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<document xmlns="http://maven.apache.org/XDOC/2.0"
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  <properties>
    <title>Download Commons Digester</title>
    <author email="dev@commons.apache.org">Commons Documentation Team</author>
  </properties>
  <body>
    <section name="Annotations">
      <p>The <code>annotations</code> package provides for Java5 Annotations
        meta-data based definition of rules for <code>Digester</code>.
        This improves maintainability of both Java code and XML documents, as
        rules are now defined in POJOs and generating <code>Digester</code>
        parsers at run-time, avoiding manual updates.</p>

       <subsection name="Introduction">
         <p>This is a brief overview of the digester-rules-in-Java5 Annotations
        feature. Inspired by the basic idea behind the JPA, BeanValidation and
        JAXB's specifications, this feature lets you define Digester rules
        directly in target POJOs, instead of creating and initializing the Rules
        objects programmatically, which can become tedious.</p>
       </subsection>

       <subsection name="Annotation Rules">
         <p>A digester rule on a POJO is expressed through one or more annotations.
        An annotation is considered a digester rule definition if its retention
        policy contains RUNTIME and if the annotation itself is annotated with
        <code>org.apache.commons.digester.annotations.DigesterRule</code>.</p>

        <p>The <code>DigesterRule</code> is defined by the combination of:</p>
        <ul>
            <li>the reflected <code>Class&lt;? extends org.apache.commons.digester.Rule&gt;</code>
            by the annotation;</li>
            <li>the <code>org.apache.commons.digester.annotations.DigesterLoaderHandler</code>
            class that has to be invoked during the target class traversal
            (if not specifyied, the annotation processor will supply the 
            <code>org.apache.commons.digester.annotations.handlers.DefaultLoaderHandler</code>);</li>
            <li>the <code>org.apache.commons.digester.annotations.AnnotationRuleProvider</code>
            provider that produces the <code>pattern, rule</code> pair.</li>
        </ul>

        <p>Digester annotations can target any of the following <code>ElementType</code>s:</p>
        <ul>
            <li><code>FIELD</code> for Digester rules concerning attributes;</li>
            <li><code>METHOD</code> for Digester rules concerning methods calls;</li>
            <li><code>PARAMETER</code> for Digester rules concerning methods parameters setting;</li>
            <li><code>TYPE</code> for Digester rules concerning types creation.</li>
        </ul>
        <p>While other <code>ElementType</code>s are not forbidden, the Digester
        annotations processor does not have to recognize and process annotation
        rules placed on such types.</p>

        <p>Every Digester rule annotation <b>must</b> define a <i>pattern</i>
        element of type <code>String</code> that represents the element matching
        path pattern.</p>

        <source>@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@CreationRule
@DigesterRule(
        reflectsRule = ObjectCreateRule.class,
        providedBy = ObjectCreateRuleProvider.class
)
public @interface ObjectCreate {

    String pattern();

}</source>
       </subsection>

       <subsection name="Applying multiple annotation rule of the same type">
         <p>It is often useful to declare the same annotation rule more than once
        to the same target, with different properties. To support this requirement,
        the Digester annotation processor treats annotations annotated by
        <code>@org.apache.commons.digester.annotations.DigesterRuleList</code>
        whose <code>value</code> element has a return type of an array of rule
        annotations in a special way. Each element in the value array are processed
        by the Digester annotation processor as regular annotation rule annotations.
        This means that each Digester rule specified in the <code>value</code>
        element is applied to the target. The annotation must have retention
        <code>RUNTIME</code> and can be applied on a type, field, method or
        method parameter. It is recommended to use the same set of targets as
        the initial Digester annotation rule.</p>

        <p>Note to designers: each Digester annotation rule should be coupled
        with its corresponding multi-valued annotation.
        It is recommended, though not mandated, the definition of
        an inner annotation named <code>List</code>.</p>

         <source>@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@CreationRule
@DigesterRule(
    reflectsRule = ObjectCreateRule.class,
    providedBy = ObjectCreateRuleProvider.class
)
public @interface ObjectCreate {

    String pattern();

    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE)
    @DigesterRuleList
    @interface List {
        ObjectCreate[] value();
    }

}</source>
       </subsection>

       <subsection name="Rule provider implementation">
         <p>A Digester rule provider implementation performs the rule creation
        of a given annotation for a given annotated element. The implementation
        classes are specified by the <code>providedBy</code> element of the
        <code>@DigesterRule</code> annotation that decorates the rule annotation
        definition. The rule provider implementation implements the
        <code>org.apache.commons.digester.annotations.AnnotationRuleProvider&lt;A extends Annotation, E extends AnnotatedElement, R extends Rule&gt;</code>
        interface.</p>

