## org.apache.commons.configuration.tree
Class DefaultExpressionEngine

    java.lang.Object
      org.apache.commons.configuration.tree.DefaultExpressionEngine

All Implemented Interfaces:
  ExpressionEngine

    public class DefaultExpressionEngine

extends Object
implements ExpressionEngine

A default implementation of the `ExpressionEngine` interface providing the "native"e; expression language for hierarchical configurations.

This class implements a rather simple expression language for navigating through a hierarchy of configuration nodes. It supports the following operations:

- Navigating from a node to one of its children using the child node delimiter, which is by the default a dot (".").
- Navigating from a node to one of its attributes using the attribute node delimiter, which by default follows the XPATH like syntax `[@<attributeName>]`.
- If there are multiple child or attribute nodes with the same name, a specific node can be selected using a numerical index. By default indices are written in paranthesis.

As an example consider the following XML document:

      <database>
        <tables>
          <table type="system">
            <name>users</name>
            <fields>
              <field>
                <name>lid</name>
                <type>long</name>
              </field>
              <field>
                <name>usrName</name>
                <type>java.lang.String</type>
              </field>
             ...
            </fields>
          </table>
          <table>
            <name>documents</name>
            <fields>
              <field>
                <name>docid</name>
                <type>long</type>
              </field>
              ...
            </fields>
          </table>
          ...
        </tables>
      </database>

If this document is parsed and stored in a hierarchical configuration object, for instance the key `tables.table(0).name` can be used to find out the name of the first table. In opposite `tables.table.name` would return a collection with the names of all available tables. Similarily the key `tables.table(1).fields.field.name` returns a collection with the names of all fields of the second table. If another index is added after the `field` element, a single field can be accessed: `tables.table(1).fields.field(0).name`. The key `tables.table(0)[@type]` would select the type attribute of the first table.

This example works with the default values for delimiters and index markers. It is also possible to set custom values for these properties so that you can adapt a `DefaultExpressionEngine` to your personal needs.

Since:
  1.3
Version:
  $Id: DefaultExpressionEngine.java 439648 2006-09-02 20:42:10Z oheger $
Author:
  Oliver Heger

Field Summary
`static String` `DEFAULT_ATTRIBUTE_END` Constant for the default attribute end marker.
`static String` `DEFAULT_ATTRIBUTE_START` Constant for the default attribute start marker.
`static String` `DEFAULT_ESCAPED_DELIMITER` Constant for the default escaped property delimiter.
`static String` `DEFAULT_INDEX_END` Constant for the default index end marker.
`static String` `DEFAULT_INDEX_START` Constant for the default index start marker.
`static String` `DEFAULT_PROPERTY_DELIMITER` Constant for the default property delimiter.

Constructor Summary
`DefaultExpressionEngine()`

Method Summary
`protected ConfigurationNode` `findLastPathNode(DefaultConfigurationKey.KeyIterator keyIt, ConfigurationNode node)` Finds the last existing node for an add operation.
`protected void` `findNodesForKey(DefaultConfigurationKey.KeyIterator keyPart, ConfigurationNode node, Collection nodes)` Recursive helper method for evaluating a key.
`String` `getAttributeEnd()` Sets the attribute end marker.
`String` `getAttributeStart()` Returns the attribute start marker.
`String` `getEscapedDelimiter()` Returns the escaped property delimiter string.
`String` `getIndexEnd()` Returns the index end marker.
`String` `getIndexStart()` Returns the index start marker.
`String` `getPropertyDelimiter()` Returns the property delimiter.
`String` `nodeKey(ConfigurationNode node, String parentKey)` Determines the key of the passed in node.
`NodeAddData` `prepareAdd(ConfigurationNode root, String key)` Prepares Adding the property with the specified key.
`List` `query(ConfigurationNode root, String key)` Evaluates the given key and returns all matching nodes.
`void` `setAttributeEnd(String attributeEnd)` Sets the attribute end marker.
`void` `setAttributeStart(String attributeStart)` Sets the attribute start marker.
`void` `setEscapedDelimiter(String escapedDelimiter)` Sets the escaped property delimiter string.
`void` `setIndexEnd(String indexEnd)` Sets the index end marker.
`void` `setIndexStart(String indexStart)` Sets the index start marker.
`void` `setPropertyDelimiter(String propertyDelimiter)` Sets the property delmiter.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### DEFAULT_PROPERTY_DELIMITER

    public static final String DEFAULT_PROPERTY_DELIMITER

  Constant for the default property delimiter.

  See Also:
    Constant Field Values

### DEFAULT_ESCAPED_DELIMITER

    public static final String DEFAULT_ESCAPED_DELIMITER

  Constant for the default escaped property delimiter.

  See Also:
    Constant Field Values

### DEFAULT_ATTRIBUTE_START

    public static final String DEFAULT_ATTRIBUTE_START

  Constant for the default attribute start marker.

  See Also:
    Constant Field Values

### DEFAULT_ATTRIBUTE_END

    public static final String DEFAULT_ATTRIBUTE_END

  Constant for the default attribute end marker.

  See Also:
    Constant Field Values

### DEFAULT_INDEX_START

    public static final String DEFAULT_INDEX_START

  Constant for the default index start marker.

  See Also:
    Constant Field Values

### DEFAULT_INDEX_END

    public static final String DEFAULT_INDEX_END

  Constant for the default index end marker.

  See Also:
    Constant Field Values

Constructor Detail

### DefaultExpressionEngine

    public DefaultExpressionEngine()

Method Detail

### getAttributeEnd

    public String getAttributeEnd()

  Sets the attribute end marker.

  Returns:
    the attribute end marker

### setAttributeEnd

    public void setAttributeEnd(String attributeEnd)

  Sets the attribute end marker.

