Parse to a Subclass by default with Jackson

问题

I have a class called Product and some subclasses extending it. Now in my annotations I have many types, like this:

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.WRAPPER_OBJECT)
@JsonSubTypes({@Type(value=RoomProduct.class, name="RoomProduct"),
           @Type(value=GroundProduct.class, name="GroundProduct"),
           })

And then the definition of my Product class. What I want to do is if Jackson cannot detect that the field is not complying with any of these structures, to return an

UnknownProduct

How can I do that with Jackson @Type annotation? It should be something like putting blank in name or some flag in value which I don't really know (I have tried creating the UnknownProduct which extends Product and putting nothing in the name value with no success.

回答1:

@JsonTypeInfo has an option to specify default implementation class but after some debugging I found that 'defaultImpl' is broken for the WrapperObject. Configuration:

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include= JsonTypeInfo.As.WRAPPER_OBJECT, defaultImpl = UnknownProduct.class)

Jackson implementation (AsWrapperTypeDeserializer):

public AsWrapperTypeDeserializer(JavaType bt, TypeIdResolver idRes,
        String typePropertyName, boolean typeIdVisible, Class<?> defaultImpl)
{
    super(bt, idRes, typePropertyName, typeIdVisible, null);
}

Note that 'defaultImpl' is passed but it is ignored and configured default class will NOT be used. I didn't find the logged ticket for this problem in jackson's Jira.

This is a problem ONLY for the WRAPPER_OBJECT, defaultImpl is working fine for other formats. But it will change JSON format. If you can change it -- you can use EXTERNAL_PROPERTY for example with default implementation:

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include= JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "type", defaultImpl = UnknownProduct.class)

Another solution: If you have to use WRAPPER_OBJECT then you can configure Jackson not to fail when it find unknown SubType:

objectMapper.configure(DeserializationFeature.FAIL_ON_INVALID_SUBTYPE, false);

It is not completely the same what you are asking but in that case your product will be null. Probably you can treat null as unknown product.

Update I've filed a Jackson bug: https://github.com/FasterXML/jackson-databind/issues/656

Update This ticket was resolved for 2.3 and 2.4 jackson and hopefully soon you should be able to use it when jars will be re-installed into the maven repository or in a new version.

来源：https://stackoverflow.com/questions/27506241/parse-to-a-subclass-by-default-with-jackson

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java

jackson

subtype