Empty (or self-closing) Element representing `List` is incorrectly deserialized as null, not Empty List

[sir4ur0n]

Following discussion between @allienna and @cowtowncoder on #1402 I'm creating an issue.

Hi guys,

While using Jackson with JAXB bindings, I found deserialization behaves unexpectedly on empty or self-closing elements: it sets sub-fields as null no matter what.

Here's a class to reproduce the issue (sorry if it's big):

import static org.hamcrest.core.Is.is;
import static org.hamcrest.core.IsNull.notNullValue;
import static org.junit.Assert.assertThat;

import com.fasterxml.jackson.annotation.JsonSetter;
import com.fasterxml.jackson.annotation.Nulls;
import com.fasterxml.jackson.databind.type.TypeFactory;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.module.jaxb.JaxbAnnotationIntrospector;
import java.util.ArrayList;
import java.util.List;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import org.junit.Before;
import org.junit.Test;

public class Main {

  private XmlMapper mapper;

  @Before
  public void setUp() throws Exception {
    mapper = new XmlMapper();
    mapper.setDefaultSetterInfo(JsonSetter.Value.forContentNulls(Nulls.SKIP));
    mapper.setAnnotationIntrospector(new JaxbAnnotationIntrospector(TypeFactory.defaultInstance()));
  }

  @Test
  public void testWithValues() throws Exception {
    String toDeserialize = "<pojos><pojo>a</pojo><pojo>b</pojo><pojo>c</pojo></pojos>";

    Pojos pojos = mapper.readValue(toDeserialize, Pojos.class);
    assertThat(pojos, notNullValue());
    assertThat(pojos.getValues(), notNullValue());
    assertThat(pojos.getValues().size(), is(3));
  }

  @Test
  public void testWithoutValues() throws Exception {
    String toDeserialize = "<pojos></pojos>";

    Pojos pojos = mapper.readValue(toDeserialize, Pojos.class);
    assertThat(pojos, notNullValue());
    assertThat(pojos.getValues(), notNullValue());
    assertThat(pojos.getValues().size(), is(0));
  }

  @Test
  public void testSelfClosingWithoutValues() throws Exception {
    String toDeserialize = "<pojos/>";

    Pojos pojos = mapper.readValue(toDeserialize, Pojos.class);
    assertThat(pojos, notNullValue());
    assertThat(pojos.getValues(), notNullValue());
    assertThat(pojos.getValues().size(), is(0));
  }

  @Test
  public void testWrapperWithValues() throws Exception {
    String toDeserialize = "<wrapper><pojos><pojo>a</pojo><pojo>b</pojo><pojo>c</pojo></pojos></wrapper>";

    Wrapper wrapper = mapper.readValue(toDeserialize, Wrapper.class);
    assertThat(wrapper, notNullValue());
    assertThat(wrapper.getPojos(), notNullValue());
    assertThat(wrapper.getPojos().getValues(), notNullValue());
    assertThat(wrapper.getPojos().getValues().size(), is(3));
    assertThat(wrapper.getPojos().getValues().get(0).getValue(), is("a"));
  }

  @Test
  public void testWrapperWithoutValues() throws Exception {
    String toDeserialize = "<wrapper><pojos></pojos></wrapper>";

    Wrapper wrapper = mapper.readValue(toDeserialize, Wrapper.class);
    assertThat(wrapper, notNullValue());
    assertThat(wrapper.getPojos(), notNullValue());
    assertThat(wrapper.getPojos().getValues(), notNullValue());
    assertThat(wrapper.getPojos().getValues().size(), is(0));
  }

  @Test
  public void testWrapperWithSelfClosingPojos() throws Exception {
    String toDeserialize = "<wrapper><pojos/></wrapper>";

    Wrapper wrapper = mapper.readValue(toDeserialize, Wrapper.class);
    assertThat(wrapper, notNullValue());
    assertThat(wrapper.getPojos(), notNullValue());
    assertThat(wrapper.getPojos().getValues(), notNullValue());
    assertThat(wrapper.getPojos().getValues().size(), is(0));
  }

  @Test
  public void testWrapperWithoutPojos() throws Exception {
    String toDeserialize = "<wrapper></wrapper>";

