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  <properties>
    <title>Apache Commons Digester | Guide | Constructor rule</title>
    <author email="dev@commons.apache.org">Commons Documentation Team</author>
  </properties>
  <body>
    <section name="Constructor based rule">
      <p>One of the missing features of the old Digester releases is that the
      <a href="../apidocs/org/apache/commons/digester3/ObjectCreateRule.html">ObjectCreateRule</a> works just with
      the default empty constructor.</p>
      <p>One limit that cannot be exceeded is the fact that constructor arguments cannot be extracted from inner
      XML elements; that's because the <code>ObjectCreateRule</code> creates the object when the related XML
      element <code>begins</code>, otherwise properties could not be set when parsing nested elements.</p>
      <p>That is no longer true :) Constructor arguments can be extracted from <i>attributes</i> and
      <i>nested elements</i> of the matching XML element for whom the <code>ObjectCreateRule</code> is triggered.</p>
      <p><b>NOTE</b> this feature is available since release 3.2.</p>

      <subsection name="Using plain old Digester APIs">
        <p><code>ObjectCreateRule</code> has new APIs to configure the constructor arguments types; given for example
        the XML snippet below:</p>
        <source>&lt;root&gt;
  &lt;bean super="true"&gt;
    &lt;rate&gt;9.99&lt;rate/&gt;
  &lt;/bean&gt;
&lt;/root&gt;</source>
        <p>That has to be mapped to the bean:</p>
        <source>class MyBean
{

    public MyBean( Double rate, Boolean super )
    {
        ...
    }

}</source>
        <p>Then the <code>Digester</code> instance can be configured as below:</p>
        <source>ObjectCreateRule createRule = new ObjectCreateRule( MyBean.class );
createRule.setConstructorArgumentTypes( boolean.class, double.class );

Digester digester = new Digester();
digester.addRule( "root/bean", createRule );
digester.addCallParam( "root/bean", 1, "super" );
digester.addCallParam( "root/bean/rate", 0 );</source>
      </subsection>

      <subsection name="Using the RulesBinder APIs">
        <p>The Binder APIs just allow expressing the same rule in a fluent way:</p>
        <source>DigesterLoader loader = ( new AbstractRulesModule()
{

    @Override
    protected void configure()
    {
        forPattern( "root/bean" )
            .createObject().ofType( MyBean.class ).usingConstructor( boolean.class, double.class )
            .then()
            .callParam().fromAttribute( "super" ).ofIndex( 1 );
        forPattern( "root/bean/rate" ).callParam().ofIndex( 0 );
    }

} );</source>
      </subsection>

      <subsection name="Using the annotations">
        <p>Since 3.2, <a href="../apidocs/org/apache/commons/digester3/annotations/rules/ObjectCreate.html">ObjectCreate</a>
        can be used to annotate constructors as well; with the introduction of
        <a href="../apidocs/org/apache/commons/digester3/annotations/rules/Attribute.html">Attribute</a> annotation,
        constructor based rules can be expressed like:</p>
        <source>class MyBean
{

    @ObjectCreate( pattern = "root/bean" )
    public MyBean( @CallParam( pattern = "root/bean/rate" ) Double rate,
                   @CallParam( pattern = "root/bean", attributeName = "super" ) Boolean super )
    {
        ...
    }

}</source>
      </subsection>

      <subsection name="Using the XML meta-descriptor">
        <p>The XML ruleset supports as well the new constructor rule, <code>&lt;object-create-rule&gt;</code> supports
        a new inner element <code>&lt;constructor-argument&gt;</code>:</p>
        <source>&lt;digester-rules&gt;
  &lt;pattern value="root/bean"&gt;
    &lt;object-create-rule classname="MyBean
          paramtypes="java.lang.Double,java.lang.Boolean"&gt;
      &lt;constructor-argument attrname="rate" type="java.lang.Double" /&gt;
      &lt;call-param-rule paramnumber="0" pattern="rate" /&gt;
      &lt;call-param-rule paramnumber="1" attrname="super" /&gt;
    &lt;/object-create-rule&gt;
  &lt;/pattern&gt;
&lt;/digester-rules&gt;</source>
      </subsection>
    </section>

    <section name="Default constructor arguments">
      <p>In order to provide that feature, Digester relies on <code>CGLIB</code> that needs to proxy the class of the
      target object, to avoid constructors issues users can use the
      <code>ObjectCreateRule#setDefaultConstructorArguments(Object...)</code> method to give safe construction
      params to the constructor when creating the proxy; that method could be useful also when one parameter needs to
      take a static value while the other is handled with a <code>CallParam</code> rule. If these are not specified,
      the super constructor is called with nulls for Objects and default values for primitives.</p>
    </section>
  </body>
</document>