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<section version="5.0"
         xml:id="marshalling-element-default-values-and-marshalling"
         xml:lang="en" xmlns="http://docbook.org/ns/docbook"
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    <title>Element default values and marshalling</title>

    <para>Because of a "strange" way element default values in XML Schema
    work, people often get confused about its behavior. This section describes
    how this works.</para>

    <para>When a class has an element property with the default value, and if
    a value is null, then the marshaller will not produce the corresponding
    element in XML:</para>

    <example>
        <title>Element defaults and XML</title>

        <programlisting language="java"><![CDATA[@XmlRootElement
class Foo {
  @XmlElement(defaultValue="value") public String a=null;
}

marshaller.marshal(new Foo(),System.out);]]></programlisting>
    </example>

    <example>
        <title>Marshalling output from above</title>

        <programlisting language="xml"><![CDATA[<foo />]]></programlisting>
    </example>

    <para>This is consistent with the XML Schema spec, where it essentially
    states that the element defaults do not kick in when the element is
    absent. Attribute default values do not have this problem, so if you can
    change the schema, changing it to an attribute is usually a better idea.
    Alternatively, depending on your expectation, setting the field to a
    default value in Java may achieve what you are looking for.</para>

    <example>
        <title>Possible changes</title>

        <programlisting language="java"><![CDATA[@XmlRootElement
class Foo {
  @XmlElement public String a="value";
}
@XmlRootElement
class Bar {
  @XmlAttribute public String a;
}

marshaller.marshal(new Foo(),System.out);
marshaller.marshal(new Bar(),System.out);]]></programlisting>
    </example>

    <example>
        <title>Marshalling output from above</title>

        <programlisting language="xml"><![CDATA[<foo>
    <a>value</a>
</foo>

<bar/>]]></programlisting>
    </example>

    <para>Also, see <xref
    linkend="unmarshalling-element-default-values-and-unmarshalling"
    xrefstyle="select:title" />.</para>
</section>
