API that returns user details:

public class User {
private Long id;
private String name;
private String email;// Getters and Setters
}

Using RestTemplate to Perform a GET Request

Here is how to perform a simple GET request:

import org.springframework.web.client.RestTemplate;
import org.springframework.http.ResponseEntity;public class UserClient {private final RestTemplate restTemplate = new RestTemplate();
private static final String BASE_URL = “https://api.example.com/users”;public User getUserById(Long id) {
String url = BASE_URL + “/” + id;
ResponseEntity<User> response = restTemplate.getForEntity(url, User.class);
return response.getBody();
}
}

Explanation:

  • getForEntity sends a GET request and returns a ResponseEntity containing the HTTP status, headers, and body.

  • The response body is automatically deserialized into the User object.

Handling POST Requests

public User createUser(User user) {
return restTemplate.postForObject(BASE_URL, user, User.class);
}

Error Handling

You can use a try-catch block or customize a ResponseErrorHandler to manage errors:

restTemplate.setErrorHandler(new DefaultResponseErrorHandler() {
@Override
public void handleError(ClientHttpResponse response) throws IOException {
// Custom error handling
System.out.println("Error: " + response.getStatusCode());
}
});

Implementation 2: Using RestClient (Spring 6+)

RestClient is the newer and more modern HTTP client introduced in Spring Framework 6 and Spring Boot 3. It provides a fluent API and better integration with reactive features while still supporting synchronous operations. Think of it as a natural evolution of RestTemplate.

Project Setup

For Spring Boot 3+ applications, the dependency remains the same as with RestTemplate:

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

Create a RestClient Bean

import org.springframework.web.client.RestClient;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;@Configuration
public class RestClientConfig {@Bean
public RestClient restClient() {
return RestClient.create();
}
}

Performing a GET Request

import org.springframework.web.client.RestClient;
import org.springframework.stereotype.Service;@Service
public class UserClient {private final RestClient restClient;
private static final String BASE_URL = “https://api.example.com/users”;public UserClient(RestClient restClient) {
this.restClient = restClient;
}public User getUserById(Long id) {
return restClient.get()
.uri(BASE_URL + “/{id}”, id)
.retrieve()
.body(User.class);
}
}

Explanation:

  • get() specifies the HTTP method.

  • uri() dynamically injects path variables.

  • retrieve() executes the call and waits for the response.

  • body(User.class) converts the JSON body to a User object.

Performing a POST Request

public User createUser(User user) {
return restClient.post()
.uri(BASE_URL)
.body(user)
.retrieve()
.body(User.class);
}

Error Handling

RestClient allows fine-grained error management:

public User getUserSafely(Long id) {
return restClient.get()
.uri(BASE_URL + "/{id}", id)
.retrieve()
.onStatus(status -> status.value() == 404,
(req, res) -> System.out.println("User not found"))
.body(User.class);
}

This fluent style enables more readable and maintainable code compared to RestTemplate.

Comparing RestTemplate and RestClient

Feature RestTemplate RestClient
API Style Verbose, template-based Fluent, modern
Maintenance In maintenance mode, no new features Actively developed, future-facing
Error Handling Requires custom error handler classes Built-in fluent error handling
Use Case Legacy projects, backward compatibility New applications with Spring 6+

For new projects, RestClient is the recommended choice, but RestTemplate remains valuable in existing applications due to its maturity and stability.

Best Practices for Client-Side REST Communication

  1. Timeouts and Retries
    Always configure connection and read timeouts to avoid indefinite blocking.

  2. Error Handling and Logging
    Implement robust error handling with clear logging to diagnose failures quickly.

  3. Serialization/Deserialization
    Use well-defined model classes and ensure proper JSON-to-object mapping with libraries like Jackson.

  4. Security
    Secure API calls with HTTPS and proper authentication headers (e.g., OAuth2, API keys).

  5. Testing
    Mock REST calls during unit testing to avoid hitting real endpoints.

Conclusion

Client-server synchronous communication via REST remains a cornerstone of modern application development. While the server exposes RESTful endpoints, the client’s role is equally critical, orchestrating the request-response cycle and handling everything from authentication to error management.

This article explored two robust Java-based client implementations:

  • RestTemplate, the long-standing solution with straightforward methods like getForEntity and postForObject. It’s reliable and well-supported but has a slightly dated programming model and is now in maintenance mode.

  • RestClient, the modern successor introduced in Spring 6 and Spring Boot 3. It offers a fluent, readable API, better error handling, and aligns with the future direction of Spring development.

For existing projects, RestTemplate continues to be a dependable option. However, for new applications or when migrating to Spring 6+, RestClient is the preferred choice thanks to its simplicity, flexibility, and maintainability.

Ultimately, the choice depends on your project’s lifecycle and requirements. Regardless of the client library, remember that successful synchronous communication hinges on:

  • Careful request construction

  • Robust error handling

  • Secure and efficient response processing

By adhering to these best practices and leveraging the strengths of the chosen client, you can ensure smooth and reliable interactions between your client application and RESTful server APIs—powering scalable, maintainable, and user-friendly systems.