Using Jackson ObjectMapper with Java 8 Optional values

I was trying to use Jackson to write a class value to JSON that has Optional as fields:

public class Test {
    Optional<String> field = Optional.of("hello, world!");

    public Optional<String> getField() {
        return field;
    }

    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(new Test()));
    }
}

When executed, this class generates the following output:

{"field":{"present":true}}

I understand the present/not present field being included and could work around it when reading the JSON data, however I can't get around the fact that the actual content of the optional is never written to the output. :(

Any workarounds here except not using ObjectMapper at all?

Solution 1:

You could use jackson-datatype-jdk8 which is described as:

Support for new JDK8-specific types, such as Optional

In order to do this:

• add com.fasterxml.jackson.datatype:jackson-datatype-jdk8 as a dependency
• register the module with your object mapper: objectMapper.registerModule(new Jdk8Module());

Solution 2:

The Optional class has a value field, but no standard getter/setter for it. By default, Jackson looks for getters/setters to find class properties.

You can add a custom Mixin to identify the field as a property

final class OptionalMixin {
    private Mixin(){}
    @JsonProperty
    private Object value;
}

and register it with your ObjectMapper.

ObjectMapper mapper = new ObjectMapper();
mapper.addMixInAnnotations(Optional.class, OptionalMixin.class);

You can now serialize your object.

System.out.println(mapper.writeValueAsString(new Test()));

will print

{"field":{"value":"hello, world!","present":true}}

Consider also looking at jackson-datatype-guava. There's a Jackson Module implementation for Guava types including their Optional. It's possibly more complete than what I've shown above.