JAXRS / RestEasy中的@Produces集合

Question

I found some strange behaviour that I cannot understand.

I have tested 4 similar examples:

1

@GET
@Produces(MediaType.APPLICATION_JSON)
public Response produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return Response.ok(books).build();
}

2

@GET
@Produces(MediaType.APPLICATION_JSON)
public List<Book> produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return books;
}

3

@GET
@Produces(MediaType.APPLICATION_XML)
public List<Book> produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return books;
}

4

@GET
@Produces(MediaType.APPLICATION_XML)
public Response produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return Response.ok(books).build();
}

Everything works in #1, #2, #3 but 4th example throws:

Could not find MessageBodyWriter for response object of type: java.util.Arrays$ArrayList of media type: application/xml.

I run it on Wildfly 9 and I wonder if it is related to RestEasy or JaxRS in general? I know that I can fix it by wrapping collection in GenericEntity, but I don't understand this inconsistent behaviour.

Answer 1

The problem is the missing of type information. This is required for JAXB, which handles the XML serialization.

1 and 2 works because Jackson is being used for JSON, and it generally doesn't need to know type information as it just introspects properties.

3 works because type information is known through the method return type.

4 doesn't work because there is no type information. It's is erased by type erasure . That's where GenericEntity comes to the rescue. It stores the type information.

GenericEntity

Normally type erasure removes generic type information such that a Response instance that contains, eg, an entity of type List<String> appears to contain a raw List<?> at runtime. When the generic type is required to select a suitable MessageBodyWriter , this class may be used to wrap the entity and capture its generic type.

Answer 2

swch, i create an example of MessageBodyWriter for Collections (Set, List etc) Also, some can analyze Annotation for xml root name, gzip, and cache... have fun!

import java.io.IOException;
import java.io.OutputStream;

import java.lang.annotation.Annotation;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;

import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

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.MessageBodyWriter;
import javax.ws.rs.ext.Provider;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBElement;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.namespace.QName;

@Provider
@Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
public class CollectionProvider
  implements MessageBodyWriter<Collection>
{
  static final byte[] COMMA = ",".getBytes();
  static final byte[] ARRAY_START = "[".getBytes();
  static final byte[] ARRAY_END = "]".getBytes();
  static final byte[] NULL = "null".getBytes();
  static final QName OBJECT = new QName(null, "object");

  @Override
  public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType)
  {
    if (!Collection.class.isAssignableFrom(type))
      return false;

    if (genericType == null || !(genericType instanceof ParameterizedType))
      return false;

    Type[] args = ((ParameterizedType) genericType).getActualTypeArguments();

    for (Type arg: args)
    {
      if (arg instanceof TypeVariable) // can't mashal Collection<T>
        return false;

      if (!(arg instanceof Class))
        return false;
    }

    String type = mediaType.getType().toLowerCase();
    String subtype = mediaType.getSubtype().toLowerCase();

    return type.equals("application") && 
      (subtype.startsWith("json") || subtype.startsWith("xml"));
  }

  @Override
  public long getSize(Collection list, Class<?> c, Type type, Annotation[] annotation, MediaType mediaType)
  {
    return -1;
  }

  @Override
  public void writeTo(Collection list, Class<?> c, Type type, Annotation[] annotation, MediaType mediaType,
                      MultivaluedMap<String, Object> multivaluedMap, OutputStream outputStream)
    throws IOException, WebApplicationException
  {
    try
    {
      boolean json = mediaType.getSubtype().toLowerCase().startsWith("json");

      if (list.isEmpty())
      {
        if(json)
        {
          outputStream.write(ARRAY_START);
          outputStream.write(ARRAY_END);
        }
        else
          outputStream.write("<list/>".getBytes());
      }
      else
      {
        Set<Class> classes = new HashSet<Class>();

        for (Type clazz: ((ParameterizedType) type).getActualTypeArguments())
          classes.add((Class) clazz);

        JAXBContext jc = JAXBContext.newInstance(classes.toArray(new Class[classes.size()]));
        Marshaller m = jc.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, false);

        if(json)
        {
          m.setProperty("eclipselink.media-type", MediaType.APPLICATION_JSON);
          m.setProperty("eclipselink.json.include-root", false);
        }

        if(json)
          outputStream.write(ARRAY_START);
        else
          outputStream.write("<list>".getBytes());

        for (Iterator it = list.iterator(); it.hasNext();)
        {
          Object object = it.next();

          if(json)
          {
            if (object == null) // Allow nullabale value collections
              outputStream.write(NULL);
            else
              m.marshal(new JAXBElement(OBJECT, object.getClass(), object), outputStream);

            if (it.hasNext())
              outputStream.write(COMMA);
          }
          else if (object != null) // null in xml? xm...
            m.marshal(object, outputStream); // <-- requered XmlRoot annotation
        }

        if(json)
          outputStream.write(ARRAY_END);
        else
          outputStream.write("</list>".getBytes());
      }
    }
    catch (JAXBException e)
    {
      throw new IOException(e);
    }
  }
}