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Combining Configuration Sources

While simple applications often store their configuration data in a single configuration file, there may be advanced requirements for more complex systems. From a certain size of configuration data it surely makes sense to divide the settings available on a set of configuration files each of which is related to a specific sub-domain. This makes it easier for users or administrators to adapt specific configuration settings. However, rather than reading multiple files and dealing with multiple Configuration objects, an application propably prefers a combined view on its configuration data. Commons Configuration supports this use case with a special configuration builder implementation: CombinedConfigurationBuilder.

CombinedConfigurationBuilder is the option of choice for applications that have to deal with multiple configuration sources. It provides the following features:

  • Various configuration sources can be combined to a single CombinedConfiguration object. This is a truly hierarchical configuration supporting enhanced query facilities.
  • As configuration sources the most relevant Configuration implementations provided by this library are supported. Sources are defined as bean declarations, so complex initializations are possible.
  • Meta data can be provided to fine-tune the constructed configuration.
  • CombinedConfigurationBuilder is extensible. Custom configuration sources can be added.

This document starts with some explanations of CombinedConfigurationBuilder basics. Then the configuration definition files processed by CombinedConfigurationBuilder are discussed in detail. Finally an advanced example is presented.

The configuration definition file

In previous chapters we have already seen how specific configuration classes like PropertiesConfiguration or XMLConfiguration can be used to load configuration data from a single source. CombinedConfigurationBuilder now allows combining multiple configuration sources to a single CombinedConfiguration object. The sources to be loaded have to be defined in an XML document with a specific structure, a so-called configuration definition file. The following listing shows a simple example of such a definition file:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration>
  <properties fileName="usergui.properties"/>
</configuration>

A configuration definition file can contain an arbitrary number of elements declaring the configuration sources to load. The <properties> element is one of these; it is used to include properties files. For this example we store the definition file in the same directory as the properties file and call it config.xml. The properties file used in this example is the same as in the section about properties files.

Setting up a CombinedConfigurationBuilder

Now we have to create a CombinedConfigurationBuilder object and let it read this definition file. This works in a similar way as the construction of other typical configuration builders: A new instance is created, and the configure() method is called with initialization parameters (CombinedConfigurationBuilder is derived from BasicConfigurationBuilder; so all features common to configuration builders are available here as well). The combined configuration collecting all sources defined in the configuration definition file can then be obtained by calling the builder's getConfiguration() method.

The easiest way to define the configuration definition file is to pass an initialized parameters object for file-based configurations to the builder. In this case, no other specific settings are set for the builder, and the file is read as an XML document:

Parameters params = new Parameters();
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(params.fileBased().setFile(new File("config.xml")));
CombinedConfiguration config = builder.getConfiguration();

Now the config object can be accessed in the usual way to query configuration properties, e.g. by using methods like getString(), or getInt(). A frequent use case is that a configuration file is shipped with an application inside its jar archive. For instance, the jar could have a special folder where all configuration files are located as in the following example:

/classes
/conf
/conf/mainConfig.xml
/conf/subConfig1.properties
/conf/subConfig2.xml

Here the conf folder in the jar contains a main configuration file - this is the definition file for the combined builder - and two configuration sources referenced from the main file. mainConfig.xml looks as follows:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration>
  <properties fileName="subConfig1.properties"/>
  <xml fileName="subConfig2.xml"/>
</configuration>

In order to load this file and the referenced configuration sources, the previous example can slightly be adapted to determine a URL to the main configuration file from the application's class loader:

Parameters params = new Parameters();
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(params.fileBased().setURL(
      getClass().getClassLoader().getResource("/conf/mainConfig.xml")
    ));
CombinedConfiguration config = builder.getConfiguration();

The point to take here is that it is possible to load a combined configuration directly from a jar file by specifying the URL to the configuration definition file. The configuration sources to be embedded are specified as relative paths; they are automatically resolved based on the URL of the main configuration file.

