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## org.apache.commons.configuration
Class CombinedConfiguration

    java.lang.Object
      org.apache.commons.configuration.event.EventSource
          org.apache.commons.configuration.AbstractConfiguration
              org.apache.commons.configuration.HierarchicalConfiguration
                  org.apache.commons.configuration.CombinedConfiguration

All Implemented Interfaces:
  Cloneable, Configuration, ConfigurationListener, Serializable

Direct Known Subclasses:
  DynamicCombinedConfiguration

public class CombinedConfiguration
extends HierarchicalConfiguration
implements ConfigurationListener, Cloneable

A hierarchical composite configuration class.

This class maintains a list of configuration objects, which can be added using the divers `addConfiguration()` methods. After that the configurations can be accessed either by name (if one was provided when the configuration was added) or by index. For the whole set of managed configurations a logical node structure is constructed. For this purpose a ``NodeCombiner`` object can be set. This makes it possible to specify different algorithms for the combination process.

The big advantage of this class is that it creates a truly hierarchical structure of all the properties stored in the contained configurations - even if some of them are no hierarchical configurations per se. So all enhanced features provided by a hierarchical configuration (e.g. choosing an expression engine) are applicable.

The class works by registering itself as an event listener at all added configurations. So it gets notified whenever one of these configurations is changed and can invalidate its internal node structure. The next time a property is accessed the node structure will be re-constructed using the current state of the managed configurations. Note that, depending on the used `NodeCombiner`, this may be a complex operation.

Because of the way a `CombinedConfiguration` is working it has more or less view character: it provides a logic view on the configurations it contains. In this constellation not all methods defined for hierarchical configurations - especially methods that update the stored properties - can be implemented in a consistent manner. Using such methods (like `addProperty()`, or `clearProperty()` on a `CombinedConfiguration` is not strictly forbidden, however, depending on the current ``NodeCombiner`` and the involved properties, the results may be different than expected. Some examples may illustrate this:

- Imagine a `CombinedConfiguration` cc containing two child configurations with the following content:
  user.properties

         gui.background = blue
         gui.position = (10, 10, 400, 200)

  default.properties

         gui.background = black
         gui.foreground = white
         home.dir = /data

  As a `NodeCombiner` a ``OverrideCombiner`` is used. This combiner will ensure that defined user settings take precedence over the default values. If the resulting `CombinedConfiguration` is queried for the background color, `blue` will be returned because this value is defined in `user.properties`. Now consider what happens if the key `gui.background` is removed from the `CombinedConfiguration`:

      cc.clearProperty("gui.background");

  Will a `cc.containsKey("gui.background")` now return false? No, it won't! The `clearProperty()` operation is executed on the node set of the combined configuration, which was constructed from the nodes of the two child configurations. It causes the value of the background node to be cleared, which is also part of the first child configuration. This modification of one of its child configurations causes the `CombinedConfiguration` to be re-constructed. This time the `OverrideCombiner` cannot find a `gui.background` property in the first child configuration, but it finds one in the second, and adds it to the resulting combined configuration. So the property is still present (with a different value now).
- `addProperty()` can also be problematic: Most node combiners use special view nodes for linking parts of the original configurations' data together. If new properties are added to such a special node, they do not belong to any of the managed configurations and thus hang in the air. Using the same configurations as in the last example, the statement

       addProperty("database.user", "scott");

  would cause such a hanging property. If now one of the child configurations is changed and the `CombinedConfiguration` is re-constructed, this property will disappear! (Add operations are not problematic if they result in a child configuration being updated. For instance an `addProperty("home.url", "localhost");` will alter the second child configuration - because the prefix home is here already present; when the `CombinedConfiguration` is re-constructed, this change is taken into account.)

Because of such problems it is recommended to perform updates only on the managed child configurations.

Whenever the node structure of a `CombinedConfiguration` becomes invalid (either because one of the contained configurations was modified or because the `invalidate()` method was directly called) an event is generated. So this can be detected by interested event listeners. This also makes it possible to add a combined configuration into another one.

Implementation note: Adding and removing configurations to and from a combined configuration is not thread-safe. If a combined configuration is manipulated by multiple threads, the developer has to take care about properly synchronization.

Since:
  1.3
Version:
  $Id: CombinedConfiguration.java 712401 2008-11-08 15:29:56Z oheger $
Author:
  Commons Configuration team
See Also:
  Serialized Form

Nested Class Summary

Nested classes inherited from class org.apache.commons.configuration.HierarchicalConfiguration
`HierarchicalConfiguration.BuilderVisitor, HierarchicalConfiguration.Node, HierarchicalConfiguration.NodeVisitor`

Field Summary
`static int` | `EVENT_COMBINED_INVALIDATE` Constant for the invalidate event that is fired when the internal node structure becomes invalid.

