Implicit default values when deserializing JSON using Jackson

问题

When deserializing a variety of JSON messages, I want to provide a default value for attributes of a certain type. It is generally suggested to simply specify the value in the Class, but this is error-prone if you have to do this across many Classes. You might forget one and end up with null instead of a default value. My intention is to set every property that is an Optional<T> to Optional.absent. Since null is exactly what Optional is trying to eliminate, using them with Jackson has proven to be frustrating.

Most features of Jackson that allow you to customize the deserialization process focus on the JSON that is the input, not around the process of instantiating the Object that you are deserializing into. The closest I seem to be getting to a general solution is by building my own ValueInstantiator, but there are two remaining issues I have:

how do I make it only instantiate Optional as absent but not interfere with the rest of the instantiation process?
how do I wire the end result into my ObjectMapper?

UPDATE: I want to clarify that I am looking for a solution that does not involve modifying each Class that contains Optional's. I'm opposed to violating the DRY principle. Me or my colleagues should not have to think about having to do something extra every time we add Optional's to a new or existing Class. I want to be able to say, "make every Optional field in every Class I deserialize into, pre-filled with Absent", only once, and be done with it.

That means the following are out:

abstract parent class (need to declare)
custom Builder/Creator/JsonDeserializer (needs annotation on each applicable class)
MixIn's? I tried this, combined with reflection, but I don't know how to access the Class I'm being mixed into...

回答1:

Specifically for java.lang.Optional, there is a module by the Jackson guys themselves: https://github.com/FasterXML/jackson-datatype-jdk8

Guava Optional is covered by https://github.com/FasterXML/jackson-datatype-guava

It will create a Optional.absent for null's, but not for absent JSON values :-(.

See https://github.com/FasterXML/jackson-databind/issues/618 and https://github.com/FasterXML/jackson-datatype-jdk8/issues/2.

So you're stuck with initializing your Optionals just as you should initialize collections. It is a good practice, so you should be able to enforce it.

private Optional<Xxx> xxx = Optional.absent();
private List<Yyy> yyys = Lists.newArrayList();

回答2:

You can write a custom deserializer to handle the default value. Effectively you will extend the appropriate deserializer for the type of object you are deserializing, get the deserialized value, and if it's null just return the appropriate default value.

Here's a quick way to do it with Strings:

public class DefaultStringModule extends SimpleModule {
    private static final String NAME = "DefaultStringModule";

    private static final String DEFAULT_VALUE = "[DEFAULT]";

    public DefaultStringModule() {
        super(NAME, ModuleVersion.instance.version());
        addDeserializer(String.class, new DefaultStringDeserializer());
    }

    private static class DefaultStringDeserializer extends StdScalarDeserializer<String> {
        public DefaultStringDeserializer() {
            super(String.class);
        }

        public String deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException, JsonProcessingException {
            String deserialized = jsonParser.getValueAsString();

            // Use a default value instead of null
            return deserialized == null ? DEFAULT_VALUE : deserialized;
        }

        @Override
        public Object deserializeWithType(JsonParser jp, DeserializationContext ctxt, TypeDeserializer typeDeserializer) throws IOException {
            return deserialize(jp, ctxt);
        }
    }
}

To use this with an ObjectMapper you can register the module on the instance:

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new DefaultStringModule());

To handle default values for fields not present in the JSON, I've typically seen this done through the use of a builder class that will construct the class using the values supplied and add any default values for the missing fields. Then, on the deserialized class (e.g. MyClass), add a @JsonDeserialize(builder = MyClass.Builder.class) annotation to indicate to Jackson to deserialize MyClass by way of the builder class.

回答3:

Your value object should initialize these values as absent. That's the way to ensure that default values have no nulls. Guava module's Optional handler really should only deserialize them as "absent" (even with explicit JSON nulls), and never as nulls, with later versions. But since Jackson only operates on JSON properties that exist (and not on things that could exist but do not), POJO still needs to have default absent assignment.

来源：https://stackoverflow.com/questions/29928841/implicit-default-values-when-deserializing-json-using-jackson

标签

java

json

jackson

deserialization