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<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?

Answer 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());

Answer 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.