Spring MVC mapping view for Google-GSON? - json

Question

Does anyone know if there is a Spring MVC mapping view for Gson? I'm looking for something similar to org.springframework.web.servlet.view.json.MappingJacksonJsonView.
Ideally it would take my ModelMap and render it as JSON, respecting my renderedAttributes set in the ContentNegotiatingViewResolver declaration
We plan to use Gson extensively in the application as it seems safer and better than Jackson. That said, we're getting hung up by the need to have two different JSON libraries in order to do native JSON views.
Thanks in advance!
[cross-posted to Spring forums]

Answer 1

aweigold got me most of the way there, but to concretely outline a solution for Spring 3.1 Java based configuration, here's what I did.
Grab GsonHttpMessageConverter.java from the spring-android-rest-template project.
Register your GsonHttpMessageConverter with the message converters in your MVC config.
#EnableWebMvc
public class WebConfig extends WebMvcConfigurerAdapter {
#Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
converters.add(new GsonHttpMessageConverter());
}
}
The Spring docs outline this process, but aren't crystal clear. In order to get this to work properly, I had to extend WebMvcConfigurerAdapter, and then override configureMesageConverters. After doing this, you should be able to do the following in your controller method:
#Controller
public class AppController {
#RequestMapping(value = "messages", produces = MediaType.APPLICATION_JSON_VALUE)
public List<Message> getMessages() {
// .. Get list of messages
return messages;
}
}
And voila! JSON output.

Answer 2

I would recommend to extend AbstractView just like the MappingJacksonJsonView does.
Personally, for JSON, I prefer to use #Responsebody, and just return the object rather than a model and view, this makes it easier to test. If you would like to use GSON for that, just create a custom HttpMessageConverter like this:
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonParseException;
import com.google.gson.reflect.TypeToken;
import com.vitalimages.string.StringUtils;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.AbstractHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.stereotype.Component;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.lang.reflect.Type;
import java.nio.charset.Charset;
import java.sql.Timestamp;
#Component
public class GSONHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");
private GsonBuilder gsonBuilder = new GsonBuilder()
.excludeFieldsWithoutExposeAnnotation()
.setDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ")
.registerTypeAdapter(Timestamp.class, new GSONTimestampConverter());
public GSONHttpMessageConverter() {
super(new MediaType("application", "json", DEFAULT_CHARSET));
}
#Override
protected boolean supports(Class<?> clazz) {
// should not be called, since we override canRead/Write instead
throw new UnsupportedOperationException();
}
#Override
public boolean canRead(Class<?> clazz, MediaType mediaType) {
return MediaType.APPLICATION_JSON.isCompatibleWith(mediaType);
}
public boolean canWrite(Class<?> clazz, MediaType mediaType) {
return MediaType.APPLICATION_JSON.isCompatibleWith(mediaType);
}
public void registerTypeAdapter(Type type, Object serializer) {
gsonBuilder.registerTypeAdapter(type, serializer);
}
#Override
protected Object readInternal(Class<? extends Object> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
try {
Gson gson = gsonBuilder.create();
return gson.fromJson(StringUtils.convertStreamToString(inputMessage.getBody()), clazz);
} catch (JsonParseException e) {
throw new HttpMessageNotReadableException("Could not read JSON: " + e.getMessage(), e);
}
}
#Override
protected void writeInternal(Object o, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
Type genericType = TypeToken.get(o.getClass()).getType();
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(outputMessage.getBody(), DEFAULT_CHARSET));
try {
// See http://code.google.com/p/google-gson/issues/detail?id=199 for details on SQLTimestamp conversion
Gson gson = gsonBuilder.create();
writer.append(gson.toJson(o, genericType));
} finally {
writer.flush();
writer.close();
}
}
}
And then add it to your converter list in your handler adapter like this:
#Bean
public HandlerAdapter handlerAdapter() {
final AnnotationMethodHandlerAdapter handlerAdapter = new AnnotationMethodHandlerAdapter();
handlerAdapter.setAlwaysUseFullPath(true);
List<HttpMessageConverter<?>> converterList = new ArrayList<HttpMessageConverter<?>>();
converterList.addAll(Arrays.asList(handlerAdapter.getMessageConverters()));
converterList.add(jibxHttpMessageConverter);
converterList.add(gsonHttpMessageConverter);
handlerAdapter.setMessageConverters(converterList.toArray(new HttpMessageConverter<?>[converterList.size()]));
return handlerAdapter;
}