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);
}
}