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<section version="5.0"
         xml:id="marshalling-interaction-between-marshalling-and-dom"
         xml:lang="en" xmlns="http://docbook.org/ns/docbook"
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    <title>Interaction between marshalling and DOM</title>

    <para>Sometimes you may notice that JAXB is producing XML with seemingly
    unnecessary namespace declarations. In this section, we'll discuss the
    possible causes and how to resolve this.</para>

    <section xml:id="Caused_by_DOM_mapping">
        <title>Caused by DOM mapping</title>

        <para>The #1 cause of extra namespace declarations is due to the DOM
        mapping. This mainly happens because of a schema construct that forces
        XJC to generate a property with DOM. This includes the use of wildcard
        <literal>&lt;xs:any/&gt;</literal> (see more about this <xref
        linkend="compiling-xml-schema-mapping-of-xs-any"
        xrefstyle="select:title" />), as well as <literal>xs:anyType</literal>
        (which can also happen by omission, such as <literal>&lt;xs:element
        name="foo"/&gt;</literal>, which is interpreted as <literal>&lt;xs:element
        name="foo" type="xs:anyType" /&gt;</literal>.</para>

        <para>During unmarshalling, when a subtree of the input XML is
        converted into XML, JAXB copies all the in-scope namespace bindings
        active at that time to the root of the DOM element. So for example,
        given the following Java class and XML, the DOM tree that the
        <literal>child</literal> field will get will look like the following:</para>

        <example>
            <title>Bean with wildcard</title>

            <programlisting language="java"><![CDATA[@XmlRootElement
class Foo {
  @XmlAnyElement
  public Element child;
}]]></programlisting>
        </example>

        <example>
            <title>Instance with subtree matching wildcard</title>

            <programlisting language="xml"><![CDATA[<foo xmlns:a="a" xmlns:b="b" xmlns:c="c">
  <subtree xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>
</foo>]]></programlisting>
        </example>

        <example>
            <title>DOM tree to be stored in Foo.child</title>

            <programlisting language="xml"><![CDATA[<subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>]]></programlisting>
        </example>

        <para>Note that the two namespace declarations are copied over, but
        <literal>c</literal> is not because it's overridden. Also not that JAXB is
        not touching the whitespace in document. This copying of namespace
        declarations is necessary to preserve the infoset in the input
        document. For example, if the <literal>&lt;data&gt;</literal> is a QName, its meaning
        would change if JAXB unmarshaller doesn't copy it.</para>

        <para>Now, imagine what happens when you marshal this back to XML.
        Despite the fact that in this example neither <literal>b</literal> nor
        <literal>c</literal> prefixes are in use, JAXB cannot delete them, because
        it doesn't know if those attributes are significant to the application
        or not. Therefore, this could end up producing XML with "extra
        namespace declarations" like:</para>

        <example>
            <title>DOM tree to be stored in Foo.child</title>

            <programlisting language="xml"><![CDATA[<foo>
  <subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>
</foo>]]></programlisting>
        </example>

        <para>Resolving this problem is not possible in the general case, but
        sometimes one of the following strategy works:</para>

        <orderedlist>
            <listitem>
                <para>Sometimes schema author incorrectly assumes that
                <literal>&lt;xs:element name="foo"/&gt;</literal> means
                <literal>&lt;xs:element name="foo" type="xs:string"/&gt;</literal>,
                because attribute declarations work somewhat like this. In
                such a case, adding explicit <literal>type</literal> attribute
                avoids the use of DOM, so things will work as expected.</para>
            </listitem>

            <listitem>
                <para>The wildcard processing mode " <literal>strict</literal>"
                would force a typed binding, and thereby eliminate any DOM
                mapping.</para>
            </listitem>

            <listitem>
                <para>You might be able to manulally go into the DOM tree and
                remove unnecessary namespace declarations, if your application
                knows what are necessary and what are not.</para>
            </listitem>
        </orderedlist>
    </section>
</section>
