How to Call a REST API in Spring Boot

Calling a REST API from a Spring Boot application is a common requirement. This can involve invoking an external service or microservice to fetch resources, submit data, or trigger actions. Spring Boot offers various methods to make REST calls, namely using RestTemplate or WebClient. In this guide, we’ll explore both methods and analyze their strengths and weaknesses.

Solution

Using RestTemplate

RestTemplate is the traditional way to make REST calls in Spring Boot. Despite being replaced by WebClient since Spring 5 in reactive applications, RestTemplate is still widely used for synchronous HTTP requests.

Sample Code (RestTemplate)

@Service
class MyService(restTemplateBuilder: RestTemplateBuilder) {
    private val restTemplate: RestTemplate = restTemplateBuilder.build()

    fun getUserDetails(username: String): UserDetails? {
        val url = "https://api.example.com/users/{username}"
        return restTemplate.getForObject(url, UserDetails::class.java, username)
    }
}

data class UserDetails(val id: String, val name: String, val email: String)

Usage Example (RestTemplate)

fun main() {
    val service = MyService(RestTemplateBuilder())
    val userDetails = service.getUserDetails("john_doe")
    println(userDetails)
}

Output:

UserDetails(id=1, name=John Doe, email=john.doe@example.com)

Advantages:

• Simplicity: Easy to use with a straightforward API for simple use cases.
• Built-in conversions: Automatically converts JSON responses to POJOs.

Disadvantages:

• Synchronous: Blocks the execution flow, which can be inefficient for I/O-bound applications.
• Deprecated in newer projects: RestTemplate is not ideal for reactive programming.

Using WebClient

WebClient is the non-blocking and reactive alternative introduced in Spring 5. It is part of Spring WebFlux and suitable for building asynchronous and reactive applications.

Sample Code (WebClient)

@Service
class MyService(webClientBuilder: WebClient.Builder) {
    private val webClient: WebClient = webClientBuilder.baseUrl("https://api.example.com").build()

    fun getUserDetails(username: String): Mono<UserDetails> {
        return webClient.get()
            .uri("/users/{username}", username)
            .retrieve()
            .bodyToMono(UserDetails::class.java)
    }
}

data class UserDetails(val id: String, val name: String, val email: String)

Usage Example (WebClient)

fun main() {
    val service = MyService(WebClient.builder())
    val userDetailsMono = service.getUserDetails("john_doe")
    userDetailsMono.subscribe { userDetails -> println(userDetails) }
}

Output:

UserDetails(id=1, name=John Doe, email=john.doe@example.com)

Advantages:

• Non-blocking: Does not block the thread, making it suitable for scalable applications.
• Reactive Streams: Supports reactive programming with Mono and Flux.

Disadvantages:

• Steep learning curve: More complex to understand and use than RestTemplate.
• Requires WebFlux: Adding WebFlux to the classpath, if not already used in the project, might introduce unwanted dependencies.

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