How do I use a custom Serializer with Jackson?

As mentioned, @JsonValue is a good way. But if you don't mind a custom serializer, there's no need to write one for Item but rather one for User -- if so, it'd be as simple as:

public void serialize(Item value, JsonGenerator jgen,
    SerializerProvider provider) throws IOException,
    JsonProcessingException {
  jgen.writeNumber(id);
}

Yet another possibility is to implement JsonSerializable, in which case no registration is needed.

As to error; that is weird -- you probably want to upgrade to a later version. But it is also safer to extend org.codehaus.jackson.map.ser.SerializerBase as it will have standard implementations of non-essential methods (i.e. everything but actual serialization call).

Use @JsonValue:

public class User {
    int id;
    String name;

    @JsonValue
    public int getId() {
        return id;
    }
}

@JsonValue only works on methods so you must add the getId method. You should be able to skip your custom serializer altogether.

I tried doing this too, and there is a mistake in the example code on the Jackson web page that fails to include the type (.class) in the call to addSerializer() method, which should read like this:

simpleModule.addSerializer(Item.class, new ItemSerializer());

In other words, these are the lines that instantiate the simpleModule and add the serializer (with the prior incorrect line commented out):

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                          new Version(1,0,0,null));
// simpleModule.addSerializer(new ItemSerializer());
simpleModule.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(simpleModule);

FYI: Here is the reference for the correct example code: http://wiki.fasterxml.com/JacksonFeatureModules

You can put @JsonSerialize(using = CustomDateSerializer.class) over any date field of object to be serialized.

public class CustomDateSerializer extends SerializerBase<Date> {

    public CustomDateSerializer() {
        super(Date.class, true);
    }

    @Override
    public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)
        throws IOException, JsonProcessingException {
        SimpleDateFormat formatter = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'ZZZ (z)");
        String format = formatter.format(value);
        jgen.writeString(format);
    }

}