        <source>class ObjectCreateRuleProvider implements AnnotationRuleProvider&lt;ObjectCreate, Class&lt;?&gt;, ObjectCreateRule&gt; {

    private Class<?> clazz;

    public void init(ObjectCreate annotation, Class<?> element) {
        this.clazz = element;
    }

    public ObjectCreateRule get() {
        return new ObjectCreateRule(this.clazz);
    }

}</source>

         <h5>Notes</h5>
        <p>A new instance of the provider will be created each time the Digester
        annotations processor will meet the relative rule that requests it.</p>
        <p>To supply the missing <code>AnnotatedElement</code> for methods
        <code>PARAMETER</code>s, the Digester annotation processor come with the
        <code>org.apache.commons.digester.annotations.reflect.MethodArgument</code>
        class.</p>
       </subsection>

       <subsection name="Digester loader handler implementation">
         <p>The Digester loader handler is an <code>AnnotatedElement</code>
        interceptor invoked when meeting a particular Digester rule annotation
        while analyzing the target class.</p>

        <p>By default, the Digester annotations processor, when meeting a
        Digester annotation rule, extracts the rule pattern and the relative
        rule provider to store it in the
        <code>org.apache.commons.digester.annotations.FromAnnotationsRuleSet</code>,
        an <code>org.apache.commons.digester.RuleSet</code> implementation.</p>

        <p>If designers have the need of a more elaborate annottaion processing,
        they can specify the <code>handledBy</code> element of the
        <code>@DigesterRule</code> annotation that decorates the rule annotation
        definition. The Digester loader handler implementation implements the
        <code>DigesterLoaderHandler&lt;A extends Annotation, E extends AnnotatedElement&gt;</code>
        interface. Follows below an example:</p>

        <source>class SetPropertiesLoaderHandler implements DigesterLoaderHandler&lt;SetProperty, Field&gt; {

    public void handle(SetProperty annotation, Field element, FromAnnotationsRuleSet ruleSet) {
        SetPropertiesRuleProvider ruleProvider = ruleSet.getProvider(annotation.pattern(), SetPropertiesRuleProvider.class);

        if (ruleProvider == null) {
            ruleProvider = new SetPropertiesRuleProvider();
            ruleSet.addRuleProvider(annotation.pattern(), ruleProvider);
        }

        ruleProvider.addAlias(annotation, element);
    }

}</source>
       </subsection>

       <subsection name="Built-in Rules">
         <p>All built-in annotation rules are in the package
        <code>org.apache.commons.digester.annotations.rules</code>.
        Here is the list of annotations and their usage.</p>

        <h4><code>TYPE</code> annotations</h4>
        <table>
            <thead>
                <tr><th>Annotation</th><th>Reflect rule</th></tr>
            </thead>
            <tbody>
                <tr><td>@ObjectCreate</td><td>org.apache.commons.digester.ObjectCreateRule</td></tr>
                <tr><td>@FactoryCreate</td><td>org.apache.commons.digester.FactoryCreateRule</td></tr>
            </tbody>
        </table>

        <h4><code>FIELD</code> annotations</h4>
        <table>
            <thead>
                <tr><th>Annotation</th><th>Reflect rule</th></tr>
            </thead>
            <tbody>
                <tr><td>@BeanPropertySetter</td><td>org.apache.commons.digester.BeanPropertySetterRule</td></tr>
                <tr><td>@SetProperty</td><td>org.apache.commons.digester.SetPropertiesRule</td></tr>
            </tbody>
        </table>

        <h4><code>METHOD</code> annotations</h4>
        <table>
            <thead>
                <tr><th>Annotation</th><th>Reflect rule</th></tr>
            </thead>
            <tbody>
                <tr><td>@CallMethod</td><td>org.apache.commons.digester.CallMethodRule</td></tr>
                <tr><td>@SetNext</td><td>org.apache.commons.digester.SetNextRule</td></tr>
                <tr><td>@SetRoot</td><td>org.apache.commons.digester.SetRootRule</td></tr>
                <tr><td>@SetTop</td><td>org.apache.commons.digester.SetTopRule</td></tr>
            </tbody>
        </table>

