Jackson: in case of multiple serializers which one is used?

Question

I am using Jackson to serialize POJOs. I wrote a custom serializer for string values, and it works fine.

However I am not sure what happens, when two serializers are registered for the same type. In my tests the last one added was used, but I am not sure whether it works that way all the time or not.

So my question is: if I add multiple serializers for the same type, which one will be used?

---

Code snippet:

objectMapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addSerializer(new CustomSerializer1());
module.addSerializer(new CustomSerializer2());

...

class CustomSerializer1 extends NonTypedScalarSerializerBase<String>
class CustomSerializer2 extends NonTypedScalarSerializerBase<String>

Answer 1

In an ideal world, something like this would be clearly specified in the Javadoc of SimpleModule, but it unfortunately doesn't seem to be the case here.

The next-best approach is to look at the source, which reveals that SimpleModule uses the class SimpleSerializers to keep track of its configured serializers.

Diving into that reveals the _addSerializer method:

protected void _addSerializer(Class<?> cls, JsonSerializer<?> ser)
{
    ClassKey key = new ClassKey(cls);
    // Interface or class type?
    if (cls.isInterface()) {
        if (_interfaceMappings == null) {
            _interfaceMappings = new HashMap<ClassKey,JsonSerializer<?>>();
        }
        _interfaceMappings.put(key, ser);
    } else { // nope, class:
        if (_classMappings == null) {
            _classMappings = new HashMap<ClassKey,JsonSerializer<?>>();
        }
        _classMappings.put(key, ser);
        if (cls == Enum.class) {
            _hasEnumSerializer = true;
        }
    }
}

Conclusion is the same as the one you've already reached: The last added serializer is used, due to them being stored in a Map with their input type as keys. Strictly speaking, there are no guarantees that this will not change in the future though, as it is all internal implementation.