The Digester package provides for rules-based processing of arbitrary XML documents.
[Dependencies] [Introduction] [Configuration Properties] [The Object Stack] [Element Matching Patterns] [Processing Rules] [Logging] [Usage Example] [Namespace Aware Parsing] [Pluggable Rules Processing] [Encapsulated Rule Sets] [FAQ] [Known Limitations]

### External Dependencies

- An XML parser conforming to JAXP , version 1.1 or later (the first one to support SAX 2.0)
- Beanutils Package (Jakarta Commons), version 1.5 or later
- Collections Package (Jakarta Commons), version 2.1 or later
- Commons Logging Package (Jakarta Commons), version 1.0.2 or later

### Introduction

In many application environments that deal with XML-formatted data, it is useful to be able to process an XML document in an "event driven" manner, where particular Java objects are created (or methods of existing objects are invoked) when particular patterns of nested XML elements have been recognized. Developers familiar with the Simple API for XML Parsing (SAX) approach to processing XML documents will recognize that the Digester provides a higher level, more developer-friendly interface to SAX events, because most of the details of navigating the XML element hierarchy are hidden -- allowing the developer to focus on the processing to be performed.

In order to use a Digester, the following basic steps are required:

- Create a new instance of the `org.apache.commons.digester.Digester` class. Previously created Digester instances may be safely reused, as long as you have completed any previously requested parse, and you do not try to utilize a particular Digester instance from more than one thread at a time.
- Set any desired configuration properties that will customize the operation of the Digester when you next initiate a parse operation.
- Push any desired initial object(s) onto the Digester's object stack.
- Register all of the element matching patterns for which you wish to have processing rules fired when this pattern is recognized in an input document. You may register as many rules as you like for any particular pattern. If there is more than one rule for a given pattern, the rules will be executed in the order that they were listed.
- Call the `digester.parse()` method, passing a reference to the XML document to be parsed in one of a variety of forms. See the Digester.parse() documentation for details. Note that you will need to be prepared to catch any `IOException` or `SAXException` that is thrown by the parser, or any runtime expression that is thrown by one of the processing rules.

### Digester Configuration Properties

A `org.apache.commons.digester.Digester` instance contains several configuration properties that can be used to customize its operation. These properties must be configured before you call one of the `parse()` variants, in order for them to take effect on that parse.

> Property | Description
> --- | ---
> classLoader | You can optionally specify the class loader that will be used to load classes when required by the `ObjectCreateRule` and `FactoryCreateRule` rules. If not specified, application classes will be loaded from the thread's context class loader (if the `useContextClassLoader` property is set to `true`) or the same class loader that was used to load the `Digester` class itself.
> debug | An integer defining the amount of debugging output that will be written to `System.out()` as the parse progresses. This is useful when tracking down where parsing problems are occurring. The default value of zero means no debugging output will be generated -- increasing values generally cause the generation of more verbose and detailed debugging information.
> errorHandler | You can optionally specify a SAX `ErrorHandler` that is notified when parsing errors occur. By default, any parsing errors that are encountered are logged, but Digester will continue processing as well.
> namespaceAware | A boolean that is set to `true` to perform parsing in a manner that is aware of XML namespaces. Among other things, this setting affects how elements are matched to processing rules. See Namespace Aware Parsing for more information.
> ruleNamespaceURI | The public URI of the namespace for which all subsequently added rules are associated, or `null` for adding rules that are not associated with any namespace. See Namespace Aware Parsing for more information.
> rules | The `Rules` component that actually performs matching of `Rule` instances against the current element nesting pattern is pluggable. By default, Digester includes a `Rules` implementation that behaves as described in this document. See Pluggable Rules Processing for more information.

> useContextClassLoader

> A boolean that is set to `true` if you want application classes required by `FactoryCreateRule` and `ObjectCreateRule` to be loaded from the context class loader of the current thread. By default, classes will be loaded from the class loader that loaded this `Digester` class. NOTE - This property is ignored if you set a value for the `classLoader` property; that class loader will be used unconditionally.

> validating | A boolean that is set to `true` if you wish to validate the XML document against a Document Type Definition (DTD) that is specified in its `DOCTYPE` declaration. The default value of `false` requests a parse that only detects "well formed" XML documents, rather than "valid" ones.