        <h4><code>PARAMETER</code> annotations</h4>
        <table>
            <thead>
                <tr><th>Annotation</th><th>Reflect rule</th></tr>
            </thead>
            <tbody>
                <tr><td>@AttributeCallParam</td><td>org.apache.commons.digester.Digester#addCallParam(String, int, String)</td></tr>
                <tr><td>@CallParam</td><td>org.apache.commons.digester.Digester#addCallParam(String, int)</td></tr>
                <tr><td>@PathCallParam</td><td>org.apache.commons.digester.Digester#addCallParamPath(String, int)</td></tr>
                <tr><td>@StackCallParam</td><td>org.apache.commons.digester.Digester#addCallParam(String, int, int)</td></tr>
            </tbody>
        </table>
       </subsection>

       <subsection name="Bootstrapping">
         <p>The core of Digester annotations rules processor is the
    <code>org.apache.commons.digester.annotations.DigesterLoader</code> class.</p>

    <p>A <code>org.apache.commons.digester.annotations.DigesterLoader</code>
    instance is able to analyze <code>Class&lt;?&gt;</code> graphs and builds
    the relative <code>org.apache.commons.digester.RuleSet</code> to create
    <code>org.apache.commons.digester.Digester</code> instances.</p>

    <p>The bootstrap sequence has been designed to be as simple as possible,
    all that's needed is creating a new
    <code>org.apache.commons.digester.annotations.DigesterLoaderBuilder</code>
    instance, plugging the desired
    <code>org.apache.commons.digester.annotations.spi.AnnotationRuleProviderFactory</code> and
    <code>org.apache.commons.digester.annotations.spi.DigesterLoaderHandlerFactory</code>.
    using a chaining builders pattern.</p>

    <p>An <code>org.apache.commons.digester.annotations.spi.AnnotationRuleProviderFactory</code>
    implementation performs the creation of
    <code>org.apache.commons.digester.annotations.AnnotationRuleProvider&lt;A extends Annotation, E extends AnnotatedElement, R extends Rule&gt;</code>
    instances; the default implementation is limited to create the provider
    by invoking the default empty constructor of the required class, but
    users are free to give their implementation if they need a more complex
    factory, i.e. providers requires components that could be injected from a
    context, etc. etc.</p>

    <h5>Note</h5>
    <p>It is strongly descouraged caching <code>AnnotationRuleProvider</code>
    instances!!!</p>

    <p>Same thing for the <code>org.apache.commons.digester.annotations.spi.DigesterLoaderHandlerFactory</code>,
    which implementation performs the creation of
    <code>DigesterLoaderHandler&lt;A extends Annotation, E extends AnnotatedElement&gt;</code>
    instances; the default implementation is limited to create the handler
    by invoking the default empty constructor of the required class, but
    users are free to give their implementation if they need a more complex
    factory, i.e. providers requires components that could be injected from a
    context, etc. etc.</p>

    <p>Said that, to obtain a fresh new
    <code>org.apache.commons.digester.annotations.DigesterLoader</code> instance
    with default factories, it is enough invoking the default empty constructor:</p>

    <source>DigesterLoader digesterLoader = new DigesterLoaderBuilder()
                                    .useDefaultAnnotationRuleProviderFactory()
                                    .useDefaultDigesterLoaderHandlerFactory();</source>

    <p>Otherwise, if users need specify theyr custom factories:</p>

    <source>DigesterLoader digesterLoader = new DigesterLoaderBuilder()
                                    .useAnnotationRuleProviderFactory(new MyAnnotationRuleProviderFactory())
                                    .useDigesterLoaderHandlerFactory(new MyDigesterLoaderHandlerFactory());</source>
       </subsection>

       <subsection name="Example: a simple RSS parser">
         <p>Let's assume there is the need to parse the following (simplified)
    XML/RSS feed:</p>

    <source>&lt;rss version="2.0"&gt;
    &lt;channel&gt;

        &lt;title&gt;Apache&lt;/title&gt;
        &lt;link&gt;http://www.apache.org&lt;/link&gt;
        &lt;description&gt;The Apache Software Foundation&lt;/description&gt;
        &lt;language&gt;en-US&lt;/language&gt;
        &lt;rating&gt;(PICS-1.1 "http://www.rsac.org/ratingsv01.html"
            2 gen true comment "RSACi North America Server"
            for "http://www.rsac.org" on "1996.04.16T08:15-0500"
            r (n 0 s 0 v 0 l 0))&lt;/rating&gt;

        &lt;image&gt;
            &lt;title&gt;Apache&lt;/title&gt;
            &lt;url&gt;http://jakarta.apache.org/images/jakarta-logo.gif&lt;/url&gt;
            &lt;link&gt;http://jakarta.apache.org&lt;/link&gt;
            &lt;width&gt;505&lt;/width&gt;
            &lt;height&gt;480&lt;/height&gt;
            &lt;description&gt;The Jakarta project. Open source, serverside java.&lt;/description&gt;
        &lt;/image&gt;