Fields inherited from class org.apache.commons.configuration.HierarchicalConfiguration
`EVENT_ADD_NODES, EVENT_CLEAR_TREE, EVENT_SUBNODE_CHANGED`

Fields inherited from class org.apache.commons.configuration.AbstractConfiguration
`END_TOKEN, EVENT_ADD_PROPERTY, EVENT_CLEAR, EVENT_CLEAR_PROPERTY, EVENT_READ_PROPERTY, EVENT_SET_PROPERTY, START_TOKEN`

Constructor Summary
`CombinedConfiguration()` Creates a new instance of `CombinedConfiguration` that uses a union combiner.
`CombinedConfiguration(NodeCombiner comb)` Creates a new instance of `CombinedConfiguration` and initializes the combiner to be used.

Method Summary
`void` | `addConfiguration(AbstractConfiguration config)` Adds a new configuration to this combined configuration.
`void` | `addConfiguration(AbstractConfiguration config, String name)` Adds a new configuration to this combined configuration with an optional name.
`void` | `addConfiguration(AbstractConfiguration config, String name, String at)` Adds a new configuration to this combined configuration.
`void` | `clear()` Clears this configuration.
`Object` | `clone()` Returns a copy of this object.
`void` | `configurationChanged(ConfigurationEvent event)` Event listener call back for configuration update events.
`protected List` | `fetchNodeList(String key)` Evaluates the passed in property key and returns a list with the matching configuration nodes.
`Configuration` | `getConfiguration(int index)` Returns the configuration at the specified index.
`Configuration` | `getConfiguration(String name)` Returns the configuration with the given name.
`Set` | `getConfigurationNames()` Returns a set with the names of all configurations contained in this combined configuration.
`ExpressionEngine` | `getConversionExpressionEngine()` Returns the `ExpressionEngine` for converting flat child configurations to hierarchical ones.
`NodeCombiner` | `getNodeCombiner()` Returns the node combiner that is used for creating the combined node structure.
`int` | `getNumberOfConfigurations()` Returns the number of configurations that are contained in this combined configuration.
`ConfigurationNode` | `getRootNode()` Returns the configuration root node of this combined configuration.
`Configuration` | `getSource(String key)` Returns the configuration source, in which the specified key is defined.
`void` | `invalidate()` Invalidates this combined configuration.
`boolean` | `isForceReloadCheck()` Returns a flag whether an enhanced reload check must be performed.
`protected void` | `performReloadCheck()` Triggers the contained configurations to perform a reload check if necessary.
`boolean` | `removeConfiguration(Configuration config)` Removes the specified configuration from this combined configuration.
`Configuration` | `removeConfiguration(String name)` Removes the configuration with the specified name.
`Configuration` | `removeConfigurationAt(int index)` Removes the configuration at the specified index.
`void` | `setConversionExpressionEngine(ExpressionEngine conversionExpressionEngine)` Sets the `ExpressionEngine` for converting flat child configurations to hierarchical ones.
`void` | `setForceReloadCheck(boolean forceReloadCheck)` Sets the force reload check flag.
`void` | `setNodeCombiner(NodeCombiner nodeCombiner)` Sets the node combiner.

Methods inherited from class org.apache.commons.configuration.HierarchicalConfiguration
`addNodes, addPropertyDirect, clearNode, clearNode, clearProperty, clearReferences, clearTree, configurationAt, configurationAt, configurationsAt, containsKey, createAddPath, createNode, createSubnodeConfiguration, createSubnodeConfiguration, fetchAddNode, findLastPathNode, findPropertyNodes, getDefaultExpressionEngine, getExpressionEngine, getKeys, getKeys, getMaxIndex, getProperty, getRoot, interpolatedConfiguration, isEmpty, nodeDefined, nodeDefined, removeNode, removeNode, setDefaultExpressionEngine, setExpressionEngine, setProperty, setRoot, setRootNode, subnodeConfigurationChanged, subset`

Methods inherited from class org.apache.commons.configuration.AbstractConfiguration
`addErrorLogListener, addProperty, append, clearPropertyDirect, copy, createInterpolator, getBigDecimal, getBigDecimal, getBigInteger, getBigInteger, getBoolean, getBoolean, getBoolean, getByte, getByte, getByte, getDefaultListDelimiter, getDelimiter, getDouble, getDouble, getDouble, getFloat, getFloat, getFloat, getInt, getInt, getInteger, getInterpolator, getList, getList, getListDelimiter, getLogger, getLong, getLong, getLong, getProperties, getProperties, getShort, getShort, getShort, getString, getString, getStringArray, getSubstitutor, interpolate, interpolate, interpolateHelper, isDelimiterParsingDisabled, isThrowExceptionOnMissing, resolveContainerStore, setDefaultListDelimiter, setDelimiter, setDelimiterParsingDisabled, setListDelimiter, setLogger, setThrowExceptionOnMissing`