Just defining the configuration definition file via a file-based parameters object is a special case. Internally, a builder for an XMLConfiguration object is constructed which is then used to load and interprete the definition file. This should be appropriate for many cases. A drawback of this method is that there is no way to set additional initialization parameters for the CombinedConfigurationBuilder. For this purpose, a special parameters object exists offering some more specialized settings. If this object is to be used, information about the file to be loaded can be passed via the definitionBuilderParameters property:

Parameters params = new Parameters();
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(params.combined().setDefinitionBuilderParameters(
        params.fileBased().setFileName("definition.xml")
    ));
CombinedConfiguration config = builder.getConfiguration();

In this example, the combined() method of Parameters is used to obtain a special parameters object for a combined configuration builder. Internally, again a builder for constructing an XMLConfiguration for the definition file is created. The object stored in the definitionBuilderParameters property is passed to this builder's configure() method.

It is even possible to construct the builder for the configuration definition file externally and then pass it to the CombinedConfigurationBuilder via the definitionBuilder property of its initialization parameters object:

Parameters params = new Parameters();
// set up the builder for the configuration definition file
ConfigurationBuilder<? extends HierarchicalConfiguration<?>> defBuilder = ...;

// Create the combined builder and pass it the definition builder
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(params.combined().setDefinitionBuilder(defBuilder));
CombinedConfiguration config = builder.getConfiguration();

This is the most flexible variant. It makes it possible in theory to read the definition for the combined configuration builder from a completely different source. In practice, most applications will stick to XML files defining a combined configuration source because this is the native format for listing configuration sources and defining additional meta data (as described in the following sections). But making use of the definitionBuilder property allows at least passing in a specially configured builder object. Please refer to the section Builder Configuration Related to Combined Configurations for additional parameters supported by builders for combined configurations.

If you do not need any specific initialization and just want to read the configuration definition from an XML document, the Configurations helper class introduced in section Making it easier is made for you. It offers convenience methods for creating a builder for combined configurations from various sources. Here is an example how a builder can be constructed from a definition file specified as a file path:

Configurations configs = new Configurations();
CombinedConfigurationBuilder builder = configs.combinedBuilder("path/to/definition/file.xml");

Overriding properties

Using CombinedConfigurationBuilder to collect configuration sources does not make much sense if there is only a single source to be loaded. So let's add another one to the example definition file used before! This time we will embedd a XML file: gui.xml which is shown in the next listing:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<gui-definition>
  <colors>
    <background>#808080</background>
    <text>#000000</text>
    <header>#008000</header>
    <link normal="#000080" visited="#800080"/>
  </colors>
  <rowsPerPage>15</rowsPerPage>
</gui-definition>

To make this XML document part of our global configuration we have to modify our configuration definition file to also include the new file. For XML documents the element <xml> can be used so that we have now:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration>
  <properties fileName="usergui.properties"/>
  <xml fileName="gui.xml"/>
</configuration>

The code for setting up the CombinedConfigurationBuilder object remains the same. From the Configuration object returned by the factory the new properties can be accessed in the usual way.

There is one open question with this example configuration setup: The color.background property is defined in both the properties and the XML file, and - to make things worse - with different values. Which value will be returned by a call to getString()?

The answer is that the configuration sources are searched in the order they are defined in the configuration definition file. Here the properties file is included first, then comes the XML file. Because the color.background property can be found in the properties file the value specified there will be returned (which happens to be #FFFFFF).

It might not be obvious why it makes sense to define the value of one and the same property in multiple configuration sources. But consider the following scenario: An application comes with a set of default properties and allows the user to override some or all of them. This can now easily be realized by saving the user's settings in one file and the default settings in another. Then in the configuration definition file the file with the user settings is included first and after that the file with the default values. The application code that queries these settings does not have to bother whether a property was overriden by the user. CombinedConfigurationBuilder takes care that properties defined in the first file (the user file) are found; other properties which the user has not changed will still be returned from the second file (the defaults file).

Optional configuration sources

The example above with two configuration sources - one for user settings and one with default values - raises an interesting question: What happens if the user has not defined specific properties yet? Or what if a new user starts our application for the first time and thus no user specific properties exist?

The default behavior of CombinedConfigurationBuilder is to throw a ConfigurationException exception if one of the sources defined in the configuration definition file cannot be loaded. For our example this behavior is not desired: the properties file with specific user settings is not required. If it cannot be loaded, the example application should still work because a complete set of configuration properties is defined in the second file.

