org.apache.commons.configuration2

## Class CombinedConfiguration

- java.lang.Object
- - org.apache.commons.configuration2.event.BaseEventSource
  - - org.apache.commons.configuration2.AbstractConfiguration
    - - org.apache.commons.configuration2.AbstractHierarchicalConfiguration<ImmutableNode>
      - - org.apache.commons.configuration2.BaseHierarchicalConfiguration
        - - org.apache.commons.configuration2.CombinedConfiguration

- All Implemented Interfaces:
    Cloneable, Configuration, EventListener<ConfigurationEvent>, EventSource, HierarchicalConfiguration<ImmutableNode>, ImmutableConfiguration, ImmutableHierarchicalConfiguration, SynchronizerSupport, InMemoryNodeModelSupport, NodeKeyResolver<ImmutableNode>, NodeModelSupport<ImmutableNode>
  Direct Known Subclasses:
    DynamicCombinedConfiguration

      public class CombinedConfiguration
      extends BaseHierarchicalConfiguration
      implements EventListener<ConfigurationEvent>

  A hierarchical composite configuration class.

  This class maintains a list of configuration objects, which can be added using the diverse `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.

  Notes about thread-safety: This configuration implementation uses a `Synchronizer` object to protect instances against concurrent access. The concrete `Synchronizer` implementation used determines whether an instance of this class is thread-safe or not. In contrast to other implementations derived from `BaseHierarchicalConfiguration`, thread-safety is an issue here because the nodes structure used by this configuration has to be constructed dynamically when a child configuration is changed. Therefore, when multiple threads are involved which also manipulate one of the child configurations, a proper `Synchronizer` object should be set. Note that the `Synchronizer` objects used by the child configurations do not really matter. Because immutable in-memory nodes structures are used for them there is no danger that updates on child configurations could interfere with read operations on the combined configuration.

  Since:
    1.3

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from class org.apache.commons.configuration2.BaseHierarchicalConfiguration

      `BaseHierarchicalConfiguration.BuilderVisitor`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static EventType<ConfigurationEvent>` | `COMBINED_INVALIDATE` Constant for the event type fired when the internal node structure of a combined configuration becomes invalid.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `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

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addConfiguration(Configuration config)` Adds a new configuration to this combined configuration.
    `void` | `addConfiguration(Configuration config, String name)` Adds a new configuration to this combined configuration with an optional name.
    `void` | `addConfiguration(Configuration config, String name, String at)` Adds a new configuration to this combined configuration.
    `protected void` | `beginRead(boolean optimize)` Notifies this configuration's `Synchronizer` that a read operation is about to start.
    `protected void` | `beginWrite(boolean optimize)` Notifies this configuration's `Synchronizer` that an update operation is about to start.
    `protected void` | `clearInternal()` Clears this configuration.
    `Object` | `clone()` Returns a copy of this object.
    `Configuration` | `getConfiguration(int index)` Returns the configuration at the specified index.
    `Configuration` | `getConfiguration(String name)` Returns the configuration with the given name.
    `List<String>` | `getConfigurationNameList()` Returns a List of the names of all the configurations that have been added in the order they were added.
    `Set<String>` | `getConfigurationNames()` Returns a set with the names of all configurations contained in this combined configuration.
    `List<Configuration>` | `getConfigurations()` Returns a List of all the configurations that have been added.
    `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.
    `Configuration` | `getSource(String key)` Returns the configuration source, in which the specified key is defined.
    `Set<Configuration>` | `getSources(String key)` Returns a set with the configuration sources, in which the specified key is defined.
    `void` | `invalidate()` Invalidates this combined configuration.
    `void` | `onEvent(ConfigurationEvent event)` Event listener call back for configuration update events.
    `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` | `setNodeCombiner(NodeCombiner nodeCombiner)` Sets the node combiner.

    - ### Methods inherited from class org.apache.commons.configuration2.BaseHierarchicalConfiguration

      `childConfigurationsAt, childConfigurationsAt, cloneNodeModel, configurationAt, configurationAt, configurationsAt, configurationsAt, createSubConfigurationForTrackedNode, getNodeModel, getSubConfigurationNodeSelector, getSubConfigurationParentModel, immutableChildConfigurationsAt, immutableConfigurationAt, immutableConfigurationAt, immutableConfigurationsAt, initSubConfigurationForThisParent, interpolatedConfiguration, subnodeConfigurationChanged, subset`

    - ### Methods inherited from class org.apache.commons.configuration2.AbstractHierarchicalConfiguration

      `addNodes, addNodesInternal, addPropertyDirect, addPropertyInternal, clearPropertyDirect, clearTree, clearTreeInternal, containsKeyInternal, fetchNodeList, getExpressionEngine, getKeysInternal, getKeysInternal, getMaxIndex, getMaxIndexInternal, getModel, getPropertyInternal, getRootElementName, getRootElementNameInternal, isEmptyInternal, nodeDefined, nodeKey, resolveAddKey, resolveKey, resolveNodeKey, resolveUpdateKey, setExpressionEngine, setPropertyInternal, sizeInternal, toString`

    - ### Methods inherited from class org.apache.commons.configuration2.AbstractConfiguration

