Using Feign to make HTTP calls in Java API development

Use Feign to make HTTP calls in Java API development

In the development of Java applications, HTTP calls are often made to obtain data from external services or call remote APIs. At this time, we usually need to choose an HTTP client library to handle these requests. In this field, Feign is undoubtedly a good choice.

Feign is a declarative HTTP client library for simplifying calls to RESTful APIs. It is inspired by Retrofit's annotation style. By using Feign, developers can easily make HTTP requests without knowing the underlying HTTP client implementation details.

In this article, we will introduce the basic usage and some examples of how to use Feign to make HTTP calls. Before we begin, we need to import Feign's library. In the Maven project, we can add the following dependencies in the pom.xml file:

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-openfeign</artifactId>
    <version>2.2.2.RELEASE</version>
</dependency>

Next, we will explain how to use it.

Basic usage

First, we need to define a remote service interface. This interface should contain methods for each HTTP endpoint we want to call. For each method, we can use the annotations provided by Feign to define various aspects of the HTTP request we want to send, such as URL, HTTP method, request body, request headers, etc. The sample code is as follows:

@FeignClient(name = "github", url = "https://api.github.com")
public interface GitHubApiClient {
    @RequestMapping(value = "/users/{username}", method = RequestMethod.GET)
    GitHubUser getUserByName(@PathVariable("username") String username);
}

In this example, we define a Feign client interface GitHubApiClient, which uses the @FeignClient annotation to interact with the remote service named "github", and the URL of the service is https: //api.github.com. There is a getUserByName method in the interface. On this method, we use the @RequestMapping and @PathVariable annotations to specify the URL and path parameters of the HTTP GET request we want to send.

Next, we can inject the interface into our code and then make HTTP requests by calling its methods. The sample code is as follows:

@RestController
public class GitHubUserController {
    @Autowired
    private GitHubApiClient gitHubApiClient;

    @GetMapping("/users/{username}")
    public GitHubUser getUserByName(@PathVariable("username") String username) {
        return gitHubApiClient.getUserByName(username);
    }
}

In this example, we inject GitHubApiClient into GitHubUserController and use this interface in the getUserByName method to obtain GitHub user information. This method will return the response object returned by Feign. In this case, the concrete class is GitHubUser.

Customized Feign configuration

Sometimes, we need to customize the behavior of Feign. For example, we might need to add a request interceptor, change the timeout, or change the log level. In this case, we can add corresponding configuration to implement these custom behaviors.

For configuring Feign, we can use the @ConfigurationProperties and @Bean annotations provided by Spring Cloud to define our configuration class. Here is an example of configuring Feign:

@ConfigurationProperties("feign.client.config.github")
public class GitHubFeignConfiguration {
    @Bean
    public RequestInterceptor requestInterceptor() {
        return new BasicAuthRequestInterceptor("username", "password");
    }
}

In this example, we add a request interceptor using Feign's BasicAuthRequestInterceptor class. Additionally, we used the @ConfigurationProperties annotation to map our custom configuration with properties in the properties file.

To connect this configuration with our Feign client interface, we need to use the @Configuration and @Import annotations on it. The sample code is as follows:

@Configuration
@Import({GitHubFeignConfiguration.class})
public class GitHubApiConfiguration {
    @Bean
    public GitHubApiClient gitHubApiClient() {
        return Feign.builder()
                .decoder(new JacksonDecoder())
                .encoder(new JacksonEncoder())
                .contract(new SpringMvcContract())
                .target(GitHubApiClient.class, "https://api.github.com");
    }
}

In this example, we use Feign's builder mode to create a GitHubApiClient object. On the build() method, we define the encoder, decoder, and contract for the returned Feign client object. Additionally, we set the target address of the GitHubApiClient type to https://api.github.com.

Summary

In this article, we introduced the usage of Feign client library and some examples. By using Feign, we can easily make HTTP calls in a declarative way. In daily development, we can customize Feign's behavior according to specific business needs. If you haven't used Feign for development yet, you might as well try it. I believe it will bring you a more convenient and efficient development experience.

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