CombinedConfigurationBuilder supports such optional configuration sources. For this purpose in the definition of a configuration source the config-optional attribute can be placed. An example of this is shown below:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration>
  <properties fileName="usersettings.properties" config-optional="true"/>
  <properties fileName="default.properties"/>
</configuration>

In this configuration definition file the first properties file with user specific settings is marked as optional. This means that if it cannot be loaded, CombinedConfigurationBuilder will not throw an exception, but only write a warning message to its logger. Note that the config-optional attribute is absent for the second properties file. Thus it is mandatory, and the getConfiguration() method of CombinedConfigurationBuilder would throw an exception if it could not be found.

A configuration source with the config-optional attribute that cannot be loaded is simply ignored; in the resulting combined configuration no reference for this source is stored. In the example with the user configuration, it would be good if in case of a failure (because the user configuration file does not yet exist) an empty configuration object is created and added to the combined configuration. This configuration object can then be used to store specific user settings which the user might define.

For configuration sources marked as optional, an additional attribute is supported providing exactly this functionality: config-forceCreate. If set to true, a configuration is created in any case for this source. If the source can be loaded successfully, this is of course the resulting configuration. Otherwise, an empty configuration (of the same type) is created. The example below shows how this attribute can be used. Here we also define a name for the configuration source, so that the produced configuration can later be retrieved from the resulting combined configuration.

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration>
  <properties fileName="usersettings.properties" config-name="userConfig"
    config-optional="true" config-forceCreate="true"/>
  <properties fileName="default.properties"/>
</configuration>

Union configuration

In an earlier section about the configuration definition file for CombinedConfigurationBuilder it was stated that configuration files included first can override properties in configuraton files included later, and an example use case for this behaviour was given. There may be cases when there are other requirements.

Let's continue the example with the application that somehow process database tables and that reads the definitions of the affected tables from its configuration. This example and the corresponding XML configuration files were introduced in the section about hierarchical configurations. Now consider that this application grows larger and must be maintained by a team of developers. Each developer works on a separated set of tables. In such a scenario it would be problematic if the definitions for all tables were kept in a single file. It can be expected that this file needs to be changed very often and thus can be a bottleneck for team development when it is nearly steadily checked out. It would be much better if each developer had an associated file with table definitions, and all these information could be linked together at the end.

CombinedConfigurationBuilder provides support for such a use case, too. It is possible to specify in the configuration definition file that from a set of configuration sources a logic union configuration is to be constructed. Then all properties defined in the provided sources are collected and can be accessed as if they had been defined in a single source. To demonstrate this feature let us assume that a developer of the database application has defined a specific XML file with a table definition named tasktables.xml:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<config>
  <table tableType="application">
    <name>tasks</name>
    <fields>
      <field>
        <name>taskid</name>
        <type>long</type>
      </field>
      <field>
        <name>name</name>
        <type>java.lang.String</type>
      </field>
      <field>
        <name>description</name>
        <type>java.lang.String</type>
      </field>
      <field>
        <name>responsibleID</name>
        <type>long</type>
      </field>
      <field>
        <name>creatorID</name>
        <type>long</type>
      </field>
      <field>
        <name>startDate</name>
        <type>java.util.Date</type>
      </field>
      <field>
        <name>endDate</name>
        <type>java.util.Date</type>
      </field>
    </fields>
  </table>
</config>

This file defines the structure of an additional table, which should be added to the so far existing table definitions. To achieve this the configuration definition file has to be changed: A new section is added that contains the declaring elements of all configuration sources which are to be combined.

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!-- Configuration definition file that demonstrates the
     override and additional sections -->

<configuration>
  <override>
    <properties fileName="usergui.properties"/>
    <xml fileName="gui.xml"/>
  </override>

  <additional>
    <xml fileName="tables.xml"/>
    <xml fileName="tasktables.xml" config-at="tables"/>
  </additional>
</configuration>

Compared to the older versions of this file some changes have been done. One major difference is that the elements for including configuration sources are no longer direct children of the root element, but are now contained in either an <override> or <additional> section. The names of these sections already imply their purpose.

The override section is not strictly necessary. Elements in this section are treated as if they were children of the root element, i.e. properties in the included configuration sources override properties in sources included later. So the <override> tags could have been ommitted, but for the sake of clearity it is recommended to use them if there is also an <additional> section.

It is the <additonal> section that introduces a new behaviour. All configuration sources listed here are combined to a union configuration. In our example we have put two xml elements in this area that load the available files with database table definitions. The syntax of elements in the additional section is analogous to the syntax described so far. In this example the config-at attribute is introduced. It specifies the position in the logic union configuration where the included properties are to be added. Here it is set for the second element to the value tables. This is because the file starts with a <table> element, but to be compatible with the other table definition file it should be accessable under the key tables.table.