In addition to the scalar properties defined above, you can also register a local copy of a Document Type Definition (DTD) that is referenced in a `DOCTYPE` declaration. Such a registration tells the XML parser that, whenever it encounters a `DOCTYPE` declaration with the specified public identifier, it should utilize the actual DTD content at the registered system identifier (a URL), rather than the one in the `DOCTYPE` declaration.

For example, the Struts framework controller servlet uses the following registration in order to tell Struts to use a local copy of the DTD for the Struts configuration file. This allows usage of Struts in environments that are not connected to the Internet, and speeds up processing even at Internet connected sites (because it avoids the need to go across the network).

        URL url = new URL("/org/apache/struts/resources/struts-config_1_0.dtd");
        digester.register
          ("-//Apache Software Foundation//DTD Struts Configuration 1.0//EN",
           url.toString());

As a side note, the system identifier used in this example is the path that would be passed to `java.lang.ClassLoader.getResource()` or `java.lang.ClassLoader.getResourceAsStream()`. The actual DTD resource is loaded through the same class loader that loads all of the Struts classes -- typically from the `struts.jar` file.

### The Object Stack

One very common use of `org.apache.commons.digester.Digester` technology is to dynamically construct a tree of Java objects, whose internal organization, as well as the details of property settings on these objects, are configured based on the contents of the XML document. In fact, the primary reason that the Digester package was created (it was originally part of Struts, and then moved to the Commons project because it was recognized as being generally useful) was to facilitate the way that the Struts controller servlet configures itself based on the contents of your application's `struts-config.xml` file.

To facilitate this usage, the Digester exposes a stack that can be manipulated by processing rules that are fired when element matching patterns are satisfied. The usual stack-related operations are made available, including the following:

- clear() - Clear the current contents of the object stack.
- peek() - Return a reference to the top object on the stack, without removing it.
- pop() - Remove the top object from the stack and return it.
- push() - Push a new object onto the top of the stack.

A typical design pattern, then, is to fire a rule that creates a new object and pushes it on the stack when the beginning of a particular XML element is encountered. The object will remain there while the nested content of this element is processed, and it will be popped off when the end of the element is encountered. As we will see, the standard "object create" processing rule supports exactly this functionalility in a very convenient way.

Several potential issues with this design pattern are addressed by other features of the Digester functionality:

- How do I relate the objects being created to each other? - The Digester supports standard processing rules that pass the top object on the stack as an argument to a named method on the next-to-top object on the stack (or vice versa). This rule makes it easy to establish parent-child relationships between these objects. One-to-one and one-to-many relationships are both easy to construct.
- How do I retain a reference to the first object that was created? As you review the description of what the "object create" processing rule does, it would appear that the first object you create (i.e. the object created by the outermost XML element you process) will disappear from the stack by the time that XML parsing is completed, because the end of the element would have been encountered. To deal with this, the normal approach is to push a reference to some application global object onto the stack before the parse begins, and arrange that a parent-child relationship be created (by appropriate processing rules) between this manually pushed object and the one that is dynamically created. In this way, the pushed object will retain a reference to the dynamically created object (and therefore all of its children) after the parse finishes.

### Element Matching Patterns

A primary feature of the `org.apache.commons.digester.Digester` parser is that the Digester automatically navigates the element hierarchy of the XML document you are parsing for you, without requiring any developer attention to this process. Instead, you focus on deciding what functions you would like to have performed whenver a certain arrangement of nested elements is encountered in the XML document being parsed. The mechanism for specifying such arrangements are called element matching patterns.

A very simple element matching pattern is a simple string like "a". This pattern is matched whenever an `<a>` top-level element is encountered in the XML document, no matter how many times it occurs. Note that nested `<a>` elements will not match this pattern -- we will describe means to support this kind of matching later.

The next step up in matching pattern complexity is "a/b". This pattern will be matched when a `<b>` element is found nested inside a top-level `<a>` element. Again, this match can occur as many times as desired, depending on the content of the XML document being parsed. You can use multiple slashes to define a hierarchy of any desired depth that will be matched appropriately.