Methods inherited from class org.apache.commons.configuration.event.EventSource
`addConfigurationListener, addErrorListener, clearConfigurationListeners, clearErrorListeners, createErrorEvent, createEvent, fireError, fireEvent, getConfigurationListeners, getErrorListeners, isDetailEvents, removeConfigurationListener, removeErrorListener, setDetailEvents`

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

Field Detail

### EVENT_COMBINED_INVALIDATE

    public static final int EVENT_COMBINED_INVALIDATE

  Constant for the invalidate event that is fired when the internal node structure becomes invalid.

  See Also:
    Constant Field Values

Constructor Detail

### CombinedConfiguration

    public CombinedConfiguration(NodeCombiner comb)

  Creates a new instance of `CombinedConfiguration` and initializes the combiner to be used.

Parameters:
  `comb` - the node combiner (can be null, then a union combiner is used as default)

### CombinedConfiguration

    public CombinedConfiguration()

  Creates a new instance of `CombinedConfiguration` that uses a union combiner.

See Also:
  `UnionCombiner`

Method Detail

### getNodeCombiner

    public NodeCombiner getNodeCombiner()

  Returns the node combiner that is used for creating the combined node structure.

  Returns:
    the node combiner

### setNodeCombiner

    public void setNodeCombiner(NodeCombiner nodeCombiner)

  Sets the node combiner. This object will be used when the combined node structure is to be constructed. It must not be null, otherwise an `IllegalArgumentException` exception is thrown. Changing the node combiner causes an invalidation of this combined configuration, so that the new combiner immediately takes effect.

  Parameters:
    `nodeCombiner` - the node combiner

### isForceReloadCheck

    public boolean isForceReloadCheck()

  Returns a flag whether an enhanced reload check must be performed.

  Returns:
    the force reload check flag
  Since:
    1.4

### setForceReloadCheck

    public void setForceReloadCheck(boolean forceReloadCheck)

  Sets the force reload check flag. If this flag is set, each property access on this configuration will cause a reload check on the contained configurations. This is a workaround for a problem with some reload implementations that only check if a reload is required when they are triggered. Per default this mode is disabled. If the force reload check flag is set to true, accessing properties will be less performant, but reloads on contained configurations will be detected.

  Parameters:
    `forceReloadCheck` - the value of the flag
  Since:
    1.4

### getConversionExpressionEngine

    public ExpressionEngine getConversionExpressionEngine()

  Returns the `ExpressionEngine` for converting flat child configurations to hierarchical ones.

  Returns:
    the conversion expression engine
  Since:
    1.6

### setConversionExpressionEngine

    public void setConversionExpressionEngine(ExpressionEngine conversionExpressionEngine)

  Sets the `ExpressionEngine` for converting flat child configurations to hierarchical ones. When constructing the root node for this combined configuration the properties of all child configurations must be combined to a single hierarchical node structure. In this process, non hierarchical configurations are converted to hierarchical ones first. This can be problematic if a child configuration contains keys that are no compatible with the default expression engine used by hierarchical configurations. Therefore it is possible to specify a specific expression engine to be used for this purpose.

  Parameters:
    `conversionExpressionEngine` - the conversion expression engine
  Since:
    1.6
  See Also:
    `ConfigurationUtils.convertToHierarchical(Configuration, ExpressionEngine)`

### addConfiguration

    public void addConfiguration(AbstractConfiguration config,
                                 String name,
                                 String at)

  Adds a new configuration to this combined configuration. It is possible (but not mandatory) to give the new configuration a name. This name must be unique, otherwise a `ConfigurationRuntimeException` will be thrown. With the optional `at` argument you can specify where in the resulting node structure the content of the added configuration should appear. This is a string that uses dots as property delimiters (independent on the current expression engine). For instance if you pass in the string `"database.tables"`, all properties of the added configuration will occur in this branch.

  Parameters:
    `config` - the configuration to add (must not be null)
    `name` - the name of this configuration (can be null)
    `at` - the position of this configuration in the combined tree (can be null)

### addConfiguration

    public void addConfiguration(AbstractConfiguration config,
                                 String name)

  Adds a new configuration to this combined configuration with an optional name. The new configuration's properties will be added under the root of the combined node structure.

  Parameters:
    `config` - the configuration to add (must not be null)
    `name` - the name of this configuration (can be null)

### addConfiguration

    public void addConfiguration(AbstractConfiguration config)

  Adds a new configuration to this combined configuration. The new configuration is not given a name. Its properties will be added under the root of the combined node structure.