After these modifications have been performed, the configuration obtained from CombinedConfigurationBuilder allows access to three database tables. A call of config.getString("tables.table(2).name"); results in a value of tasks. In an analogous way it is possible to retrieve the fields of the third table.

Note that it is also possible to override properties defined in an additonal section. This can be done by placing a configuration source in the override section that defines properties that are also defined in one of the sources listed in the additional section. The example does not make use of that. Note also that the order of the override and additional sections in a configuration definition file does not matter. Sources in an override section are always treated with higher priority (otherwise they could not override the values of other sources).

Configuration definition file reference

Configuration definition files are XML documents telling CombinedConfigurationBuilder which configuration sources to load and how to process them in order to create the resulting combined configuration.

Overall structure of a configuration definition file

A configuration definition file for CombinedConfigurationBuilder can contain three sections, all of which are optional. A skeleton looks as follows:

<?xml version="1.0" encoding="ISO-8859-1" ?>

<configuration systemProperties="path to property file">
  <header>
    <!-- Meta data about the resulting combined configuration -->
  </header>
  <override>
    <!-- Configuration declarations with override semantics -->
  </override>
  <additional>
    <!-- Configuration declarations that form a union configuration -->
  </additional>
</configuration>

Declaring configuration sources

The override and additional sections have already been introduced when the basics of CombinedConfigurationBuilder were discussed. They contain declarations for the configuration sources to be embedded. For convenience reasons it is also possible to declare configuration sources outside these sections; they are then treated as if they were placed inside the override section.

Each declaration of a configuration source is represented by an XML element whose name determines the type of the configuration source. Attributes or nested elements can be used to provide additional configuration options for the sources to be included (e.g. a name of a file to be loaded or further flags). Below is a list of all tags which can be used out of the box:

properties
With this element properties files can be included. The name of the file to load is specified using the fileName attribute. Which configuration class is created by this tag depends on the extension of the file to load: If the extension is ".xml", a XMLPropertiesConfiguration object is created, which is able to process the XML properties format introduced in Java 5.0. Otherwise a PropertiesConfiguration object is created, the default reader for properties files.
xml
The xml element can be used to load XML configuration files. It also uses the fileName attribute to determine the name of the file to load and creates an instance of XMLConfiguration.
jndi
As the name implies, with this element JNDI resources can be included in the resulting configuration. Under the hood this is done by an instance of the JndiConfiguration class. The prefix attribute can be used to select a subset of the JNDI tree.
plist
The plist element allows embedding configuration files in the NeXT / OpenStep or Mac OS X format. Again the name of the file to load is specified through the fileName attribute. If a XML file is specified, a XMLPropertyListConfiguration object is created to process the file. Otherwise, this task is delegated to a PropertyListConfiguration instance.
system
With this element an instance of SystemConfiguration is added to the resulting configuration allowing access to system properties. Note: Using this element system properties are directly made available. Alternatively the interpolation features (see Variable Interpolation for more details) can be used for referencing system properties.
ini
This tag can be used to include an ini file into the resulting combined configuration. Behind the scenes an instance of INIConfiguration is used to load the ini file.
env
With this tag direct access to environment properties can be enabled. This works in the same way as the <system> tag for Java system properties.
multFile
Using this tag, a builder for a multi-file configuration can be integrated into the resulting combined configuration. This is described in a later chapter.
configuration
The configuration tag allows other configuration definition files to be included. This makes it possible to nest these definition files up to an arbitrary depth. In fact, this tag will create another CombinedConfigurationBuilder object, initialize it, and obtain the CombinedConfiguation from it. This combined configuration will then be added to the resulting combined configuration. Like all file-based configurations the fileName attribute can be used to specify the configuration definition file to be loaded. This file must be an XML document that conforms to the format described here. Some of the most important settings are copied from the original CombinedConfigurationBuilder object to the newly created builder:
  • the base path under which configuration files are searched
  • some flags, e.g. for controlling delimiter parsing or throwing exceptions on missing properties
  • the logger
  • the configuration and error listeners

In the declaration of a configuration source it is possible to set properties on the corresponding configuration objects. Configuration declarations are indeed Bean declarations. That means they can have attributes matching simple properties of the configuration object to create, and sub elements matching complex properties. The following example fragment shows how complex initialization can be performed in a configuration declaration:

