Usando Gson ao invés de Jackson em uma aplicação baseada no JAX-RS

by Jennifer Presant / from beautifuldecay.com

Jersey, framework para construção de webservices RESTful baseado na definição JSR 311 & JSR 339, faz uso do framework Jackson para que os recursos retornem dados no formato Json. Porém, eu tive a necessidade de usar o framework Gson para converter os objetos em Json. Mas como fazer isso se o Jersey usa o Jackson implicitamente? Encontrei a resposta nesse site:

http://eclipsesource.com/blogs/2012/11/02/integrating-gson-into-a-jax-rs-based-application/

Basta criar uma classe provedora que implemente as interfaces javax.ws.rs.ext.MessageBodyWriter e javax.ws.rs.ext.MessageBodyReader e possua a annotation @Provider. Nessa nossa classe realizaremos a conversão dos de formato objeto para formato json e vice-versa, e para isso usaremos o framework Gson, substituindo assim o Jackson.

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.io.OutputStream;

import java.io.OutputStreamWriter;

import java.lang.annotation.Annotation;

import java.lang.reflect.Type;

import javax.ws.rs.Consumes;

import javax.ws.rs.Produces;

import javax.ws.rs.WebApplicationException;

import javax.ws.rs.core.MediaType;

import javax.ws.rs.core.MultivaluedMap;

import javax.ws.rs.ext.MessageBodyReader;

import javax.ws.rs.ext.MessageBodyWriter;

import javax.ws.rs.ext.Provider;

import com.google.gson.Gson;

import com.google.gson.GsonBuilder;

@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();

        }

    }

}