  Parameters:
    `config` - the configuration to add (must not be null)

### getNumberOfConfigurations

    public int getNumberOfConfigurations()

  Returns the number of configurations that are contained in this combined configuration.

  Returns:
    the number of contained configurations

### getConfiguration

    public Configuration getConfiguration(int index)

  Returns the configuration at the specified index. The contained configurations are numbered in the order they were added to this combined configuration. The index of the first configuration is 0.

  Parameters:
    `index` - the index
  Returns:
    the configuration at this index

### getConfiguration

    public Configuration getConfiguration(String name)

  Returns the configuration with the given name. This can be null if no such configuration exists.

  Parameters:
    `name` - the name of the configuration
  Returns:
    the configuration with this name

### removeConfiguration

    public boolean removeConfiguration(Configuration config)

  Removes the specified configuration from this combined configuration.

  Parameters:
    `config` - the configuration to be removed
  Returns:
    a flag whether this configuration was found and could be removed

### removeConfigurationAt

    public Configuration removeConfigurationAt(int index)

  Removes the configuration at the specified index.

  Parameters:
    `index` - the index
  Returns:
    the removed configuration

### removeConfiguration

    public Configuration removeConfiguration(String name)

  Removes the configuration with the specified name.

  Parameters:
    `name` - the name of the configuration to be removed
  Returns:
    the removed configuration (null if this configuration was not found)

### getConfigurationNames

    public Set getConfigurationNames()

  Returns a set with the names of all configurations contained in this combined configuration. Of course here are only these configurations listed, for which a name was specified when they were added.

  Returns:
    a set with the names of the contained configurations (never null)

### invalidate

    public void invalidate()

  Invalidates this combined configuration. This means that the next time a property is accessed the combined node structure must be re-constructed. Invalidation of a combined configuration also means that an event of type `EVENT_COMBINED_INVALIDATE` is fired. Note that while other events most times appear twice (once before and once after an update), this event is only fired once (after update).

### configurationChanged

    public void configurationChanged(ConfigurationEvent event)

  Event listener call back for configuration update events. This method is called whenever one of the contained configurations was modified. It invalidates this combined configuration.

  Specified by:
    `configurationChanged` in interface `ConfigurationListener`
  Parameters:
    `event` - the update event

### getRootNode

    public ConfigurationNode getRootNode()

  Returns the configuration root node of this combined configuration. This method will construct a combined node structure using the current node combiner if necessary.

  Overrides:
    `getRootNode` in class `HierarchicalConfiguration`
  Returns:
    the combined root node

### clear

    public void clear()

  Clears this configuration. All contained configurations will be removed.

  Specified by:
    `clear` in interface `Configuration`
  Overrides:
    `clear` in class `AbstractConfiguration`

### clone

    public Object clone()

  Returns a copy of this object. This implementation performs a deep clone, i.e. all contained configurations will be cloned, too. For this to work, all contained configurations must be cloneable. Registered event listeners won't be cloned. The clone will use the same node combiner than the original.

  Overrides:
    `clone` in class `HierarchicalConfiguration`
  Returns:
    the copied object

### getSource

    public Configuration getSource(String key)

  Returns the configuration source, in which the specified key is defined. This method will determine the configuration node that is identified by the given key. The following constellations are possible:
  - If no node object is found for this key, null is returned.
  - If the key maps to multiple nodes belonging to different configuration sources, a `IllegalArgumentException` is thrown (in this case no unique source can be determined).
  - If exactly one node is found for the key, the (child) configuration object, to which the node belongs is determined and returned.
  - For keys that have been added directly to this combined configuration and that do not belong to the namespaces defined by existing child configurations this configuration will be returned.

  Parameters:
    `key` - the key of a configuration property
  Returns:
    the configuration, to which this property belongs or null if the key cannot be resolved
  Throws:
    `IllegalArgumentException` - if the key maps to multiple properties and the source cannot be determined, or if the key is null
  Since:
    1.5

### fetchNodeList

    protected List fetchNodeList(String key)

  Evaluates the passed in property key and returns a list with the matching configuration nodes. This implementation also evaluates the force reload check flag. If it is set, `performReloadCheck()` is invoked.

  Overrides:
    `fetchNodeList` in class `HierarchicalConfiguration`
  Parameters:
    `key` - the property key
  Returns:
    a list with the matching configuration nodes

### performReloadCheck

    protected void performReloadCheck()

  Triggers the contained configurations to perform a reload check if necessary. This method is called when a property of this combined configuration is accessed and the `forceReloadCheck` property is set to true.

  Since:
    1.6
  See Also:
    `setForceReloadCheck(boolean)`

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