For example, assume you have registered processing rules that match patterns "a", "a/b", and "a/b/c". For an input XML document with the following contents, the indicated patterns will be matched when the corresponding element is parsed:

      <a>         -- Matches pattern "a"
        <b>       -- Matches pattern "a/b"
          <c/>    -- Matches pattern "a/b/c"
          <c/>    -- Matches pattern "a/b/c"
        </b>
        <b>       -- Matches pattern "a/b"
          <c/>    -- Matches pattern "a/b/c"
          <c/>    -- Matches pattern "a/b/c"
          <c/>    -- Matches pattern "a/b/c"
        </b>
      </a>

It is also possible to match a particular XML element, no matter how it is nested (or not nested) in the XML document, by using the "*" wildcard character in your matching pattern strings. For example, an element matching pattern of "*/a" will match an `<a>` element at any nesting position within the document.

It is quite possible that, when a particular XML element is being parsed, the pattern for more than one registered processing rule will be matched either because you registered more than one processing rule with the same matching pattern, or because one more more exact pattern matches and wildcard pattern matches are satisfied by the same element.

When this occurs, the corresponding processing rules will all be fired in order. `begin` (and `body`) method calls are executed in the order that the `Rules` where initially registered with the `Digester`, whilst `end` method calls are execute in reverse order. In other words - the order is first in, last out.

### Processing Rules

The previous section documented how you identify when you wish to have certain actions take place. The purpose of processing rules is to define what should happen when the patterns are matched.

Formally, a processing rule is a Java class that subclasses the org.apache.commons.digester.Rule interface. Each Rule implements one or more of the following event methods that are called at well-defined times when the matching patterns corresponding to this rule trigger it:

- begin() - Called when the beginning of the matched XML element is encountered. A data structure containing all of the attributes corresponding to this element are passed as well.
- body() - Called when nested content (that is not itself XML elements) of the matched element is encountered. Any leading or trailing whitespace will have been removed as part of the parsing process.
- end() - Called when the ending of the matched XML element is encountered. If nested XML elements that matched other processing rules was included in the body of this element, the appropriate processing rules for the matched rules will have already been completed before this method is called.
- finish() - Called when the parse has been completed, to give each rule a chance to clean up any temporary data they might have created and cached.

As you are configuring your digester, you can call the `addRule()` method to register a specific element matching pattern, along with an instance of a `Rule` class that will have its event handling methods called at the appropriate times, as described above. This mechanism allows you to create `Rule` implementation classes dynamically, to implement any desired application specific functionality.

In addition, a set of processing rule implementation classes are provided, which deal with many common programming scenarios. These classes include the following:

- ObjectCreateRule - When the `begin()` method is called, this rule instantiates a new instance of a specified Java class, and pushes it on the stack. The class name to be used is defaulted according to a parameter passed to this rule's constructor, but can optionally be overridden by a classname passed via the specified attribute to the XML element being processed. When the `end()` method is called, the top object on the stack (presumably, the one we added in the `begin()` method) will be popped, and any reference to it (within the Digester) will be discarded.
- FactoryCreateRule - A variation of `ObjectCreateRule` that is useful when the Java class with which you wish to create an object instance does not have a no-arguments constructor, or where you wish to perform other setup processing before the object is handed over to the Digester.
- SetPropertiesRule - When the `begin()` method is called, the digester uses the standard Java Reflection API to identify any JavaBeans property setter methods (on the object at the top of the digester's stack) who have property names that match the attributes specified on this XML element, and then call them individually, passing the corresponding attribute values. These natural mappings can be overridden. This allows (for example) a `class` attribute to be mapped correctly. It is recommended that this feature should not be overused - in most cases, it's better to use the standard `BeanInfo` mechanism. A very common idiom is to define an object create rule, followed by a set properties rule, with the same element matching pattern. This causes the creation of a new Java object, followed by "configuration" of that object's properties based on the attributes of the same XML element that created this object.
- SetPropertyRule - When the `begin()` method is called, the digester calls a specified property setter (where the property itself is named by an attribute) with a specified value (where the value is named by another attribute), on the object at the top of the digester's stack. This is useful when your XML file conforms to a particular DTD, and you wish to configure a particular property that does not have a corresponding attribute in the DTD.
- SetNextRule - When the `end()` method is called, the digester analyzes the next-to-top element on the stack, looking for a property setter method for a specified property. It then calls this method, passing the object at the top of the stack as an argument. This rule is commonly used to establish one-to-many relationships between the two objects, with the method name commonly being something like "addChild".
- SetTopRule - When the `end()` method is called, the digester analyzes the top element on the stack, looking for a property setter method for a specified property. It then calls this method, passing the next-to-top object on the stack as an argument. This rule would be used as an alternative to a SetNextRule, with a typical method name "setParent", if the API supported by your object classes prefers this approach.
- CallMethodRule - This rule sets up a method call to a named method of the top object on the digester's stack, which will actually take place when the `end()` method is called. You configure this rule by specifying the name of the method to be called, the number of arguments it takes, and (optionally) the Java class name(s) defining the type(s) of the method's arguments. The actual parameter values, if any, will typically be accumulated from the body content of nested elements within the element that triggered this rule, using the CallParamRule discussed next.
- CallParamRule - This rule identifies the source of a particular numbered (zero-relative) parameter for a CallMethodRule within which we are nested. You can specify that the parameter value be taken from a particular named attribute, or from the nested body content of this element.
- NodeCreateRule - A specialized rule that converts part of the tree into a `DOM Node` and then pushes it onto the stack.