      `addErrorLogListener, addProperty, append, clear, clearProperty, cloneInterpolator, containsKey, copy, endRead, endWrite, get, get, getArray, getArray, getBigDecimal, getBigDecimal, getBigInteger, getBigInteger, getBoolean, getBoolean, getBoolean, getByte, getByte, getByte, getCollection, getCollection, getConfigurationDecoder, getConversionHandler, getDouble, getDouble, getDouble, getEncodedString, getEncodedString, getFloat, getFloat, getFloat, getInt, getInt, getInteger, getInterpolator, getKeys, getKeys, getList, getList, getList, getList, getListDelimiterHandler, getLogger, getLong, getLong, getLong, getProperties, getProperties, getProperty, getShort, getShort, getShort, getString, getString, getStringArray, getSynchronizer, immutableSubset, initLogger, installInterpolator, interpolate, interpolate, isEmpty, isScalarValue, isThrowExceptionOnMissing, lock, setConfigurationDecoder, setConversionHandler, setDefaultLookups, setInterpolator, setListDelimiterHandler, setLogger, setParentInterpolator, setPrefixLookups, setProperty, setSynchronizer, setThrowExceptionOnMissing, size, unlock`

    - ### Methods inherited from class org.apache.commons.configuration2.event.BaseEventSource

      `addEventListener, clearErrorListeners, clearEventListeners, copyEventListeners, createErrorEvent, createEvent, fireError, fireEvent, getEventListenerRegistrations, getEventListeners, isDetailEvents, removeEventListener, setDetailEvents`

    - ### Methods inherited from class java.lang.Object

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

    - ### Methods inherited from interface org.apache.commons.configuration2.Configuration

      `addProperty, clear, clearProperty, getInterpolator, installInterpolator, setInterpolator, setProperty`

    - ### Methods inherited from interface org.apache.commons.configuration2.sync.SynchronizerSupport

      `getSynchronizer, lock, setSynchronizer, unlock`

    - ### Methods inherited from interface org.apache.commons.configuration2.ImmutableConfiguration

      `containsKey, get, get, getArray, getArray, getBigDecimal, getBigDecimal, getBigInteger, getBigInteger, getBoolean, getBoolean, getBoolean, getByte, getByte, getByte, getCollection, getCollection, getDouble, getDouble, getDouble, getEncodedString, getEncodedString, getFloat, getFloat, getFloat, getInt, getInt, getInteger, getKeys, getKeys, getList, getList, getList, getList, getLong, getLong, getLong, getProperties, getProperty, getShort, getShort, getShort, getString, getString, getStringArray, immutableSubset, isEmpty, size`

- - ### Field Detail

    - #### COMBINED_INVALIDATE

          public static final EventType<ConfigurationEvent> COMBINED_INVALIDATE

      Constant for the event type fired when the internal node structure of a combined configuration becomes invalid.
      Since:
        2.0

  - ### 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

    - #### 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(Configuration 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(Configuration 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(Configuration 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

    - #### getConfigurations

          public List<Configuration> getConfigurations()

      Returns a List of all the configurations that have been added.
      Returns:
        A List of all the configurations.
      Since:
        1.7

    - #### getConfigurationNameList

          public List<String> getConfigurationNameList()

      Returns a List of the names of all the configurations that have been added in the order they were added. A NULL value will be present in the list for each configuration that was added without a name.
      Returns:
        A List of all the configuration names.
      Since:
        1.7

    - #### 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<String> 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).

    - #### onEvent

          public void onEvent(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:
        `onEvent` in interface `EventListener<ConfigurationEvent>`
      Parameters:
        `event` - the update event

    - #### clearInternal

          protected void clearInternal()

      Clears this configuration. All contained configurations will be removed.
      Overrides:
        `clearInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`

    - #### 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 `AbstractHierarchicalConfiguration<ImmutableNode>`
      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

    - #### getSources

          public Set<Configuration> getSources(String key)

      Returns a set with the configuration sources, in which the specified key is defined. This method determines the configuration nodes that are identified by the given key. It then determines the configuration sources to which these nodes belong and adds them to the result set. Note the following points:
      - If no node object is found for this key, an empty set is 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 combined configuration is contained in the result set.
      Parameters:
        `key` - the key of a configuration property
      Returns:
        a set with the configuration sources, which contain this property
      Since:
        2.0

    - #### beginRead

          protected void beginRead(boolean optimize)

      Notifies this configuration's `Synchronizer` that a read operation is about to start. This method is called by all methods which access this configuration in a read-only mode. Subclasses may override it to perform additional actions before this read operation. The boolean optimize argument can be evaluated by overridden methods in derived classes. Some operations which require a lock do not need a fully initialized configuration object. By setting this flag to true, such operations can give a corresponding hint. An overridden implementation of `beginRead()` can then decide to skip some initialization steps. All basic operations in this class (and most of the basic `Configuration` implementations) call this method with a parameter value of false. In any case the inherited method must be called! Otherwise, proper synchronization is not guaranteed. This implementation checks whether a combined root node is available. If not, it is constructed by requesting a write lock.
      Overrides:
        `beginRead` in class `AbstractConfiguration`
      Parameters:
        `optimize` - a flag whether optimization can be performed

    - #### beginWrite

          protected void beginWrite(boolean optimize)

      Notifies this configuration's `Synchronizer` that an update operation is about to start. This method is called by all methods which modify this configuration. Subclasses may override it to perform additional operations before an update. For a description of the boolean optimize argument refer to the documentation of `beginRead()`. In any case the inherited method must be called! Otherwise, proper synchronization is not guaranteed. This implementation checks whether a combined root node is available. If not, it is constructed now.
      Overrides:
        `beginWrite` in class `AbstractConfiguration`
      Parameters:
        `optimize` - a flag whether optimization can be performed
      See Also:
        `AbstractConfiguration.beginRead(boolean)`

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