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();
	}
	}
	}