You can create instances of the standard `Rule` classes and register them by calling `digester.addRule()`, as described above. However, because their usage is so common, shorthand registration methods are defined for each of the standard rules, directly on the `Digester` class. For example, the following code sequence:

        Rule rule = new SetNextRule(digester, "addChild",
                                    "com.mycompany.mypackage.MyChildClass");
        digester.addRule("a/b/c", rule);

can be replaced by:

        digester.addSetNext("a/b/c", "addChild",
                            "com.mycompany.mypackage.MyChildClass");

### Logging

Logging is a vital tool for debugging Digester rulesets. Digester can log copious amounts of debugging information. So, you need to know how logging works before you start using Digester seriously.

Digester uses Jakarta Commons Logging. This component is not really a logging framework - rather an extensible, configurable bridge. It can be configured to swallow all log messages, to provide very basic logging by itself or to pass logging messages on to more sophisticated logging frameworks. Commons-logging comes with connectors for many popular logging frameworks. Consult the commons-logging documentation for more information.

Two main logs are used by Digester.

- SAX-related messages are logged to `org.apache.commons.digester.Digester.sax`. This log gives information about the basic SAX events received by digester.
- `org.apache.commons.digester.Digester` is used for everything else. You'll probably want to have this log turned up during debugging but turned down during production due to the high message volume.

### Usage Examples

##### Processing A Struts Configuration File

As stated earlier, the primary reason that the `Digester` package was created is because the Struts controller servlet itself needed a robust, flexible, easy to extend mechanism for processing the contents of the `struts-config.xml` configuration that describes nearly every aspect of a Struts-based application. Because of this, the controller servlet contains a comprehensive, real world, example of how the Digester can be employed for this type of a use case. See the `initDigester()` method of class `org.apache.struts.action.ActionServlet` for the code that creates and configures the Digester to be used, and the `initMapping()` method for where the parsing actually takes place.

(Struts binary and source distributions can be acquired at http://jakarta.apache.org/struts/.)

The following discussion highlights a few of the matching patterns and processing rules that are configured, to illustrate the use of some of the Digester features. First, let's look at how the Digester instance is created and initialized:

        Digester digester = new Digester();
        digester.push(this);
        digester.setDebug(detail);
        digester.setValidating(true);

We see that a new Digester instance is created, and is configured to use a validating parser. Validation will occur against the struts-config_1_0.dtd DTD that is included with Struts (as discussed earlier). In order to provide a means of tracking the configured objects, the controller servlet instance itself will be added to the digester's stack.

        digester.addObjectCreate("struts-config/global-forwards/forward",
                                 forwardClass, "className");
        digester.addSetProperties("struts-config/global-forwards/forward");
        digester.addSetNext("struts-config/global-forwards/forward",
                            "addForward",
                            "org.apache.struts.action.ActionForward");
        digester.addSetProperty
          ("struts-config/global-forwards/forward/set-property",
           "property", "value");

