org.apache.commons.digester
Class SetNestedPropertiesRule

java.lang.Object
  org.apache.commons.digester.Rule
      org.apache.commons.digester.SetNestedPropertiesRule

public class SetNestedPropertiesRule
extends Rule

Rule implementation that sets properties on the object at the top of the stack, based on child elements with names matching properties on that object.

Example input that can be processed by this rule:

   [widget]
    [height]7[/height]
    [width]8[/width]
    [label]Hello, world[/label]
   [/widget]
 

For each child element of [widget], a corresponding setter method is located on the object on the top of the digester stack, the body text of the child element is converted to the type specified for the (sole) parameter to the setter method, then the setter method is invoked.

This rule supports custom mapping of xml element names to property names. The default mapping for particular elements can be overridden by using SetNestedPropertiesRule(String[] elementNames, String[] propertyNames). This allows child elements to be mapped to properties with different names. Certain elements can also be marked to be ignored.

A very similar effect can be achieved using a combination of the BeanPropertySetterRule and the ExtendedBaseRules rules manager; this Rule, however, works fine with the default RulesBase rules manager.

Note that this rule is designed to be used to set only "primitive" bean properties, eg String, int, boolean. If some of the child xml elements match ObjectCreateRule rules (ie cause objects to be created) then you must use one of the more complex constructors to this rule to explicitly skip processing of that xml element, and define a SetNextRule (or equivalent) to handle assigning the child object to the appropriate property instead.

Implementation Notes

This class works by creating its own simple Rules implementation. When begin is invoked on this rule, the digester's current rules object is replaced by a custom one. When end is invoked for this rule, the original rules object is restored. The digester rules objects therefore behave in a stack-like manner.

For each child element encountered, the custom Rules implementation ensures that a special AnyChildRule instance is included in the matches returned to the digester, and it is this rule instance that is responsible for setting the appropriate property on the target object (if such a property exists). The effect is therefore like a "trailing wildcard pattern". The custom Rules implementation also returns the matches provided by the underlying Rules implementation for the same pattern, so other rules are not "disabled" during processing of a SetNestedPropertiesRule.

TODO: Optimise this class. Currently, each time begin is called, new AnyChildRules and AnyChildRule objects are created. It should be possible to cache these in normal use (though watch out for when a rule instance is invoked re-entrantly!).

Since:

1.6

Field Summary

Fields inherited from class org.apache.commons.digester.Rule

digester, namespaceURI

Constructor Summary

SetNestedPropertiesRule()
Base constructor, which maps every child element into a bean property with the same name as the xml element.

SetNestedPropertiesRule(String[] elementNames, String[] propertyNames)
Constructor which allows element->property mapping to be overridden.

SetNestedPropertiesRule(String elementName, String propertyName)
Convenience constructor which overrides the default mappings for just one property.

Method Summary

void	addAlias(String elementName, String propertyName) Add an additional custom xml-element -> property mapping.
void	begin(String namespace, String name, Attributes attributes) Process the beginning of this element.
void	body(String bodyText) This is only invoked after all child elements have been processed, so we can remove the custom Rules object that does the child-element-matching.
boolean	getAllowUnknownChildElements() See setAllowUnknownChildElements(boolean).
boolean	getTrimData() See setTrimData(boolean).
void	setAllowUnknownChildElements(boolean allowUnknownChildElements) Determines whether an error is reported when a nested element is encountered for which there is no corresponding property-setter method.
void	setDigester(Digester digester) Invoked when rule is added to digester.
void	setTrimData(boolean trimData) When set to true, any text within child elements will have leading and trailing whitespace removed before assignment to the target object.
String	toString() Render a printable version of this Rule.

Methods inherited from class org.apache.commons.digester.Rule

begin, body, end, end, finish, getDigester, getNamespaceURI, setNamespaceURI

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

SetNestedPropertiesRule

public SetNestedPropertiesRule()

Base constructor, which maps every child element into a bean property with the same name as the xml element.

It is an error if a child xml element exists but the target java bean has no such property (unless setAllowUnknownChildElements has been set to true).

SetNestedPropertiesRule

public SetNestedPropertiesRule(String elementName,
                               String propertyName)

Convenience constructor which overrides the default mappings for just one property.

For details about how this works, see SetNestedPropertiesRule(String[] elementNames, String[] propertyNames).

Parameters:

elementName - is the child xml element to match

propertyName - is the java bean property to be assigned the value of the specified xml element. This may be null, in which case the specified xml element will be ignored.

SetNestedPropertiesRule

public SetNestedPropertiesRule(String[] elementNames,
                               String[] propertyNames)

Constructor which allows element->property mapping to be overridden.

Two arrays are passed in. One contains xml element names and the other java bean property names. The element name / property name pairs are matched by position; in order words, the first string in the element name array corresponds to the first string in the property name array and so on.

If a property name is null or the xml element name has no matching property name due to the arrays being of different lengths then this indicates that the xml element should be ignored.

Example One

The following constructs a rule that maps the alt-city element to the city property and the alt-state to the state property. All other child elements are mapped as usual using exact name matching.

      SetNestedPropertiesRule(
                new String[] {"alt-city", "alt-state"}, 
                new String[] {"city", "state"});
 

Example Two

The following constructs a rule that maps the class xml element to the className property. The xml element ignore-me is not mapped, ie is ignored. All other elements are mapped as usual using exact name matching.

      SetPropertiesRule(
                new String[] {"class", "ignore-me"}, 
                new String[] {"className"});
 

Parameters:

elementNames - names of elements to map

propertyNames - names of properties mapped to

Method Detail

setDigester

public void setDigester(Digester digester)

Invoked when rule is added to digester.

Overrides:

setDigester in class Rule

setTrimData

public void setTrimData(boolean trimData)

When set to true, any text within child elements will have leading and trailing whitespace removed before assignment to the target object. The default value for this attribute is true.

getTrimData

public boolean getTrimData()

See setTrimData(boolean).

setAllowUnknownChildElements

public void setAllowUnknownChildElements(boolean allowUnknownChildElements)

Determines whether an error is reported when a nested element is encountered for which there is no corresponding property-setter method.

When set to false, any child element for which there is no corresponding object property will cause an error to be reported.

When set to true, any child element for which there is no corresponding object property will simply be ignored.

The default value of this attribute is false (unknown child elements are not allowed).

getAllowUnknownChildElements

public boolean getAllowUnknownChildElements()

See setAllowUnknownChildElements(boolean).

begin

public void begin(String namespace,
                  String name,
                  Attributes attributes)
           throws Exception

Process the beginning of this element.

Overrides:

begin in class Rule

Parameters:

namespace - is the namespace this attribute is in, or null

name - is the name of the current xml element

attributes - is the attribute list of this element

Throws:

Exception

body

public void body(String bodyText)
          throws Exception

This is only invoked after all child elements have been processed, so we can remove the custom Rules object that does the child-element-matching.

Overrides:

body in class Rule

Parameters:

bodyText - The text of the body of this element

Throws:

Exception

addAlias

public void addAlias(String elementName,
                     String propertyName)

Add an additional custom xml-element -> property mapping.

This is primarily intended to be used from the xml rules module (as it is not possible there to pass the necessary parameters to the constructor for this class). However it is valid to use this method directly if desired.

toString

public String toString()

Render a printable version of this Rule.

Overrides:

toString in class Object