    Wrapper wrapper = mapper.readValue(toDeserialize, Wrapper.class);
    assertThat(wrapper, notNullValue());
    assertThat(wrapper.getPojos(), notNullValue());
    assertThat(wrapper.getPojos().getValues(), notNullValue());
    assertThat(wrapper.getPojos().getValues().size(), is(0));
  }

  @Test
  public void testSelfClosingWrapper() throws Exception {
    String toDeserialize = "<wrapper/>";

    Wrapper wrapper = mapper.readValue(toDeserialize, Wrapper.class);
    assertThat(wrapper, notNullValue());
    assertThat(wrapper.getPojos(), notNullValue());
    assertThat(wrapper.getPojos().getValues(), notNullValue());
    assertThat(wrapper.getPojos().getValues().size(), is(0));
  }

  @XmlRootElement(name = "wrapper")
  @XmlAccessorType(XmlAccessType.FIELD)
  private static class Wrapper {
    @XmlElement(name = "pojos")
    private Pojos pojos = new Pojos();

    @java.beans.ConstructorProperties({"pojos"})
    private Wrapper(Pojos pojos) {
      this.pojos = pojos;
    }

    public Wrapper() {
    }

    public Pojos getPojos() {
      return this.pojos;
    }

    public void setPojos(Pojos pojos) {
      this.pojos = pojos;
    }

  }

  @XmlAccessorType(XmlAccessType.FIELD)
  private static class Pojos {
    @XmlElement(name = "pojo")
    private java.util.List<Pojo> values = new ArrayList<>();

    @java.beans.ConstructorProperties({"values"})
    private Pojos(List<Pojo> values) {
      this.values = values;
    }

    public Pojos() {
    }

    public List<Pojo> getValues() {
      return this.values;
    }

    public void setValues(List<Pojo> values) {
      this.values = values;
    }

  }

  @XmlAccessorType(XmlAccessType.FIELD)
  private static class Pojo {
    private String value = "";

    @java.beans.ConstructorProperties({"value"})
    private Pojo(String value) {
      this.value = value;
    }

    public Pojo() {
    }

    public String getValue() {
      return this.value;
    }

    public void setValue(String value) {
      this.value = value;
    }

  }
}

And the Maven pom.xml (without Lombok as requested by @cowtowncoder):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>test</groupId>
  <artifactId>test</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.1</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
          <encoding>UTF-8</encoding>
        </configuration>
      </plugin>
    </plugins>
  </build>

  <dependencies>
    <dependency>
      <groupId>com.fasterxml.jackson.dataformat</groupId>
      <artifactId>jackson-dataformat-xml</artifactId>
      <version>2.9.0.pr4</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-core</artifactId>
      <version>2.9.0.pr4</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.9.0.pr4</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-annotations</artifactId>
      <version>2.9.0.pr4</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.module</groupId>
      <artifactId>jackson-module-jaxb-annotations</artifactId>
      <version>2.9.0.pr4</version>
    </dependency>
    <dependency>
      <groupId>org.mockito</groupId>
      <artifactId>mockito-core</artifactId>
      <version>2.7.9</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.12</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

</project>

If you run it, you will see that 2 tests fail:

• testWrapperWithoutValues
• testWrapperWithSelfClosingPojos

Now I'm anything but a JAXB or Jackson expert, so I definitely may have done mistakes or misconfigured my mapper or POJOs, in that case, any hint is appreciated.
To my surprise, <wrapper><pojos></pojos></wrapper> returns a null Pojos object, while <wrapper></wrapper> returns an instance of Pojos (as you can see in testWrapperWithoutPojos test).
I think this is a bug.

Dependencies to ensure we test in the same conditions:
org.projectlombok:lombok:jar:1.16.14
org.mockito:mockito-core:jar:2.7.9
com.fasterxml.jackson.dataformat:jackson-dataformat-xml:jar:2.9.0.pr4
com.fasterxml.jackson.core:jackson-core:jar:2.9.0.pr4
com.fasterxml.jackson.core:jackson-annotations:jar:2.9.0.pr4
com.fasterxml.jackson.module:jackson-module-jaxb-annotations:jar:2.9.0.pr4

Note I also tried with jackson 2.8.9 version (for all Jackson-related libraries obviously) without this line:

mapper.setDefaultSetterInfo(JsonSetter.Value.forContentNulls(Nulls.SKIP));

and the problem remains.