The rules created by these lines are used to process the global forward declarations. When a `<forward>` element is encountered, the following actions take place:

- A new object instance is created -- the `ActionForward` instance that will represent this definition. The Java class name defaults to that specified as an initialization parameter (which we have stored in the String variable `forwardClass`), but can be overridden by using the "className" attribute (if it is present in the XML element we are currently parsing). The new `ActionForward` instance is pushed onto the stack.
- The properties of the `ActionForward` instance (at the top of the stack) are configured based on the attributes of the `<forward>` element.
- Nested occurrences of the `<set-property>` element cause calls to additional property setter methods to occur. This is required only if you have provided a custom implementation of the `ActionForward` class with additional properties that are not included in the DTD.
- The `addForward()` method of the next-to-top object on the stack (i.e. the controller servlet itself) will be called, passing the object at the top of the stack (i.e. the `ActionForward` instance) as an argument. This causes the global forward to be registered, and as a result of this it will be remembered even after the stack is popped.
- At the end of the `<forward>` element, the top element (i.e. the `ActionForward` instance) will be popped off the stack.

Later on, the digester is actually executed as follows:

        InputStream input =
          getServletContext().getResourceAsStream(config);
        ...
        try {
            digester.parse(input);
            input.close();
        } catch (SAXException e) {
            ... deal with the problem ...
        }

As a result of the call to `parse()`, all of the configuration information that was defined in the `struts-config.xml` file is now represented as collections of objects cached within the Struts controller servlet, as well as being exposed as servlet context attributes.

##### Parsing Body Text In XML Files

The Digester module also allows you to process the nested body text in an XML file, not just the elements and attributes that are encountered. The following example is based on an assumed need to parse the web application deployment descriptor (`/WEB-INF/web.xml`) for the current web application, and record the configuration information for a particular servlet. To record this information, assume the existence of a bean class with the following method signatures (among others):

      package com.mycompany;
      public class ServletBean {
        public void setServletName(String servletName);
        public void setServletClass(String servletClass);
        public void addInitParam(String name, String value);
      }

We are going to process the `web.xml` file that declares the controller servlet in a typical Struts-based application (abridged for brevity in this example):

      <web-app>
        ...
        <servlet>
          <servlet-name>action</servlet-name>
          <servlet-class>org.apache.struts.action.ActionServlet<servlet-class>
          <init-param>
            <param-name>application</param-name>
            <param-value>org.apache.struts.example.ApplicationResources<param-value>
          </init-param>
          <init-param>
            <param-name>config</param-name>
            <param-value>/WEB-INF/struts-config.xml<param-value>
          </init-param>
        </servlet>
        ...
      </web-app>

Next, lets define some Digester processing rules for this input file:

      digester.addObjectCreate("web-app/servlet",
                               "com.mycompany.ServletBean");
      digester.addCallMethod("web-app/servlet/servlet-name", "setServletName", 0);
      digester.addCallMethod("web-app/servlet/servlet-class",
                             "setServletClass", 0);
      digester.addCallMethod("web-app/servlet/init-param",
                             "addInitParam", 2);
      digester.addCallParam("web-app/servlet/init-param/param-name", 0);
      digester.addCallParam("web-app/servlet/init-param/param-value", 1);

Now, as elements are parsed, the following processing occurs:

- <servlet> - A new `com.mycompany.ServletBean` object is created, and pushed on to the object stack.
- <servlet-name> - The `setServletName()` method of the top object on the stack (our `ServletBean`) is called, passing the body content of this element as a single parameter.
- <servlet-class> - The `setServletClass()` method of the top object on the stack (our `ServletBean`) is called, passing the body content of this element as a single parameter.
- <init-param> - A call to the `addInitParam` method of the top object on the stack (our `ServletBean`) is set up, but it is not called yet. The call will be expecting two `String` parameters, which must be set up by subsequent call parameter rules.
- <param-name> - The body content of this element is assigned as the first (zero-relative) argument to the call we are setting up.
- <param-value> - The body content of this element is assigned as the second (zero-relative) argument to the call we are setting up.
- </init-param> - The call to `addInitParam()` that we have set up is now executed, which will cause a new name-value combination to be recorded in our bean.
- <init-param> - The same set of processing rules are fired again, causing a second call to `addInitParam()` with the second parameter's name and value.
- </servlet> - The element on the top of the object stack (which should be the `ServletBean` we pushed earlier) is popped off the object stack.

