Retrofit — Define a Custom Response Converter

This post describes how to set your custom JSON converter in Retrofit. There are more posts in the Retrofit series. Find previous posts in the list below

Retrofit Series Overview

  1. Introduction to Call Adapters (Coming soon)
  2. Custom Call Adapter to Separate OnResponse Callback (Coming soon)
  3. How to Integrate RxJava 1.x Call Adapter (Coming soon)
  4. How to Integrate RxJava 2.x Call Adapter (Coming soon)
  5. How to Integrate Guava Call Adapter (Coming soon)
  6. Custom Call Adapter to Separate Network and Gson Errors (Coming soon)
  1. Callbacks (Coming soon)
  2. Annotations (Coming soon)
  3. Fluent Interface with Builders (Coming soon)

Basics

Retrofit ships with Google's JSON by default. Every JSON mapping is done with the help of GSON. Sometimes, your framework or attitude requires to change the integrated JSON converter. One of the well known converters is Jackson. We'll use Jackson for the code examples.

Existing Retrofit Converters

Besides GSON, Retrofit can be configured with various content formats. The retrofit-converters directory lists existing response converts:

The integration with gradle can be done with these commands:

compile 'com.squareup.retrofit:converter-<converter-name>:1.9.0'

# e.g. Jackson
compile 'com.squareup.retrofit:converter-jackson:1.9.0'

# e.g. XML
compile 'com.squareup.retrofit:converter-simplexml:1.9.0'  

You probably know how to do with protocol buffers :)

Create Your Own Converter

In case you need or want to create your own Retrofit response converter, you can follow the steps below. We'll use Jackson library to illustrate the concrete definition and handling.

Remember: There are existing Retrofit converters and you can use them directly by integrating the respective gradle dependency. The list above depicts existing converters which are shipped as a separate gradle dependency and can be easily integrated with Retrofit.

Jackson Gradle Dependency

At first, define the Jackson dependency. This example uses Jackson 2.x which ships with a different group and artifact id than previous Jackson 1.x. Add the required Maven repository and compile dependency.

repositories {  
    maven { url "http://repository.codehaus.org/org/codehaus" }
}

dependencies {  
    compile 'com.fasterxml.jackson.core:jackson-databind:2.4.3'
}

Implement Your Custom JSON Converter

The important part to replace the JSON converter of Retrofit is to implement the Converter interface and override its two methods: fromBody and toBody. Both methods handle the conversion from and to JSON.

public class JacksonConverter implements Converter {  
    private ObjectMapper mapper = new ObjectMapper();

    @Override
    public Object fromBody(TypedInput body, Type type) throws ConversionException {
        JavaType javaType = mapper.getTypeFactory().constructType(type);

        try {
            return mapper.readValue(body.in(), javaType);
        } catch (IOException e) {
            throw new ConversionException(e);
        }
    }

    @Override
    public TypedOutput toBody(Object object) {
        try {
            String charset = "UTF-8";
            String json = mapper.writeValueAsString(object);
            return new JsonTypedOutput(json.getBytes(charset));
        } catch (IOException e) {
            throw new AssertionError(e);
        }
    }

    private static class JsonTypedOutput implements TypedOutput {
        private final byte[] jsonBytes;

        JsonTypedOutput(byte[] jsonBytes) { this.jsonBytes = jsonBytes; }

        @Override public String fileName() { return null; }
        @Override public String mimeType() { return "application/json; charset=UTF-8"; }
        @Override public long length() { return jsonBytes.length; }
        @Override public void writeTo(OutputStream out) throws IOException { out.write(jsonBytes); }
}

The JsonTypedOutput class is used to set the correct mimeType. Since the output type is JSON, the mime type should be application/json, too.

Set Your Custom JSON Converter

The JSON converter of Retrofit is integrated in the RestAdapter. Changing the converter can be done by calling the setConverter() method on your rest adapter object. Further, when creating a rest client, you can integrate the jackson converter by setting the converter directly during rest adapters build process.

// Create our Converter
JacksonConverter jacksonConverter = new JacksonConverter();

// Build the Retrofit REST adaptor pointing to the URL specified, with the Converter.
// Note: The Converter must be set before the .build() command
RestAdapter restAdapter = new RestAdapter.Builder()  
        .setConverter(jacksonConverter)
        .setEndpoint("https://api.example.com/")
        .build();

Great! Now, all your created Rest adapters will use the JacksonConverter for JSON handling.


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