无法按照 Javadoc 中的描述使用 @JsonCreator 正确反序列化 JSON

Question

I've got a hard time understanding how Jackson's @JsonCreator annotation works (esp. with the different modes).

I tried to simplify at most:

public class JacksonDeserialization {

    private ObjectMapper om = new ObjectMapper();

    @Test // 1
    public void test_deserialization_emptyJson() throws JsonParseException, JsonMappingException, IOException {
        Wrapper read = om.readValue("{}", Wrapper.class);
        // Throws here: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `Inner` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)

        assertThat(read).isNotNull();
        assertThat(read.getInner()).isNull();
    }

    @Test // 2
    public void test_deserialization_innerIsEmpty() throws JsonParseException, JsonMappingException, IOException {
        Wrapper read = om.readValue("{\"inner\":{}}", Wrapper.class);
        // Throws here: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "inner" (class Inner), not marked as ignorable (one known property: "prop"])

        assertThat(read).isNotNull();
        assertThat(read.getInner()).isNotNull();
    }

    @Test // 3
    public void test_deserialization_innerIsSet() throws JsonParseException, JsonMappingException, IOException {
        Wrapper read = om.readValue("{\"inner\":{\"prop\":\"42\"}}", Wrapper.class);
        // Throws here: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "inner" (class Inner), not marked as ignorable (one known property: "prop"])

        assertThat(read).isNotNull();
        assertThat(read.getInner()).isNotNull();
        assertThat(read.getInner().getProp()).isEqualTo("42");
    }

}

My object classes:

public class Wrapper {

    private Inner inner;

    @JsonCreator
    public Wrapper(Inner inner) {
        this.inner = inner;
    }

    public Inner getInner() {
        return inner;
    }

}

and

public class Inner {

    private String prop;

    @JsonCreator
    public Inner(String prop) {
        this.prop = prop;
    }

    public String getProp() {
        return prop;
    }

}

Excerpts, from @JsonCreator Javadoc:

NOTE: when annotating creator methods (constructors, factory methods),method must either be:

•Single-argument constructor/factory method without JsonPropertyannotation for the argument: if so, this is so-called "delegate creator",in which case Jackson first binds JSON into type of the argument, andthen calls creator. This is often used in conjunction with JsonValue(used for serialization).

...and from its mode argument:

Default value of Mode.DEFAULT means that caller is to use standardheuristics for choosing mode to use.

com.fasterxml.jackson.annotation.JsonCreator.Mode.DEFAULT Javadoc:

Pseudo-mode that indicates that caller is to use default heuristics for choosing mode to use. This typically favors use of delegating mode for single-argument creators that take structured types.

What did I do wrong regarding these explanations?

I use Jackson 2.9.9.20190807 as indicated by one pom.xml from Spring Boot we're using.

Answer 1

I admit I am not knowledgeable of JsonCreator modes. However, the tests pass if you annotate the constructors arguments with property names.

The rationale is that argument names is an optional compiler settings, so it is possible that Jackson is unable to read argument name through reflection and hence is unable to assign a deserialized value to that argument

public class Wrapper {

    private Inner inner;

    @JsonCreator
    public Wrapper(@JsonProperty("inner") Inner inner) {
        this.inner = inner;
    }

    public Inner getInner() {
        return inner;
    }

}

public class Inner {

    private String prop;

    public Inner(@JsonProperty("prop") String prop) {
        this.prop = prop;
    }

    public String getProp() {
        return prop;
    }

}