[jetersen]

This is still an issue 😭

    val xml = """
<?xml version="1.0" encoding="utf-8"?>
<AcResponse
    Command="show depots"
    TaskId="1260">
  <Element
      Number="1"
      Name="accurev"
      Slice="1"
      exclusiveLocking="false"
      case="insensitive"
      locWidth="128" />
  <Element
      Number="2"
      Name="second accurev"
      Slice="2"
      exclusiveLocking="false"
      case="insensitive"
      locWidth="128" />
</AcResponse>
"""
    val mapper : ObjectMapper = XmlMapper()
    mapper.registerModule(KotlinModule())
    mapper.configure(MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES, true)
    val response : Depots = mapper.readValue(xml)
    println(response)

[akuhtz]

This is still an issue 😭 😭 😭

[saurabhkumar2015]

is this issue now resolved ?

[cowtowncoder]

@saurabhkumar2015 what do you think, based on the fact issue is open and there havn't been comments?

And to everyone else -- please not add arbitrary "still an issue" notes unless you add some useful information: simpler reproduction, something else you think is related. Or at very least, version in which you observe it that is more recent than earlier versions mentioned.
Same goes for asking "is this still an issue": unless there is some additional potentially useful information, you are just littering and making it bit more cumbersome to follow discussion.

Conversely, if you do find issue DOES NOT occur any more on some version, adding that does add information and help.

[leluna]

With Version 2.9.9, the issue can be reproduced with the following quite minimal test:

@Test
public void testSelfclosingTag() throws JsonParseException, JsonMappingException, IOException {
	XmlMapper mapper = new XmlMapper();
	TestPojo readValue = mapper.readValue("<rootelement><elements/></rootelement>", TestPojo.class);
	Assert.assertTrue(readValue.getElements() != null);
}

@Test
public void testEmptyTag() throws JsonParseException, JsonMappingException, IOException {
	XmlMapper mapper = new XmlMapper();
	TestPojo readValue = mapper.readValue("<rootelement><elements></elements></rootelement>", TestPojo.class);
	Assert.assertTrue(readValue.getElements() != null);
}

@XmlRootElement(name = "rootelement")
@XmlAccessorType(XmlAccessType.NONE)
public static class TestPojo {

	@XmlElementWrapper(name = "elements")
	@XmlElement(name = "element")
	private List<String> elements = new ArrayList<>();

	public List<String> getElements() {
		return elements;
	}
}

[the-rockkk]

Isn't this already resolved:
#246

[hd42]

No. Setting new XmlFactoryBuilder(xmlFactory).configure(FromXmlParser.Feature.EMPTY_ELEMENT_AS_NULL, false).build(); creates a MismatchedInputException instead of creating null. An empty element(either '<elements></elements>' or '<elements/>') should be an empty list instead of null, which I would only expect if there was no XML tag.

[cowtowncoder]

Currently this can be done, I think, with combination of 2 things:

1. Passing List via Creator (constructor or static factory method annotated with @JsonCreator, parameter names annotated with @JsonProperty or auto-detected with Java 8 module)
2. Defining "null replacement" of @JsonSetter(nulls=Nulls.AS_EMPTY) (explained in https://medium.com/@cowtowncoder/jackson-2-9-features-b2a19029e9ff )

This would make both null values and missing values to be set (due to constructors; setters are ONLY called if there is matching input value), and then map missing/null into "empty" value, which in case of Lists would be empty list.

[hd42]

@JsonSetter works for self-closing tags('<elements/>'), but an empty wrapper tag('<elements></elements>') still produces a MismatchedInputException.

[cowtowncoder]

I am hoping to tackle this for Jackson 2.12, fwtw; including both handling "blank" text and empty element the same and allowing either null or "empty" List; defaulting (I think) to empty/blank become empty List, unless configured differently to produce null.

[cowtowncoder]

Good news! As far as I can see, tests all pass with current 2.12 branch. Even without setting null-strategy, now that empty String by default coerces into empty values for XML.