How to Call or Consume External API in Spring Boot?

To call or consume an external API in a Spring Boot application, you can use the RestTemplate class or the newer WebClient class which is part of the Spring WebFlux module. Below are examples of how to use both methods:

1. Using `RestTemplate`:

Step 1: Add the necessary dependencies to your pom.xml or build.gradle:

<!-- Spring Boot Web Starter -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

Step 2: Create or inject a RestTemplate bean:

import org.springframework.web.client.RestTemplate;

@Configuration
public class AppConfig {
    
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

Step 3: Use RestTemplate to consume the external API:

@Service
public class MyService {

    @Autowired
    private RestTemplate restTemplate;

    public String fetchDataFromExternalApi() {
        String apiUrl = "https://api.example.com/data";
        return restTemplate.getForObject(apiUrl, String.class);
    }
}

2. Using `WebClient` (Reactive approach):

Step 1: Add the necessary dependencies:

<!-- Spring Boot WebFlux Starter -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

Step 2: Create or inject a WebClient bean:

import org.springframework.web.reactive.function.client.WebClient;

@Configuration
public class AppConfig {

    @Bean
    public WebClient.Builder webClientBuilder() {
        return WebClient.builder();
    }
}

Step 3: Use WebClient to consume the external API:

@Service
public class MyService {

    @Autowired
    private WebClient.Builder webClientBuilder;

    public Mono<String> fetchDataFromExternalApi() {
        return webClientBuilder.build()
                .get()
                .uri("https://api.example.com/data")
                .retrieve()
                .bodyToMono(String.class);
    }
}

Between the two, WebClient is more modern and provides a non-blocking, reactive way to handle HTTP requests, which can be more performant especially under high load. However, if you're working in a more traditional, blocking application or if you're not familiar with reactive programming, RestTemplate may be easier to get started with.

REST template usage for consuming external APIs in Spring Boot:

Description: Learn how to use the RestTemplate class in Spring Boot to make HTTP requests and consume data from external APIs.

Code:

@Service
public class ExternalApiService {

    @Autowired
    private RestTemplate restTemplate;

    public String fetchDataFromExternalApi() {
        return restTemplate.getForObject("https://api.example.com/data", String.class);
    }
}

Making HTTP requests in Spring Boot to external services:

Description: Understand the basics of making HTTP requests to external services in Spring Boot, covering different methods and configurations.

Code:

@RestController
public class ApiController {

    @GetMapping("/fetchData")
    public String fetchData() {
        // Make HTTP request using HttpURLConnection or other libraries
        return "Data from external service";
    }
}

Spring Boot WebClient for consuming external APIs:

Description: Explore the WebClient class in Spring Boot for a more reactive and non-blocking way of consuming external APIs.

Code:

@Service
public class WebClientService {

    @Autowired
    private WebClient webClient;

    public Mono<String> fetchDataFromExternalApi() {
        return webClient.get()
                .uri("https://api.example.com/data")
                .retrieve()
                .bodyToMono(String.class);
    }
}

Using RestTemplate to fetch data from external API in Spring Boot:

Description: A detailed example of using RestTemplate to fetch data from an external API in a Spring Boot application.

Code:

@Service
public class ExternalApiService {

    @Autowired
    private RestTemplate restTemplate;

    public String fetchDataFromExternalApi() {
        ResponseEntity<String> response = restTemplate.getForEntity("https://api.example.com/data", String.class);
        return response.getBody();
    }
}

Handling JSON responses from external API in Spring Boot:

Description: Learn how to handle JSON responses from external APIs in Spring Boot, including deserialization into Java objects.

Code:

@Service
public class ExternalApiService {

    @Autowired
    private RestTemplate restTemplate;

    public MyResponseObject fetchDataFromExternalApi() {
        ResponseEntity<MyResponseObject> response = restTemplate.getForEntity("https://api.example.com/data", MyResponseObject.class);
        return response.getBody();
    }
}

Concurrency and asynchronous requests when calling external APIs in Spring Boot:
- Description: Learn how to make asynchronous and concurrent requests to external APIs in Spring Boot for improved performance.
- Code:
```
@Service
public class AsyncExternalApiService {

    @Async
    public CompletableFuture<String> fetchDataAsync() {
        // Asynchronous API call logic
    }
}
```
Using Feign Client for declarative REST client in Spring Boot:
- Description: Explore the Feign Client in Spring Boot for a declarative way of defining REST clients and making API calls.
- Code:
```
@FeignClient(name = "external-api", url = "https://api.example.com")
public interface ExternalApiClient {

    @GetMapping("/data")
    String fetchData();
}
```

How to Call or Consume External API in Spring Boot?

1. Using RestTemplate:

2. Using WebClient (Reactive approach):

1. Using `RestTemplate`:

2. Using `WebClient` (Reactive approach):