List serialization without @XmlElementWrapper not working correctly (need to enable XmlMapper.defaultUseWrapper(false))

[toellrich]

When serializing a list without @XmlElementWrapper, the list is still wrapped inside another element. The following unit test reproduces the problem:

public class ListWithoutElementWrapperTest {

  @XmlRootElement(name = "employee")
  @XmlType(propOrder = {"hobbies", "id", "firstName", "lastName"})
  @XmlAccessorType(FIELD)
  public static class Employee {

    @XmlElement(name = "hobby")
    List<String> hobbies;

    Integer id;
    String firstName;
    String lastName;
  }

  private Employee employee;

  @BeforeEach
  public void setUp() {
    employee = new Employee();
    employee.id = 1;
    employee.firstName = "Lokesh";
    employee.lastName = "Gupta";
    employee.hobbies = List.of("Swimming", "Playing", "Karate");
  }

  @Test
  public void marshallUsingJaxb() throws JAXBException {
    JAXBContext jaxbContext = JAXBContext.newInstance(Employee.class);
    Marshaller marshaller = jaxbContext.createMarshaller();
    marshaller.setProperty(JAXB_FORMATTED_OUTPUT, TRUE);
    marshaller.setProperty(JAXB_FRAGMENT, Boolean.TRUE);

    String expected =
        """
        <employee>
            <hobby>Swimming</hobby>
            <hobby>Playing</hobby>
            <hobby>Karate</hobby>
            <id>1</id>
            <firstName>Lokesh</firstName>
            <lastName>Gupta</lastName>
        </employee>""";

    StringWriter sw = new StringWriter();
    marshaller.marshal(employee, sw);
    String actual = sw.toString();
    assertEquals(expected, actual);
  }

  /**
   * This test fails
   */
  @Test
  public void writeValueAsStringUsingJackson() throws JsonProcessingException {
    XmlMapper xmlMapper = new XmlMapper();
    xmlMapper.enable(INDENT_OUTPUT);
    xmlMapper.registerModule(new JakartaXmlBindAnnotationModule());

    String expected =
        """
        <employee>
          <hobby>Swimming</hobby>
          <hobby>Playing</hobby>
          <hobby>Karate</hobby>
          <id>1</id>
          <firstName>Lokesh</firstName>
          <lastName>Gupta</lastName>
        </employee>""";

    String actual = xmlMapper.writeValueAsString(employee);
    assertEquals(expected, actual);
  }
}

The second test when using Jackson instead of JAXB fails because the actual output is as follows:

<employee>
  <hobby>
    <hobby>Swimming</hobby>
    <hobby>Playing</hobby>
    <hobby>Karate</hobby>
  </hobby>
  <id>1</id>
  <firstName>Lokesh</firstName>
  <lastName>Gupta</lastName>
</employee>

but I would expect it to be like this:

<employee>
  <hobby>Swimming</hobby>
  <hobby>Playing</hobby>
  <hobby>Karate</hobby>
  <id>1</id>
  <firstName>Lokesh</firstName>
  <lastName>Gupta</lastName>
</employee>

I'm using java 17 and jackson 2.15.0-rc1.

test.zip

[cowtowncoder]

Yes, Jackson defaults to wrapping Lists by default.
This is different from JAXB defaults, but Jackson is not a JAXB implementation; it has partial support for JAXB annotations but does not have exactly same defaulting.

There is a way to change this default; I forget the method but there's one in XMLMapper.

[toellrich]

Thanks, found it. It is

XmlMapper mapper = XmlMapper.builder()
   .defaultUseWrapper(false)
   .build();

The issue can be closed.

[cowtowncoder]

Thank you for confirming @toellrich !