Using Jackson ObjectMapper with Java 8 Optional values

Question

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

public class Test {
    Optional field = Optional.of(\"         


        

Answer 1

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}}

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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.