### Namespace Aware Parsing

For digesting XML documents that do not use XML namespaces, the default behavior of `Digester`, as described above, is generally sufficient. However, if the document you are processing uses namespaces, it is often convenient to have sets of `Rule` instances that are only matched on elements that use the prefix of a particular namespace. This approach, for example, elegantly deals with element names that are the same in different namespaces, but where you want to perform different processing for each namespace. To accomplish this, follow these steps:

1. Tell `Digester` that you will be doing namespace aware parsing, by adding this statement in your initalization of the Digester's properties:

           digester.setNamespaceAware(true);

2. Declare the public namespace URI of the namespace with which following rules will be associated. Note that you do not make any assumptions about the prefix - the XML document author is free to pick whatever prefix they want:

           digester.setRuleNamespaceURI("http://www.mycompany.com/MyNamespace");

3. Add the rules that correspond to this namespace, in the usual way, by calling methods like `addObjectCreate()` or `addSetProperties()`. In the matching patterns you specify, use only the local name portion of the elements (i.e. the part after the prefix and associated colon (":") character:

           digester.addObjectCreate("foo/bar", "com.mycompany.MyFoo");
           digester.addSetProperties("foo/bar");

4. Repeat the previous two steps for each additional public namespace URI that should be recognized on this `Digester` run.

Now, consider that you might wish to digest the following document, using the rules that were set up in the steps above:

    <m:foo
       xmlns:m="http://www.mycompany.com/MyNamespace"
       xmlns:y="http://www.yourcompany.com/YourNamespace">

      <m:bar name="My Name" value="My Value"/>

      <y:bar id="123" product="Product Description"/>L

    </x:foo>

Note that your object create and set properties rules will be fired for the first occurrence of the `bar` element, but not the second one. This is because we declared that our rules only matched for the particular namespace we are interested in. Any elements in the document that are associated with other namespaces (or no namespaces at all) will not be processed. In this way, you can easily create rules that digest only the portions of a compound document that they understand, without placing any restrictions on what other content is present in the document.

You might also want to look at Encapsulated Rule Sets if you wish to reuse a particular set of rules, associated with a particular namespace, in more than one application context.

#### Using Namespace Prefixes In Pattern Matching

Using rules with namespaces is very useful when you have orthogonal rulesets. One ruleset applies to a namespace and is independent of other rulesets applying to other namespaces. However, if your rule logic requires mixed namespaces, then matching namespace prefix patterns might be a better strategy.

When you set the `NamespaceAware` property to false, digester uses the qualified element name (which includes the namespace prefix) rather than the local name as the patten component for the element. This means that your pattern matches can include namespace prefixes as well as element names. So, rather than create namespace-aware rules, create pattern matches including the namespace prefixes.

For example, (with `NamespaceAware` false), the pattern `'foo:bar'` will match a top level element named `'bar'` in the namespace with (local) prefix `'foo'`.

### Pluggable Rules Processing

By default, `Digester` selects the rules that match a particular pattern of nested elements as described under Element Matching Patterns. If you prefer to use different selection policies, however, you can create your own implementation of the org.apache.commons.digester.Rules interface, or subclass the corresponding convenience base class org.apache.commons.digester.RulesBase. Your implementation of the `match()` method will be called when the processing for a particular element is started or ended, and you must return a `List` of the rules that are relevant for the current nesting pattern. The order of the rules you return is significant, and should match the order in which rules were initally added.

Your policy for rule selection should generally be sensitive to whether Namespace Aware Parsing is taking place. In general, if `namespaceAware` is true, you should select only rules that:

- Are registered for the public namespace URI that corresponds to the prefix being used on this element.
- Match on the "local name" portion of the element (so that the document creator can use any prefix that they like).