  Parameters:
    `attributeEnd` - the attribute end marker; can be null if no end marker is needed

### getAttributeStart

    public String getAttributeStart()

  Returns the attribute start marker.

  Returns:
    the attribute start marker

### setAttributeStart

    public void setAttributeStart(String attributeStart)

  Sets the attribute start marker. Attribute start and end marker are used together to detect attributes in a property key.

  Parameters:
    `attributeStart` - the attribute start marker

### getEscapedDelimiter

    public String getEscapedDelimiter()

  Returns the escaped property delimiter string.

  Returns:
    the escaped property delimiter

### setEscapedDelimiter

    public void setEscapedDelimiter(String escapedDelimiter)

  Sets the escaped property delimiter string. With this string a delimiter that belongs to the key of a property can be escaped. If for instance "." is used as property delimiter, you can set the escaped delimiter to "\." and can then escape the delimiter with a back slash.

  Parameters:
    `escapedDelimiter` - the escaped delimiter string

### getIndexEnd

    public String getIndexEnd()

  Returns the index end marker.

  Returns:
    the index end marker

### setIndexEnd

    public void setIndexEnd(String indexEnd)

  Sets the index end marker.

  Parameters:
    `indexEnd` - the index end marker

### getIndexStart

    public String getIndexStart()

  Returns the index start marker.

  Returns:
    the index start marker

### setIndexStart

    public void setIndexStart(String indexStart)

  Sets the index start marker. Index start and end marker are used together to detect indices in a property key.

  Parameters:
    `indexStart` - the index start marker

### getPropertyDelimiter

    public String getPropertyDelimiter()

  Returns the property delimiter.

  Returns:
    the property delimiter

### setPropertyDelimiter

    public void setPropertyDelimiter(String propertyDelimiter)

  Sets the property delmiter. This string is used to split the parts of a property key.

  Parameters:
    `propertyDelimiter` - the property delimiter

### query

    public List query(ConfigurationNode root,
                      String key)

  Evaluates the given key and returns all matching nodes. This method supports the syntax as described in the class comment.

  Specified by:
    `query` in interface `ExpressionEngine`
  Parameters:
    `root` - the root node
    `key` - the key
  Returns:
    a list with the matching nodes

### nodeKey

    public String nodeKey(ConfigurationNode node,
                          String parentKey)

  Determines the key of the passed in node. This implementation takes the given parent key, adds a property delimiter, and then adds the node's name. (For attribute nodes the attribute delimiters are used instead.) The name of the root node is a blanc string. Note that no indices will be returned.

  Specified by:
    `nodeKey` in interface `ExpressionEngine`
  Parameters:
    `node` - the node whose key is to be determined
    `parentKey` - the key of this node's parent
  Returns:
    the key for the given node

### prepareAdd

    public NodeAddData prepareAdd(ConfigurationNode root,
                                  String key)

  Prepares Adding the property with the specified key.

  To be able to deal with the structure supported by hierarchical configuration implementations the passed in key is of importance, especially the indices it might contain. The following example should clearify this: Suppose the actual node structure looks like the following:

        tables
           +-- table
                   +-- name = user
                   +-- fields
                           +-- field
                                   +-- name = uid
                           +-- field
                                   +-- name = firstName
                           ...
           +-- table
                   +-- name = documents
                   +-- fields
                          ...

  In this example a database structure is defined, e.g. all fields of the first table could be accessed using the key `tables.table(0).fields.field.name`. If now properties are to be added, it must be exactly specified at which position in the hierarchy the new property is to be inserted. So to add a new field name to a table it is not enough to say just

       config.addProperty("tables.table.fields.field.name", "newField");

  The statement given above contains some ambiguity. For instance it is not clear, to which table the new field should be added. If this method finds such an ambiguity, it is resolved by following the last valid path. Here this would be the last table. The same is true for the `field`; because there are multiple fields and no explicit index is provided, a new `name` property would be added to the last field - which is propably not what was desired.

  To make things clear explicit indices should be provided whenever possible. In the example above the exact table could be specified by providing an index for the `table` element as in `tables.table(1).fields`. By specifying an index it can also be expressed that at a given position in the configuration tree a new branch should be added. In the example above we did not want to add an additional `name` element to the last field of the table, but we want a complete new `field` element. This can be achieved by specifying an invalid index (like -1) after the element where a new branch should be created. Given this our example would run:

       config.addProperty("tables.table(1).fields.field(-1).name", "newField");

  With this notation it is possible to add new branches everywhere. We could for instance create a new `table` element by specifying

       config.addProperty("tables.table(-1).fields.field.name", "newField2");

  (Note that because after the `table` element a new branch is created indices in following elements are not relevant; the branch is new so there cannot be any ambiguities.)

  Specified by:
    `prepareAdd` in interface `ExpressionEngine`
  Parameters:
    `root` - the root node of the nodes hierarchy
    `key` - the key of the new property
  Returns:
    a data object with information needed for the add operation

### findNodesForKey

    protected void findNodesForKey(DefaultConfigurationKey.KeyIterator keyPart,
                                   ConfigurationNode node,
                                   Collection nodes)

  Recursive helper method for evaluating a key. This method processes all facets of a configuration key, traverses the tree of properties and fetches the the nodes of all matching properties.

  Parameters:
    `keyPart` - the configuration key iterator
    `node` - the actual node
    `nodes` - here the found nodes are stored

### findLastPathNode

    protected ConfigurationNode findLastPathNode(DefaultConfigurationKey.KeyIterator keyIt,
                                                 ConfigurationNode node)

  Finds the last existing node for an add operation. This method traverses the configuration node tree along the specified key. The last existing node on this path is returned.

  Parameters:
    `keyIt` - the key iterator
    `node` - the actual node
  Returns:
    the last existing node on the given path

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