Decode an OkHttp JSON Response

1. Introduction

In this article, we’ll see several techniques for decoding a JSON response while using OkHttp.

2. OkHttp Response

OkHttp is an HTTP client for Java and Android with features like transparent handling of GZIP, response caching, and recovery from network problems.

In spite of these great features, OkHttp doesn’t have a built-in encoder/decoder for JSON, XML, and other content types. However, we can implement these with the help of XML/JSON binding libraries, or we can use high-level libraries like Feign or Retrofit.

To implement our JSON decoder, we need to extract the JSON from the result of the service call. For this, we can access the body via the body() method of the Response object. The ResponseBody class has several options for extracting this data:

• byteStream(): exposes the raw bytes of the body as an InputStream; we can use this for all formats, but usually it is used for binaries and files
  
• charStream(): when we have a text response, charStream() wraps its InputStream in a Reader and handles encoding according to the response’s content type or “UTF-8” if charset isn’t set in the response header; however, when using charStream(), we can’t change the Reader‘s encoding
• string(): returns the whole response body as a String; manages the encoding the same as charStream(), but if we need a different encoding, we can use source().readString(charset) instead

In this article, we’re going to use string() since our response is small and we don’t have memory or performance concerns. The byteStream() and charStream() methods are better choices in production systems when performance and memory matter.

To start, let’ s add okhttp to our pom.xml file:

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId> 
    <version>3.14.2</version> 
</dependency>

And then, we model the SimpleEntity to test our decoders:

public class SimpleEntity {
    protected String name;

    public SimpleEntity(String name) {
        this.name = name;
    }
    
    // no-arg constructor, getters, and setters
}

Now, we’re going to initiate our test:

SimpleEntity sampleResponse = new SimpleEntity("Baeldung");

OkHttpClient client = // build an instance;
MockWebServer server = // build an instance;
Request request = new Request.Builder().url(server.url("...")).build();

3. Decode the ResponseBody with Jackson

Jackson is one of the most popular libraries for JSON-Object binding.

Let’s add jackson-databind to our pom.xml:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.9</version>
</dependency>

Jackson’s ObjectMapper lets us convert JSON to an object. Thus, we can decode the response using ObjectMapper.readValue():

ObjectMapper objectMapper = new ObjectMapper(); 
ResponseBody responseBody = client.newCall(request).execute().body(); 
SimpleEntity entity = objectMapper.readValue(responseBody.string(), SimpleEntity.class);

Assert.assertNotNull(entity);
Assert.assertEquals(sampleResponse.getName(), entity.getName());

4. Decode the ResponseBody with Gson

Gson is another useful library for mapping JSON to Objects and vice versa.

Let’s add gson to our pom.xml file:

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.5</version>
</dependency>

Let’s see how we can use Gson.fromJson() to decode the response body:

Gson gson = new Gson(); 
ResponseBody responseBody = client.newCall(request).execute().body();
SimpleEntity entity = gson.fromJson(responseBody.string(), SimpleEntity.class);

Assert.assertNotNull(entity);
Assert.assertEquals(sampleResponse.getName(), entity.getName());

5. Conclusion

In this article, we’ve explored several ways to decode the JSON response of OkHttp with Jackson and Gson.

The complete sample is available on GitHub.