为什么Jackson NON_NULL不能使用可选的？

Question

为了区分(a)任何值、(b)显式null和(c)无值的情况，我使用java.util.optional和non_null注释。

public interface MyProjection {

    @JsonInclude(Include.NON_NULL)
    Optional<String> getMyAttribute();
}

这在Spring Boot 2.1.0/Jackson 2.9.7中非常适用。

目前（更新后）null似乎像optional.empty()一样处理。它呈现{“myattribute”：null}，尽管该字段是用@jsoninclude(NON_NULL)注释的。在Spring Boot 2.5.5/Jackson 2.12.5中，它呈现：

Answer 1

经过几次搜索，我找到了感兴趣的测试用例。

以下是jackson Projects中带有可选选项的测试用例摘录：

public void testConfigAbsentsAsNullsTrue() throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(true));

        OptionalData data = new OptionalData();
        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL).writeValueAsString(data);
        assertEquals("{}", value);
    }

    public void testConfigAbsentsAsNullsFalse() throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(false));

        OptionalData data = new OptionalData();
        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL).writeValueAsString(data);
        assertEquals("{\"myString\":null}", value);
    }

    class OptionalData {
        public Optional<String> myString = Optional.empty();
    }

还有一个注释ConfigureAbsentSasNulls。https://github.com/fasterxml/jackson-datatype-jdk8/blob/master/src/main/java/com/fasterxml/jackson/datatype/jdk8/jdk8module.java

For compatibility with older versions
     * of other "optional" values (like Guava optionals), it can be set to 'true'. The
     * default is `false` for backwards compatibility. 
public Jdk8Module configureAbsentsAsNulls(boolean state) {
    _cfgHandleAbsentAsNull = state;
    return this;
}