  <properties fileName="test.properties" throwExceptionOnMissing="true">
    <reloadingDetectorFactory
    config-class="com.foo.MyReloadingDetector" strict=true"/>
  </properties>
  <xml fileName="test.xml" validating="true">
    <expressionEngine config-class="org.apache.commons.configuration2.tree.xpath.XPathExpressionEngine"/>
  </xml>

In this example a configuration source for a properties file and one for an XML document are defined. For the properties source the throwExceptionOnMissing property is set to true, which means that it should throw an exception if a requested property is not found. In addition, it is assigned a custom reloading detector, which is declared and configured in a sub element. The XML configuration source is initialized in a similar way: a simple property is set, and an expression engine is assigned. In fact, the properties defined in these declarations are not directly set on configuration instances, but on initialization parameters objects for configuration builders. These builders are created for the declared configuration sources and configured with the defined properties. Then their managed configurations are obtained and combined into the resulting configuration. Nevertheless, the declarations share the same syntax as other bean declarations supported by Commons Configuration. More information about the format for declaring beans and initializing their properties can be found in the section about bean declarations.

In addition to the attributes that correspond to properties of the configuration sources to be created, a configuration declaration can have a set of special attributes that are evaluated by CombinedConfigurationBuilder when it creates the objects. These attributes are listed in the following table:

Attribute Meaning
config-name Allows a name to be specified for this configuration. This name can be used to obtain a reference to the configuration from the resulting combined configuration (see below).
config-at With this attribute an optional prefix can be specified for the properties of the corresponding configuration.
config-optional Declares a configuration as optional. This means that errors that occur when creating the configuration are silently ignored. The default behavior when an error occurs is that no configuration is added to the resulting combined configuration. This behavior can be used to find out whether an optional configuration could be successfully created or not. If you specify a name for the optional configuration (using the config-name attribute), you can later check whether the combined configuration contains a configuration with this name. With the config-forceCreate attribute (see below) this default behavior can be changed.
config-forceCreate This boolean attribute is only evaluated for configurations declared as optional. It determines the behavior of the combined configuration builder when the optional configuration could not be created. If set to true, the builder tries to create an empty, uninitialized configuration of the correct type and add it to the resulting combined configuration. This is especially useful for file-based configurations. Consider a use case where an application wants to store user specific configuration files in the users' home directories. When a user starts this application for the first time, the user configuration does not exist yet. If it is declared as optional and forceCreate, the missing configuration file won't cause an error, but an empty configuration will be created. The application can then obtain this configuration, add properties to it (e.g. user specific settings) and save it. Without the config-forceCreate attribute the application would have to check whether the user configuration exists in the combined configuration and eventually create it manually. Note that not all configuration providers support this mechanism. Sometimes it may not be possible to create an empty configuration if the standard initialization fails. In this case no configuration will be added to the combined configuration (with other words: the config-forceCreate attribute will not have any effect).

Note: In older versions of Commons Configuration the attributes config-at and config-optional were named at and optional respective. They have been renamed in order to avoid possible name clashes with property names for configuration sources. However, for reasons of backwards compatibility, the old attribute names can still be used.

Another useful feature is the built-in support for interpolation (i.e. variable substitution): You can use variables in your configuration definition file that are defined in declared configuration sources. For instance, if the name of a configuration file to be loaded is defined by the system property CONFIG_FILE, you can do something like this:

<configuration>
  <!-- Load the system properties -->
  <system/>
  <!-- Now load the config file, using a system property as file name -->
  <properties fileName="${CONFIG_FILE}"/>
</configuration>

Note that you can refer only to properties that have already been loaded. If you change the order of the <system> and the <properties> elements in the example above, an error will occur because the ${CONFIG_FILE} variable will then be undefined at the moment it is evaluated.

<configuration systemProperties="systemProperties.xml">
  <!-- Load the system properties -->
  <system/>
  <!-- Now load the config file, using a system property as file name -->
  <properties fileName="${CONFIG_FILE}"/>
</configuration>

This example differs from the previous one by the systemProperties attribute added to the root element. It causes the specified properties file to be read and all properties defined therein to be added to the system properties. So properties like CONFIG_FILE can be defined in a properties file and are then treated as if they were system properties.

