[CXF-3149] WebClient getCollection() doesn't work with Jackson's JSON Provider because it passes the wrong types to readFrom()

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.3
• Fix Version/s: 2.3.1, 2.4
• Component/s: JAX-RS
• Assignee: Sergey Beryozkin
• Reporter: Dobes Vandermeer
• Created: 24/Nov/10 00:07
• Updated: 06/Dec/10 22:49
• Resolved: 25/Nov/10 22:13

Description

When you request a collection from the WebClient it calls readFrom() using the type == Collection.class and the genericType == the member type of the class.

Unfortunately, this isn't what is normally passed as the genericType when reflection or a GenericEntity was used to write out an object. Normally, the genericType == Collection<X>. In order to be more compatible with third-party providers, it would be best if the generic type was set consistently in each case, rather than having a special case for the collections.

To support this, the genericType should be set to a subclass of ParameterizedType, for example an instance of this class:

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Collection;

public final class ParameterizedCollectionType<T> implements ParameterizedType {
	private final Class<T> collectionMemberClass;
	private final Type[] typeArgs;

	public ParameterizedCollectionType(Class<T> collectionMemberClass) {
		this.collectionMemberClass=collectionMemberClass;
		this.typeArgs=new Type[] { collectionMemberClass };
	}
	 
	public Type[] getActualTypeArguments() {
	    return typeArgs;
	}

	public Type getOwnerType() {
	    return null;
	}

	public Type getRawType() {
	    return Collection.class;
	}

	public String toString() {
		return "java.util.Collection<"+collectionMemberClass.getName()+">";
	}
}

If this class was used in org.apache.cxf.jaxrs.client.WebClient.invokeAndGetCollection(String, Object, Class<T>) then the client would be able to read a JSON collection response using the JacksonJsonProvider.

As a workaround I have created a subclass of JacksonJsonProvider that "fixes" this issue by overriding readFrom():

	/**
	 * For collections, CXF passes type == Collection.class and genericType == the member class type.
	 * 
	 * But actually, it should pass genericType = Collection<X> as the generic type in order for Jackson
	 * to understand it.
	 */
	@SuppressWarnings("unchecked")
	@Override
	public Object readFrom(Class<Object> type, Type genericType, Annotation[] annotations, MediaType mediaType,
			MultivaluedMap<String, String> httpHeaders, InputStream entityStream) throws IOException {
		if(type.equals(Collection.class) && !(genericType instanceof ParameterizedType)) {
			genericType = new ParameterizedCollectionType<Object>((Class)genericType);
		}
		return super.readFrom(type, genericType, annotations, mediaType, httpHeaders, entityStream);
	}