Using Gson instead of Jackson in web service that makes use of Jersey

Question

I found out that Jersey by default makes use of the Jackson framework for resources to return data in Json format. However, I had the need to use the Gson framework to convert objects to Json. How to do this if Jersey uses Jackson implicitly?

Answer 1

Just create a provider class that implements the interfaces javax.ws.rs.ext.MessageBodyWriter, for serialization of a Json Java object, and javax.ws.rs.ext.MessageBodyReader, for deserialization of Json to Java object, and that has Annotation @Provider. Below is an example:

@Provider
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public final class GsonMessageBodyHandler implements MessageBodyWriter, MessageBodyReader {

    private static final String UTF_8 = "UTF-8";

    private Gson gson;

    private Gson getGson() {
        if (gson == null) {
            final GsonBuilder gsonBuilder = new GsonBuilder();
            gson = gsonBuilder.create();
        }
        return gson;
    }

    @Override
    public boolean isReadable(Class type, Type genericType, java.lang.annotation.Annotation[] annotations, MediaType mediaType) {
        return true;
    }

    @Override
    public Object readFrom(Class type, Type genericType, Annotation[] annotations, MediaType mediaType, MultivaluedMap httpHeaders, InputStream entityStream) throws Exception, ApplicationException {
        InputStreamReader streamReader = new InputStreamReader(entityStream, UTF_8);
        try {
            Type jsonType;
            if (type.equals(genericType)) {
                jsonType = type;
            } else {
                jsonType = genericType;
            }
            return getGson().fromJson(streamReader, jsonType);
        } finally {
            streamReader.close();
        }
    }

    @Override
    public boolean isWriteable(Class type, Type genericType, Annotation[] annotations, MediaType mediaType) {
        return true;
    }

    @Override
    public long getSize(Object object, Class type, Type genericType, Annotation[] annotations, MediaType mediaType) {
        return -1;
    }

    @Override
    public void writeTo(Object object, Class type, Type genericType, Annotation[] annotations, MediaType mediaType, MultivaluedMap httpHeaders, OutputStream entityStream) throws IOException, WebApplicationException {
        OutputStreamWriter writer = new OutputStreamWriter(entityStream, UTF_8);
        try {
            Type jsonType;
            if (type.equals(genericType)) {
                jsonType = type;
            } else {
                jsonType = genericType;
            }
            getGson().toJson(object, jsonType, writer);
        } finally {
            writer.close();
        }
    }
}

Then, in this class we will convert from object to json format and vice versa, and for that we will use the Gson framework, thus replacing Jackson.there and any conversion related to Json will run it.