The header section

In the header section properties of the resulting combined configuration object can be set. The main part of this section is a bean declaration that is used for creating the resulting configuration object. Other elements can be used for customizing the Node combiners used by the override and the union combined configuration. The following example shows a header section that uses some of the supported properties:

  <header>
    <result throwExceptionOnMissing="true">
      <nodeCombiner config-class="org.apache.commons.configuration2.tree.OverrideCombiner"/>
      <expressionEngine config-class="org.apache.commons.configuration2.tree.xpath.XPathExpressionEngine"/>
    </result>
    <combiner>
      <override>
        <list-nodes>
          <node>table</node>
          <node>list</node>
        </list-nodes>
      </override>
      <additional>
        <list-nodes>
          <node>table</node>
        </list-nodes>
      </additional>
    </combiner>
  </header>

The result element points to the bean declaration for the resulting combined configuration. In this example we set some attributes and initialize the node combiner (which is not necessary because the default override combiner is specified), and the expression engine to be used. Note that the config-class attribute makes it possible to inject custom classes for the resulting configuration or the node combiner.

The combiner section allows nodes to be defined as list nodes. This can be necessary for certain node combiner implementations to work correctly. More information can be found in the section about Node combiners.

There are some more tags for specific use cases which can occur in the header section of a configuration declaration:

providers
This tag can be used to define new tags for including custom configuration sources. An example is provided later in this document.
fileSystem
Allows defining the File System to be used for file-based configuration sources:
<configuration>
  <header>
    <fileSystem
      config-class="org.apache.commons.configuration2.io.VFSFileSystem"/>
  </header>
  <xml fileName="test.xml" config-name="xml"/>
</configuration>
lookups
In the sub section Customizing interpolation it was described how variable interpolation can be extended by defining custom lookup objects. In the configuration definition file of CombinedConfigurationBuilder it is possible to declare such lookup objects and make them available for the processing of this definition file and the resulting combined configuration. For this purpose, the header section of the definition file can contain a <lookup> element in which an arbitrary number of lookups can be defined. Have a look at the following example:
<configuration>
  <header>
    <lookups>
      <lookup config-prefix="file"
              config-class="com.foo.FileLookup"/>
    </lookups>
  </header>
  <!-- Fetch the file name from a variable -->
  <xml fileName="${file:config_file}" config-name="xml"/>
</configuration>
Here a custom lookup class is declared and registered under the prefix file (as defined by the config-prefix attribute). The lookup is immediately active. It is then used to obtain the concrete file name for the embedded XML configuration. Note that the variable prefix matches the prefix provided in the lookup declaration. The variable name is passed to the lookup object, and the custom implementation can compute a suitable value.

Note: From time to time the question is raised whether there is a document type definition or a schema defining exactly the structure of a configuration definition file. Frankly, the answer is no. This is due to the fact that the format is extensible. As will be shown below, it is possible to register your own tags in order to embedd custom configuration sources.

Reloading Support

The chapter Automatic Reloading of Configuration Sources explains how reloading facilities can be enabled for single configuration sources. This feature is also useful when working with combined configuration sources. Here you might want to support reloading for some or all of the configuration sources referenced in the configuration definition file. And - to be really flexible - external changes in the configuration definition file itself should also be detected and cause a re-construction of the whole combined configuration.

To enable reloading support for combined configurations, Commons Configuration provides a special extension of CombinedConfigurationBuilder: ReloadingCombinedConfigurationBuilder. The relation between the two is analogous to the relation between FileBasedConfigurationBuilder and ReloadingFileBasedConfigurationBuilder. The reloading-enabled variant of combined configuration builder manages and exposes a special ReloadingController that can be used to trigger reloading checks. (Please refer to the chapter about reloading for a detailed describtion of the reloading implementation in Commons Configuration and the components involved.)

The reloading controller exposed by a ReloadingCombinedConfigurationBuilder is special because it is also a combined reloading controller. It manages the specific reloading controllers of all configuration sources with reloading support. So when the reloading controller of the combined builder is triggered, in fact all reloading-aware configuration sources perform a reload check; sources that have actually been changed generate a reloading event causing the invalidation of the combined builder's managed configuration - it has to be re-constructed the next time it is accessed making the reloaded changes visible.

