Jackson Serialize Map to List

Question

Internally, I use a map for a quick lookup of values, e.g. private Map<String, CustomObject> lookupTable, where the CustomObject has some id field that is being used as the key in the lookupTable. It is silly to serialize the map's keys, so I want to annotate the internal variable to serialize only the values. This doesn't work, obviously:

@JsonSerialize(as = List.class)
private final Map<String, CustomObject> lookupTable;

---

Why a custom serializer won't work

I created a custom serializer like:

public static class MapValuesJsonSerializer extends JsonSerializer<Map<?, ?>> {

    @Override
    public void serialize(
        Map<?, ?> value,
        JsonGenerator generator,
        SerializerProvider provider)
        throws IOException, JsonProcessingException {

        generator.writeObject(value.values());
    }
}

Then, I annotated the field accordingly:

@JsonSerialize(using = MapValuesJsonSerializer.class)
public Map<String, CustomObject> lookupTable;

This works with some Java test code, but it breaks when using MongoDB's driver and MongoJack. I get some unhelpful exception, which I believe is because it is not actually serializing the returned object and is, instead, just trying to pass that raw object to the BSON serializer:

Caused by: java.lang.IllegalArgumentException: can't serialize class CustomObject
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:270)
    at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:295)
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:234)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:174)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:120)
    at com.mongodb.DefaultDBEncoder.writeObject(DefaultDBEncoder.java:27)
    at com.mongodb.OutMessage.putObject(OutMessage.java:289)
    at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:239)
    at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:204)
    at com.mongodb.DBCollection.insert(DBCollection.java:148)
    at com.mongodb.DBCollection.insert(DBCollection.java:91)
    at org.mongojack.JacksonDBCollection.insert(JacksonDBCollection.java:255)
    // My code (not the serializer)...

Answer 1

Of course, as soon as I ask, I figure out a solution.

Using the serializer above, use the provider to serialize the values instead of the generator directly. Here's the code:

/**
 * <p>
 * Serializes the values of some map into a list.
 * <p>
 *
 * @author John Jenkins
 */
public class MapValuesJsonSerializer extends JsonSerializer<Map<?, ?>> {
    /*
     * (non-Javadoc)
     * @see com.fasterxml.jackson.databind.JsonSerializer#serialize(java.lang.Object, com.fasterxml.jackson.core.JsonGenerator, com.fasterxml.jackson.databind.SerializerProvider)
     */
    @Override
    public void serialize(
        final Map<?, ?> value,
        final JsonGenerator generator,
        final SerializerProvider provider)
        throws IOException, JsonProcessingException {

        provider.defaultSerializeValue(value.values(), generator);
    }
}