Xml cannot read in Optional.empty

[HenryYihengXu]

I have an object

class MyObject {
  private final int a;
  public MyObject() {
    a = 1;
  }

  // Getter
}

then another object with an optional field of the object above

class MyObject2 {
  private final Optional<MyObject> myObject;
  public MyObject2() {
    myObject = Optional.empty();
  }
  // Getter
}

I serialize MyObject2 as

MyObject2 myObject2 = new MyObject2();
XmlMapper xmlMapper = new XmlMapper();
xmlMapper.registerModule(new Jdk8Module());
xmlMapper.writeValue(new File("myObject2.xml"), myObject2);

I got

<MyObject2>
  <MyObject/>
</MyObject2>

I'm assuming a single <xxx/> denotes optional empty? Is this correct?
Assume it is correct, I deserialize it as

MyObject2 deserializedMyObject2 =
                xmlMapper.readValue(new File("myObject2.xml"), MyObject2.class);

But then I found the deserialized one is not Optional.empty. It seems to call the default constructor of MyObject so it has a = 1 in it. Do you know how should I actually deserialize Optional.empty?

[cowtowncoder]

Intent with Jackson XML module is that it should be able to read what it writes: so the XML structure created with XmlMapper.writeValue(myObject2) should be readable with XmlMapper.readValue(xmlDoc, MyObject2.class).
If not, that is considered bug.

Package does not necessarily allow mapping different kinds of XML representations.

[HenryYihengXu]

Then this should be a bug. I tried reading back what it writes. It's readable, but the deserialized object is different from the original one.

Here is the code:

    @Test
    void testJacksonXml2() throws IOException {
        MyObject2 myObject2 = new MyObject2();

        XmlMapper xmlMapper = new XmlMapper();
        xmlMapper.registerModule(new Jdk8Module());
        xmlMapper.writeValue(new File("test-jackson.xml"), myObject2);

        MyObject2 deserializedMyObject2 = xmlMapper.readValue(new File("test-jackson.xml"), MyObject2.class);
        xmlMapper.writeValue(new File("deserialized-test-jackson.xml"), deserializedMyObject2);
        assertThat(myObject2).isEqualTo(deserializedMyObject2);
    }

The test is failing. And I checked "test-jackson.xml" and "deserialized-test-jackson.xml". They are indeed different. The former is just

<MyObject2>
  <MyObject/>
</MyObject2>

but the later one is

<MyObject2>
  <myObject>
    <a>1</a>
  </myObject>
</MyObject2>

Could you fix this bug? I would appreciate it!

[cowtowncoder]

That sounds like a bug then.

Unfortunately I doubt I have time to tackle this issue at this point. But if you or anyone else has time and interest, I can help get PR reviewed and so on.

One practical challenge here is that of testing, as the issue requires both XML module and JDK 8 datatype module -- neither of which should have dependency to the other.

But I think same problem would affect use of AtomicReference type, too, and if reproducing the issue with it, test could be added in jackson-dataformat-xml.
And fix, if any, would likely work for Optional too as their handling is very similar internally.

[HenryYihengXu]

Do you have an idea on how to fix it? I looked at your OptionalDeserializer, looks like it should deserialize to Optional.empty()?

[cowtowncoder]

A major problem is that empty XML element can mean either "empty" value (like "" for java.lang.String or, for POJOs, instance constructed with default constructor); OR null value.
In this case I think you get "empty" Object, then wrapped in Optional.

One way to force "null-ness" is to add xsi:nil attribute like so:

<!-- NOTE: xmlns:xsi could be declared in parent to avoid using multiple times
   -->
<MyObject2>
  <MyObject xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true" />
</MyObject2>

[cowtowncoder]

Ok, so, above xsi:nil works. But to produce it on writing, you need to enable

ToXmlGenerator.Feature.WRITE_NULLS_AS_XSI_NIL

after which things work.

I don't see a working way to solve this problem in other way but I suggest you try this.