Jackson module signature prevents addings serializers for self-referencing generic types

问题

I want to add a custom serializer and deserializer for JSR 363 javax.measure.Quantity<Q extends Quantity<Q>>, which basically encapsulates a "value" and a "unit". Creating the serializer (extends JsonSerializer<Quantity<?>>) and the deserializer (extends StdDeserializer<Quantity<?>>) is easy.

But it's not easy to register them. For deserializers, it's OK; look at the signature:

SimpleModule.addDeserializer(Class<T> type, JsonDeserializer<? extends T> deser)

Notice that the deserializer allows the generic type to be extended. So I can do this:

module.addDeserializer(Quantity.class, new MyQuantityJsonDeserializer());

But the serializer is another story; look at the signature for its registration:

SimpleModule.addSerializer(Class<? extends T> type, JsonSerializer<T> ser)

Oh, the pain this causes because of the restricted generic type. I cannot do this:

module.addSerializer(Quantity.class, new MyQuantityJsonSerializer());

This is because Quantity.class will never give me a Class<Quantity<Q extends Quantity<Q>>. And there is no easy cast around this, without introducing some arbitrary generic variable in the module method and using acrobatic casts. Even this won't work:

module.addSerializer((Class<Quantity<?>>) Quantity.class, new MyQuantityJsonSerializer());

The best I've been able to do is to make my serializer class QuantityJsonSerializer<Q extends Quantity<Q>> extends JsonSerializer<Q>, and then in the module register it as the raw type:

@SuppressWarnings({"rawtypes", "unchecked"})
final JsonSerializer<Quantity> quantitySerializer =
    (JsonSerializer<Quantity>) new MyQuantityJsonSerializer();
module.addSerializer(Quantity.class, quantitySerializer);

Ugh, how ugly is that? But that is one of the only ways I can find to even get it to work, and I had to go through more gymnastics to remove the warnings.

Surely SimpleModule.addSerializer() could have been a bit more flexible on its generic parameters, analogous to SimpleModule.addDeserializer()?

I'm reporting this here because the Jackson project said to report bugs here --- and I'm also asking for better workaround ideas. Thanks.

回答1:

In SimpleModule there is an overloaded version of addSerializer():

public SimpleModule addSerializer(JsonSerializer<?> ser)

which allows this to work:

module.addSerializer(new MyQuantityJsonSerializer());

as long as you define your serializer as:

public class MyQuantityJsonSerializer extends StdSerializer<Quantity<?>> {
    public MyQuantityJsonSerializer() {
        // Note: second argument here is ignored.
        super(Quantity.class, false);
    }

    @Override
    public void serialize(Quantity<?> value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        // Serialize value here...
    }
}

来源：https://stackoverflow.com/questions/42723846/jackson-module-signature-prevents-addings-serializers-for-self-referencing-gener

标签

java

generics

jackson

jsr363