#### ExtendedBaseRules

ExtendedBaseRules, adds some additional expression syntax for pattern matching to the default mechanism, but it also executes more slowly. See the JavaDocs for more details on the new pattern matching syntax, and suggestions on when this implementation should be used. To use it, simply do the following as part of your Digester initialization:

      Digester digester = ...
      ...
      digester.setRules(new ExtendedBaseRules());
      ...

#### RegexRules

RegexRules is an advanced `Rules` implementation which does not build on the default pattern matching rules. It uses a pluggable RegexMatcher implementation to test if a path matches the pattern for a Rule. All matching rules are returned (note that this behaviour differs from longest matching rule of the default pattern matching rules). See the Java Docs for more details.

Example usage:

      Digester digester = ...
      ...
      digester.setRules(new RegexRules(new SimpleRegexMatcher()));
      ...

### Encapsulated Rule Sets

All of the examples above have described a scenario where the rules to be processed are registered with a `Digester` instance immediately after it is created. However, this approach makes it difficult to reuse the same set of rules in more than one application environment. Ideally, one could package a set of rules into a single class, which could be easily loaded and registered with a `Digester` instance in one easy step.

The RuleSet interface (and the convenience base class RuleSetBase) make it possible to do this. In addition, the rule instances registered with a particular `RuleSet` can optionally be associated with a particular namespace, as described under Namespace Aware Processing.

An example of creating a `RuleSet` might be something like this:

    public class MyRuleSet extends RuleSetBase {

      public MyRuleSet() {
        this("");
      }

      public MyRuleSet(String prefix) {
        super();
        this.prefix = prefix;
        this.namespaceURI = "http://www.mycompany.com/MyNamespace";
      }

      protected String prefix = null;

      public void addRuleInstances(Digester digester) {
        digester.addObjectCreate(prefix + "foo/bar",
          "com.mycompany.MyFoo");
        digester.addSetProperties(prefix + "foo/bar");
      }

    }

You might use this `RuleSet` as follow to initialize a `Digester` instance:

      Digester digester = new Digester();
      ... configure Digester properties ...
      digester.addRuleSet(new MyRuleSet("baz/"));

A couple of interesting notes about this approach:

- The application that is using these rules does not need to know anything about the fact that the `RuleSet` being used is associated with a particular namespace URI. That knowledge is emedded inside the `RuleSet` class itself.
- If desired, you could make a set of rules work for more than one namespace URI by providing constructors on the `RuleSet` to allow this to be specified dynamically.
- The `MyRuleSet` example above illustrates another technique that increases reusability -- you can specify (as an argument to the constructor) the leading portion of the matching pattern to be used. In this way, you can construct a `Digester` that recognizes the same set of nested elements at different nesting levels within an XML document.

### Frequently Asked Questions

- Why do I get warnings when using a JAXP 1.1 parser?

  If you're using a JAXP 1.1 parser, you might see the following warning (in your log): ``

      [WARN] Digester - -Error: JAXP SAXParser property not recognized: http://java.sun.com/xml/jaxp/properties/schemaLanguage

  This property is needed for JAXP 1.2 (XML Schema support) as required for the Servlet Spec. 2.4 but is not recognized by JAXP 1.1 parsers. This warning is harmless.
- Where Can I Find Example Code?

  Digester ships with a sample application: a mapping for the Rich Site Summary format used by many newsfeeds. Download the source distribution to see how it works.

- When Are You Going To Support Rich Site Summary Version x.y.z?

  The Rich Site Summary application is intended to be a sample application. It works but we have no plans to add support for other versions of the format.

  We would consider donations of standard digester applications but it's unlikely that these would ever be shipped with the base digester distribution. If you want to discuss this, please post to common-dev mailing list

### Known Limitations

#### Accessing Public Methods In A Default Access Superclass

There is an issue when invoking public methods contained in a default access superclass. Reflection locates these methods fine and correctly assigns them as public. However, an `IllegalAccessException` is thrown if the method is invoked.

`MethodUtils` contains a workaround for this situation. It will attempt to call `setAccessible` on this method. If this call succeeds, then the method can be invoked as normal. This call will only succeed when the application has sufficient security privilages. If this call fails then a warning will be logged and the method may fail.

`Digester` uses `MethodUtils` and so there may be an issue accessing methods of this kind from a high security environment. If you think that you might be experiencing this problem, please ask on the mailing list.