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  <properties>
    <title>Download Commons Digester</title>
    <author email="dev@commons.apache.org">Commons Documentation Team</author>
  </properties>
  <body>
    <section name="Documentation for org.apache.commons.digester.xmlrules">
      <p>The <code>xmlrules</code>  package provides for XML-based definition of
rules for <code>Digester</code>. This improves maintainability of Java code,
as rules are now defined in XML and read into <code>Digester</code> at run-time.
</p>

      <subsection name="Introduction">
        <p>This is a brief overview of the digester-rules-in-XML feature. Briefly,
this feature lets you define Digester rules in XML, instead of
creating and initializing the Rules objects programmatically, which can become
tedious.  In addition, it allows for including of one XML rules file within
another, inclusion of programmatically created rule sets within an XML file
(via reflection), and recursively nested matching pattern specifications.
</p>
      </subsection>

      <subsection name="Overview of digester-rules.dtd">
        <p>A DTD, named <code>digester-rules.dtd</code> has been defined to help in the
understanding of how the loader operates.
</p>
<p>
The DTD is distributed in the <code>commons-digester.jar</code>. It can be found at
<code>org/apache/commons/digester/xmlrules/digester-rules.dtd</code>. It is not available
for download from the Apache website since users are best advised to use a copy stored 
on their local system. 
</p>

<p>Digester input documents wishing to cite this DTD should include the
following DOCTYPE declaration:</p>
<source>
  &lt;!DOCTYPE digester-rules PUBLIC
   "-//Jakarta Apache //DTD digester-rules XML V1.0//EN"
   "digester-rules.dtd">
</source>
      </subsection>

      <subsection name="Rule elements">
        <p>The DTD defines an element type corresponding to each predefined Digester
rule. Each rule element type includes attributes for values needed to
initialize the rule, and an optional <code>pattern</code> attribute
specifying the pattern to associate with the rule.</p>
<p>The <code>DigesterLoader</code> adds the rules to the digester in the order in
which they occur in the XML.</p>
<p>The use of each rule element type should be self-explanatory, if you compare
them to the API documentation for the <code>Digester</code> rules classes.
</p>
      </subsection>

      <subsection name="Defining matching patterns">
        <p>The matching pattern is a simple, xpath-like string which the
<code>Digester</code> uses to determine which elements to apply each rule to.
See the <code>Digester</code> <a href="/core.html">documentation</a> for
more details.</p>
<p>
There are two methods for associating patterns to rules in the XML file. One
is for each rule element to directly define its pattern in a
<code>pattern</code> attribute. An example would like something like:</p>
<source>
      &lt;digester-rules&gt;
        &lt;object-create-rule pattern="*/foo" classname="Foo"/&gt;
        &lt;set-properties-rule pattern="*/foo"/&gt;
      &lt;/digester-rules&gt;
</source>
<p>
In the above example, an <code>ObjectCreateRule</code> is created and
associated with the pattern "*/foo"; then a <code>SetPropertiesRule</code> is
created and associated with the pattern "*/foo".</p>
<p>The other method is to nest rules elements inside a
<code>&lt;pattern&gt;</code> element. In this way, the same pattern can be
defined for a group of rules. The following example has the same effect as the
previous example:</p>
<source>
       &lt;digester-rules&gt;
         &lt;pattern value="*/foo"&gt;
           &lt;object-create-rule classname="Foo"/&gt;
           &lt;set-properties-rule/&gt;
         &lt;/pattern&gt;
       &lt;/digester-rules&gt;
</source>
<p>Pattern elements can be recursively nested. If patterns are nested, the pattern
string is formed by concatenating all the patterns together. Example:</p>
<source>
       &lt;digester-rules&gt;
         &lt;pattern value="*/foo"&gt;
           &lt;pattern value="bar"&gt;
             &lt;object-create-rule classname="Foobar"/&gt;
             &lt;set-properties-rule/&gt;
           &lt;/pattern&gt;
         &lt;/pattern&gt;
       &lt;/digester-rules&gt;
</source>
<p>
In the above example, an <code>ObjectCreateRule</code> and a
<code>SetPropertiesRule</code> are associated with the matching pattern
"*/foo/bar".</p>
<p>The use of pattern elements and the use of the pattern attribute inside rules
elements can be freely mixed. The next example has the same effect as the
previous example:</p>
<source>
       &lt;digester-rules&gt;
         &lt;pattern value="*/foo"&gt;
           &lt;object-create-rule pattern="bar" classname="Foobar"/&gt;
           &lt;set-properties-rule pattern="bar"/&gt;
         &lt;/pattern&gt;
       &lt;/digester-rules&gt;
</source>
      </subsection>