The configuration builder providing access to the configuration definition file is treated in the same way as a configuration source in respect to reloading, i.e. if this builder supports reloading, it becomes part of the combined reloading controller. ReloadingCombinedConfigurationBuilder per default creates a reloading-aware builder for its definition configuration when no specific builder was defined in the initialization parameters. (If a builder for the definition configuration is explicitly passed in the initialization parameters, it lies in the responsibility of the caller to use a builder with reloading support.)

With a ReloadingCombinedConfigurationBuilder in place, enabling reload support for specific configuration sources is as simple as adding another attribute to the source declaration: config-reload. Remember that a combined configuration builder internally creates child configuration builders for each of the configuration sources to be loaded. For sources having a config-reload attribute with a value of true a builder with reloading support is created if possible. This may not be supported by all types of configuration sources, but it is for instance for file-based configuration sources; in this case the builder created for a source with enabled reloading support is of type ReloadingFileBasedConfigurationBuilder.

A more complex example of a combined configuration populated from multiple sources with reloading support is presented below.

An example

After all that theory let's go through a more complex example! We start with the configuration definition file that looks like the following:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!-- Test configuration definition file that demonstrates complex initialization -->
<configuration>
  <header>
    <result>
      <expressionEngine config-class="org.apache.commons.configuration2.tree.xpath.XPathExpressionEngine"/>
      <listDelimiterHandler
        config-class="org.apache.commons.configuration2.convert.DefaultListDelimiterHandler">
        <config-constrarg config-value=","/>
      </listDelimiterHandler>
    </result>
    <combiner>
      <additional>
        <list-nodes>
          <node>table</node>
        </list-nodes>
      </additional>
    </combiner>
  </header>
  <override>
    <properties fileName="user.properties" throwExceptionOnMissing="true"
      reloadingRefreshDelay="10000" config-name="properties"
      config-reload="true" config-optional="true"/>
    <xml fileName="settings.xml" config-name="xml"/>
  </override>
  <additional>
    <xml config-name="tab1" fileName="table1.xml" config-at="database.tables"/>
    <xml config-name="tab2" fileName="table2.xml" config-at="database.tables"
        validating="true"/>
  </additional>
</configuration>

This configuration definition file includes four configuration sources and sets some properties for the resulting CombinedConfiguration. We also set some properties for the configurations to be loaded; for instance we declare that one of the XML configurations should be validated.

With the following code we can create a CombinedConfigurationBuilder and load this file; because one of the configuration sources supports reloading we use the reloading-aware variant of a combined configuration builder:

Parameters params = new Parameters();
ReloadingCombinedConfigurationBuilder builder = new ReloadingCombinedConfigurationBuilder()
    .configure(params.fileBased().setFile(new File("configuration.xml")));
CombinedConfiguration cc = builder.getConfiguration();

Here the easiest way to specify the configuration definition file was used: a parameters object for a file-based configuration. This is sufficient for this example because no other properties for the builder have to be set. As described under Setting up a CombinedConfigurationBuilder, other options are available. If we wanted a special processing of the XML document with the configuration definition - e.g. enabling validation to a schema -, we could have passed a correspondingly initialized XML parameters object in the definitionBuilderParameters property of a combined parameters object or even use a specially configured builder for the definition configuration.

In the header section we have chosen an XPATH expression engine for the resulting configuration. So we can query our properties using the convenient XPATH syntax. We also enabled list delimiter parsing by specifying a DefaultListDelimiterHandler object. (Note the syntax for creating a bean instance via its constructor.) By providing the config-name attribute we have given all configuration sources a name. These names can be used to obtain the corresponding sources from the combined configuration. For configurations in the override section this is directly possible:

Configuration propertiesConfig = cc.getConfiguration("properties");
Configuration xmlConfig = cc.getConfiguration("xml");

Configurations in the additional section are treated a bit differently: they are all packed together in another combined configuration and then added to the resulting combined configuration. So in our example the combined configuration cc will contain three configurations: the two configurations from the override section, and the combined configuration with the additional configurations. The latter is stored under a name determined by the ADDITIONAL_NAME constant of CombinedConfigurationBuilder. The following code shows how the configurations of the additional section can be accessed:

CombinedConfiguration ccAdd = (CombinedConfiguration)
  cc.getConfiguration(CombinedConfigurationBuilder.ADDITIONAL_NAME);
Configuration tab1Config = ccAdd.getConfiguration("tab1");
Configuration tab2Config = ccAdd.getConfiguration("tab2");

