Java backend development: API calls using Retrofit

Java back-end development: Using Retrofit for API calls

With the rapid development of Internet technology, API has become a standard protocol for communication between applications and services, and is widely used in various scenarios. , such as mobile application and website development. In the field of Java back-end development, Retrofit is currently a very popular framework for implementing API calls. This article will introduce what Retrofit is and how to use Retrofit to make API calls.

1. What is Retrofit

Retrofit is a framework based on Java that implements server-side API calls. It uses annotations to describe HTTP requests, parameters and response bodies, and uses Java interfaces to Implement server-side API calls. It uses OkHttp as the underlying network request library, supports synchronous and asynchronous network request methods, and provides a large number of auxiliary functions, such as request retry, request caching, file upload, etc. Retrofit also supports a variety of data converters, such as Gson, Jackson, Moshi, etc., which can easily convert request and response bodies into Java objects.

2. How to use Retrofit to make API calls

1. Import dependencies

To use Retrofit to make API calls, you first need to add relevant dependencies to the project. In the Maven project, you can add the following dependencies in the pom.xml file:

<dependency>
  <groupId>com.squareup.retrofit2</groupId>
  <artifactId>retrofit</artifactId>
  <version>2.9.0</version>
</dependency>
<dependency>
  <groupId>com.squareup.retrofit2</groupId>
  <artifactId>converter-gson</artifactId>
  <version>2.9.0</version>
</dependency>

Among them, retrofit is a dependency of the Retrofit framework itself, and converter-gson is a dependency of Retrofit's Gson data converter.

2. Create API interface

When using Retrofit to make API calls, you need to create the corresponding API interface first. The interface describes the URL, request method, request parameters, return data type and other information of the server API. For example, the following code defines a basic interface for sending a GET request to the server and returning a string:

public interface ApiService {
    @GET("/api/hello")
    Call<String> getHello();
}

In this interface, the @GET annotation provided by Retrofit is used to describe HTTP Request type and URL address, use Call to describe the returned data type, where T can be any Java type, such as string, custom object, etc.

3. Create a Retrofit object

After the interface is defined, you need to use Retrofit to create the corresponding service instance. When creating a Retrofit object, you can specify the request URL, data converter, network request library and other related properties. For example, the following code creates a Retrofit instance and specifies the request URL, Gson data converter and OkHttp network request library:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("http://localhost:8080")
    .addConverterFactory(GsonConverterFactory.create())
    .client(new OkHttpClient.Builder().build())
    .build();

Among them, baseUrl specifies the base URL address of the server, and addConverterFactory specifies the data conversion The converter is GsonConverter, and the client specifies the use of OkHttp as the underlying network request library. The default configuration of OkHttpClient is used here, but you can also configure related parameters yourself, such as connection timeout, read and write timeout, etc.

4. Create an API instance

Retrofit creates the implementation class of the API interface through dynamic proxy, making API calls very simple. For example, the following code creates an API instance and calls the getHello method:

ApiService apiService = retrofit.create(ApiService.class);
Call<String> call = apiService.getHello();
Response<String> response = call.execute();
System.out.println(response.body());

In this code, an ApiService implementation class is dynamically generated through the retrofit.create method, and the getHello method is used to obtain the Call Object, and finally call the execute method of Call to execute the request synchronously. The execute method will return a Response object, which contains all the information returned by the server, in which the body attribute is the data returned by the server.

If you want to execute the request asynchronously, you can use Call's enqueue method. For example:

ApiService apiService = retrofit.create(ApiService.class);
Call<String> call = apiService.getHello();
call.enqueue(new Callback<String>() {
  @Override
  public void onResponse(Call<String> call, Response<String> response) {
    System.out.println(response.body());
  }
  
  @Override
  public void onFailure(Call<String> call, Throwable t) {
    t.printStackTrace();
  }
});

In this code, Call's enqueue method is used to asynchronously execute the request, and a Callback interface is implemented to process the request results. The onResponse method will be called when the request is successful, and the onFailure method will be called when the request fails.

3. Summary

This article introduces the basic usage of Retrofit, including the process of creating API interfaces, creating Retrofit instances, creating API instances and performing network requests. Retrofit simplifies API calls by using annotations, making it very convenient for the front-end and back-end to interact with data. You need to pay attention to thread safety issues when using Retrofit, because Retrofit is not thread-safe and requires proper synchronization in a multi-threaded environment.

The above is the detailed content of Java backend development: API calls using Retrofit. For more information, please follow other related articles on the PHP Chinese website!