Documentation for org.apache.commons.digester.xmlrules

The `xmlrules` package provides for XML-based definition of rules for `Digester`. This improves maintainability of Java code, as rules are now defined in XML and read into `Digester` at run-time.

[Introduction]
[DTD Overview]
[Rule Elements]
[Matching Patterns]
[Including rules files in other rules files]
[Including programmatically-created rules]
[Creating a digester from XML]

### Introduction

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.

## Overview of digester-rules.dtd

A DTD, named `digester-rules.dtd` has been defined to help in the understanding of how the loader operates.

The DTD is distributed in the `commons-digester.jar`. It can be found at `org/apache/commons/digester/xmlrules/digester-rules.dtd`. It is not available for download from the Apache website since users are best advised to use a copy stored on their local system.

Digester input documents wishing to cite this DTD should include the following DOCTYPE declaration:

      <!DOCTYPE digester-rules PUBLIC
       "-//Jakarta Apache //DTD digester-rules XML V1.0//EN"
       "digester-rules.dtd">

### Rule elements:

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 `pattern` attribute specifying the pattern to associate with the rule.
The `DigesterLoader` adds the rules to the digester in the order in which they occur in the XML.
The use of each rule element type should be self-explanatory, if you compare them to the API documentation for the `Digester` rules classes.

### Defining matching patterns:

The matching pattern is a simple, xpath-like string which the `Digester` uses to determine which elements to apply each rule to. See the `Digester` documentation for more details.
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 `pattern` attribute. An example would like something like:

          <digester-rules>
            <object-create-rule pattern="*/foo" classname="Foo"/>
            <set-properties-rule pattern="*/foo"/>
          </digester-rules>

In the above example, an `ObjectCreateRule` is created and associated with the pattern "*/foo"; then a `SetPropertiesRule` is created and associated with the pattern "*/foo".
The other method is to nest rules elements inside a `<pattern>` 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:

           <digester-rules>
             <pattern value="*/foo">
               <object-create-rule classname="Foo"/>
               <set-properties-rule/>
             </pattern>
           </digester-rules>

Pattern elements can be recursively nested. If patterns are nested, the pattern string is formed by concatenating all the patterns together. Example:

           <digester-rules>
             <pattern value="*/foo">
               <pattern value="bar">
                 <object-create-rule classname="Foobar"/>
                 <set-properties-rule/>
               </pattern>
             </pattern>
           </digester-rules>

In the above example, an `ObjectCreateRule` and a `SetPropertiesRule` are associated with the matching pattern "*/foo/bar".
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:

           <digester-rules>
             <pattern value="*/foo">
               <object-create-rule pattern="bar" classname="Foobar"/>
               <set-properties-rule pattern="bar"/>
             </pattern>
           </digester-rules>

### Including rules XML files within other rules XML files:

The `<include>` element lets you include one rules file within another. With respect to pattern concatenation, the `DigesterLoader` behaves as if the include file was 'macro-expanded'. Example:

          File rules1.xml:
             <?xml version="1.0"?>
             <!DOCTYPE digester-rules SYSTEM "digester-rules.dtd">

             <digester-rules>
               <pattern value="root/foo">
                 <object-create-rule classname="Foo"/>

                 <include path="rules2.xml"/>
               </pattern>
             </digester-rules>

          File rules2.xml:
             <?xml version="1.0"?>
             <!DOCTYPE digester-rules SYSTEM "digester-rules.dtd">

             <digester-rules>
               <pattern value="bar">
                 <object-create-rule classname="Bar"/>
               </pattern>
             </digester-rules>

Parsing rule1.xml would result in a `Digester` initialized with these pattern/rule pairs:
root/foo -> ObjectCreateRule(Foo)
root/foo/bar -> ObjectCreateRule(Bar)
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 `Digester` initialized with this pattern/rule:
bar -> ObjectCreateRule(Bar)

### Including programmatically-created rules:

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 `<include>` element with a `class` attribute, containing the name of a class that implements `org.apache.commons.digester.xmlrules.DigesterRulesSource`. This interface defines one method, `getRules(Digester)`, 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:

          File rules3.xml:
             <?xml version="1.0"?>
             <!DOCTYPE digester-rules SYSTEM "digester-rules.dtd">

             <digester-rules>
               <pattern value="root/foo">
                 <object-create-rule classname="Foo"/>

                 <include class="BarRuleCreator"/>
               </pattern>
             </digester-rules>

BarRuleCreator class definition:

              public class BarRuleCreator implements DigesterRulesSource {
                  public void getRules(Digester digester) {
                      digester.addObjectCreate("bar", "Bar");
                  }
              }

Parsing rules3.xml yields the same results as rules1.xml above:
root/foo -> ObjectCreateRule(Foo)
root/foo/bar -> ObjectCreateRule(Bar)

### Creating a digester from XML:

`FromXmlRuleSet` is a `RuleSet` implementation that initializes its `Digester` from rules defined in an XML file. The path to the XML file is passed to constructor.
Alternatively, the convenience class DigesterLoader defines a static method, `Digester createDigester(String rulesXml) throws DigesterLoaderException`". When passing the name of the file that contains your digester rules, this method returns a `Digester` instance initialized with the rules.
To add your own rules, you need to:

- Update the DTD
  You should add an element type for your rule. The element should have an attribute corresponding to each of the rule's initialization parameters.
- Define an `ObjectCreationFactory`
- Extend `DigesterRuleParser`
  `DigesterRuleParser` is a `RuleSet` for parsing a rules XML file. You should extend this, and override the `addRuleInstances()` method to add the rules for parsing your new element. Look in DigesterRuleParser.java to see how its done.