      <subsection name="Including rules XML files within other rules XML files">
        <p>
The <code>&lt;include&gt;</code> element lets you include one rules file within
another. With respect to pattern concatenation, the <code>DigesterLoader</code>
behaves as if the include file was 'macro-expanded'. Example:</p>
<source>
      File rules1.xml:
         &lt;?xml version="1.0"?&gt;
         &lt;!DOCTYPE digester-rules SYSTEM "digester-rules.dtd"&gt;

         &lt;digester-rules&gt;
           &lt;pattern value="root/foo"&gt;
             &lt;object-create-rule classname="Foo"/&gt;

             &lt;include path="rules2.xml"/&gt;
           &lt;/pattern&gt;
         &lt;/digester-rules&gt;


      File rules2.xml:
         &lt;?xml version="1.0"?&gt;
         &lt;!DOCTYPE digester-rules SYSTEM "digester-rules.dtd"&gt;

         &lt;digester-rules&gt;
           &lt;pattern value="bar"&gt;
             &lt;object-create-rule classname="Bar"/&gt;
           &lt;/pattern&gt;
         &lt;/digester-rules&gt;
</source>
<p>
Parsing rule1.xml would result in a <code>Digester</code> initialized with these
pattern/rule pairs:</p>
<source>
    root/foo -> ObjectCreateRule(Foo)

    root/foo/bar -> ObjectCreateRule(Bar)
</source>
<p>Note that the pattern for the 'bar' rule has been prepended with the 'root/foo'
pattern. If rule2.xml was parsed by itself, it would yield a <code>Digester</code>
initialized with this pattern/rule:</p>
<source>
    bar -> ObjectCreateRule(Bar)
</source>
      </subsection>

      <subsection name="Including programmatically-created rules">
        <p>Sometimes rules cannot be easily defined via XML. Rule sets that are created
programmatically can still be included within a digester-rules XML file. This
is done by using an <code>&lt;include&gt;</code> element with a
<code>class</code> attribute, containing the name of a class that implements
<code>org.apache.commons.digester.xmlrules.DigesterRulesSource</code>.
This interface defines one method, <code>getRules(Digester)</code>, which
creates rules and adds them to the supplied Digester. The pattern concatenation
works exactly as if the rules had been included from an XML file. Example:</p>
<source>
      File rules3.xml:
         &lt;?xml version="1.0"?&gt;
         &lt;!DOCTYPE digester-rules SYSTEM "digester-rules.dtd"&gt;

         &lt;digester-rules&gt;
           &lt;pattern value="root/foo"&gt;
             &lt;object-create-rule classname="Foo"/&gt;

             &lt;include class="BarRuleCreator"/&gt;
           &lt;/pattern&gt;
         &lt;/digester-rules&gt;
</source>
<p>BarRuleCreator class definition:</p>
<source>
          public class BarRuleCreator implements DigesterRulesSource {
              public void getRules(Digester digester) {
                  digester.addObjectCreate("bar", "Bar");
              }
          }
</source>
<p>
Parsing rules3.xml yields the same results as rules1.xml above:
</p>
<source>
    root/foo -> ObjectCreateRule(Foo)

    root/foo/bar -> ObjectCreateRule(Bar)
</source>
      </subsection>

      <subsection name="Creating a digester from XML">
        <p><code>FromXmlRuleSet</code> is a <code>RuleSet</code> implementation that
initializes its <code>Digester</code> from rules defined in an XML file. The
path to the XML file is passed to constructor.</p>
<p>Alternatively, the convenience class <code>DigesterLoader</code> defines a
static method,
<code>Digester createDigester(String rulesXml) throws DigesterLoaderException</code>".
When passing the name of the file that contains your digester rules, this
method returns a <code>Digester</code> instance initialized with the rules.</p>
<p>To add your own rules, you need to:</p>
<ul>
<li>Update the DTD<br />You should add an element type for your rule. The
element should have an attribute corresponding to each of the rule's
initialization parameters.
</li>
<li>Define an <code>ObjectCreationFactory</code>
</li>
<li>Extend <code>DigesterRuleParser</code><br /><code>DigesterRuleParser</code>
is a <code>RuleSet</code> for parsing a rules XML file. You should extend this,
and override the <code>addRuleInstances()</code> method to add the rules for
parsing your new element. Look in DigesterRuleParser.java to see how its done.
</li>
</ul>
      </subsection>
    </section>
  </body>
</document>