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<!DOCTYPE book [
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<section version="5.0"
         xml:id="unmarshalling-element-default-values-and-unmarshalling"
         xml:lang="en" xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
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    <title>Element default values and unmarshalling</title>

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

    <para>When a class has an element property with the default value, and if
    the document you are reading is missing the element, then the unmarshaller
    does <emphasis>not</emphasis> fill the field with the default value.
    Instead, the unmarshaller fills in the field when the element is present
    but the content is missing. See below:</para>

    <example>
        <title>XML instance 1</title>

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

    <example>
        <title>XML instance 2</title>

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

    <example>
        <title>XML instance 3</title>

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

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

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

Foo foo = unmarshaller.unmarshal("instance1.xml");
System.out.println(foo.a);   // null

Foo foo = unmarshaller.unmarshal("instance2.xml");
System.out.println(foo.a);   // "value". The default kicked in.

Foo foo = unmarshaller.unmarshal("instance3.xml");
System.out.println(foo.a);   // "abc". Read from the instance.]]></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, so unfortunately we can't change this behavior.</para>

    <para>Depending on your expectation, using a field initializer may achieve
    what you are looking for. See below:</para>

    <example>
        <title>Possible changes by using field initializer</title>

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

Foo foo = unmarshaller.unmarshal("instance1.xml");
System.out.println(foo.a);   // "value", because JAXB didn't overwrite the value

Foo foo = unmarshaller.unmarshal("instance2.xml");
System.out.println(foo.a);   // "", because <a> element had 0-length string in it

Foo foo = unmarshaller.unmarshal("instance3.xml");
System.out.println(foo.a);   // "abc". Read from the instance.]]></programlisting>
    </example>

    <para>Alternatively, attribute default values work in a way that agrees
    with the typical expectation, so consider using that. Also, see <xref
    linkend="marshalling-element-default-values-and-marshalling"
    xrefstyle="select:title" />.</para>
</section>