        &lt;item&gt;
            &lt;title&gt;Commons Attributes 2.1 Released&lt;/title&gt;
            &lt;link&gt;http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040815.1&lt;/link&gt;
            &lt;description&gt;The Apache Commons team is happy to announce the release of Commons Attributes 2.1.
            This is the first release of the new Commons-Attributes code.&lt;/description&gt;
        &lt;/item&gt;

        &lt;item&gt;
            &lt;title&gt;Cloudscape Becomes Apache Derby&lt;/title&gt;
            &lt;link&gt;http://jakarta.apache.org/site/news/elsewhere-2004-2ndHalf.html#20040803.1&lt;/link&gt;
            &lt;description&gt;IBM has submitted a proposal to the Apache DB project for a Java-based package to be called 'Derby'.&lt;/description&gt;
        &lt;/item&gt;

        &lt;item&gt;
            &lt;title&gt;Commons BeanUtils 1.7 Released&lt;/title&gt;
            &lt;link&gt;http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040802.1&lt;/link&gt;
            &lt;description/&gt;
        &lt;/item&gt;

        &lt;item&gt;
            &lt;title&gt;Commons JXPath 1.2 Released&lt;/title&gt;
            &lt;link&gt;http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040801.2&lt;/link&gt;
            &lt;description/&gt;
        &lt;/item&gt;
    &lt;/channel&gt;
&lt;/rss&gt;</source>

    <p>So, let's define the Java entities and annotate them; first the <code>Channel</code> entity:</p>

    <source>@ObjectCreate(pattern = "rss/channel")
class Channel {

    private final List&lt;Item&gt; items = new ArrayList&lt;Item&gt;();

    @BeanPropertySetter(pattern = "rss/channel/title")
    private String title;

    @BeanPropertySetter(pattern = "rss/channel/link")
    private String link;

    @BeanPropertySetter(pattern = "rss/channel/description")
    private String description;

    @BeanPropertySetter(pattern = "rss/channel/language")
    private String language;

    private Image image;

    // getters and setters

    @SetNext
    public void setImage(Image image) {
        this.image = image;
    }

    @SetNext
    public void addItem(Item item) {
        this.items.add(item);
    }

}</source>

    <p>Then the <code>Image</code> entity:</p>

    <source>@ObjectCreate(pattern = "rss/channel/image")
class Image {

    @BeanPropertySetter(pattern = "rss/channel/image/description")
    private String description;

    @BeanPropertySetter(pattern = "rss/channel/image/width")
    private int width;

    @BeanPropertySetter(pattern = "rss/channel/image/height")
    private int height;

    @BeanPropertySetter(pattern = "rss/channel/image/link")
    private String link;

    @BeanPropertySetter(pattern = "rss/channel/image/title")
    private String title;

    @BeanPropertySetter(pattern = "rss/channel/image/url")
    private String url;

    // getters and setters

}</source>

    <p>and finally the <code>Item</code> entity:</p>

    <source>@ObjectCreate(pattern = "rss/channel/item")
class Item {

    @BeanPropertySetter(pattern = "rss/channel/item/description")
    private String description;

    @BeanPropertySetter(pattern = "rss/channel/item/link")
    private String link;

    @BeanPropertySetter(pattern = "rss/channel/item/title")
    private String title;

    // getters and setters

}</source>

    <p>It is now possible to create the <code>Digester</code> instance and parse the XML:</p>

    <source>DigesterLoader digesterLoader = new DigesterLoaderBuilder()
                                    .useDefaultAnnotationRuleProviderFactory()
                                    .useDefaultDigesterLoaderHandlerFactory();
...
Digester digester = digesterLoader.createDigester(Channel.class);
try {
    Channel channel = (Channel) digester.parse(new URL("http://www.myfeedprovider.com/rss.xml").openStream());
} catch (Exception e) {
    // do something
}
    </source>

    <h5>Notes</h5>
    <p>If asking to the <code>DigesterLoader</code> instance more then twice the
    <code>Digester</code> for the same <code>Class&lt;?&gt;</code>, the
    <code>DigesterLoader</code> won't analize the target class for each request,
    but rather will reuse cached results.</p>

    <p>The same <code>DigesterLoader</code> instance can be reused to create
    other <code>Digester</code> instances.</p>
       </subsection>
    </section>
  </body>
</document>