To make sure that reloading checks are periodically performed, a suitable trigger has to be used to ensure that the reloading controller managed by the builder is called in regular intervals. This can be done in the same way as for simple configuration sources, for instance by setting up a PeriodicReloadingTrigger object:

PeriodicReloadingTrigger trigger = new PeriodicReloadingTrigger(builder.getReloadingController(),
    null, 1, TimeUnit.MINUTES);
trigger.start();

Extending the configuration definition file format

If you have written a custom configuration class, you might want to declare instances of this class in a configuration definition file, too. CombinedConfigurationBuilder supports this use case by registering a ConfigurationBuilderProvider object.

The task of a ConfigurationBuilderProvider is to create and initialize a configuration builder object which can then be used to obtain a configuration source. Whenever CombinedConfigurationBuilder encounters a tag in the override or the additional section it checks whether for this tag a ConfigurationBuilderProvider has been registered. If this is the case, the provider is asked to create a new configuration builder instance; otherwise an exception is thrown.

So for adding support for a new configuration class you have to create an implementation of ConfigurationBuilderProvider and register an instance of it. Registration can be done via the combined parameters object passed to the builder which offers a registerProvider() method. This method expects the name of the associated tag and the provider instance as arguments.

There is already a fully functional implementation of the ConfigurationBuilderProvider interface available in the class BaseConfigurationBuilderProvider class. When creating an instance the following information has to be passed:

  • The fully-qualified name of the configuration builder class to be used for the associated configuration source.
  • The fully-qualified name of the configuration builder class to be used if reloading is enabled for this source. This is optional; if a configuration source does not support reloading, null can be passed here. Then an exception is thrown if a configuration source of this type is declared with the config-reload attribute set to true.
  • The fully-qualified name of the configuration class created by this builder.
  • A collection with parameter object classes supported by this builder.
With this information, BaseConfigurationBuilderProvider can create and configure a correct configuration builder object for a specific configuration source. In detail, it performs the following steps:
  • It determines the builder class to be used based on the presence and value of the config-reload attribute; this means that either the normal or the reloading builder - if defined - is used.
  • An instance of the builder class is created via reflection.
  • Instances of all of the parameter object classes are created.
  • The parameter objects are initialized from the properties defined for the current configuration source.
  • The initialized parameter objects are passed to the configuration builder's configure() method.

For most cases, the functionality provided by BaseConfigurationBuilderProvider should be sufficient. So a properly initialized instance can be directly used for the registration of a new builder provider. Let's take a look at an example where we want to add support for a configuration class called com.foo.MyConfiguration. The class is a file-based configuration; therefore, already existing standard builder classes can be used to construct instances. (Otherwise, a custom builder implementation has to be created, and its name has to be passed to the provider instance.) The corresponding tag in the configuration definition file should have the name myconfig. The code for registering the new provider and loading the configuration definition file looks as follows:

ConfigurationProvider provider = new BaseConfigurationProvider(
    /* normal builder */
    "org.apache.commons.configuration2.builder.FileBasedConfigurationBuilder",
    /* reloading builder */
    "org.apache.commons.configuration2.builder.ReloadingFileBasedConfigurationBuilder",
    /* configuration class */
    "com.foo.MyConfig",
    /* Parameters; here we assume that we have a custom parameters class
       derived from FileBasedBuilderParametersImpl */
    Collections.singleton(MyConfigParams.class.getName()));

Parameters params = new Parameters();
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(
        params.combined()
            .setDefinitionBuilderParameters(
                params.fileBased().setFileName("definition.xml"))
            .registerProvider("myconfig", provider)
    );
CombinedConfiguration config = builder.getConfiguration();

If your configuration provider is registered this way, your configuration definition file can contain the myconfig tag just as any other tag for declaring a configuration source:

<configuration>
  <additional>
    <xml fileName="settings.xml"/>
    <myconfig fileName="special.cfg" throwExceptionOnMissing="false"/>
  </additional>
</configuration>

Alternatively, it is also possible to declare and register custom configuration builder providers directly in the configuration definition file. This is an interesting option because it makes the definition file self-contained; no special initialization is required on the configuration builder in order to load them. The registration of builder providers is done in the <providers> section in the header of the definition file. Each builder provider to be added has to be defined by specifying the tag name and the fully-qualified provider class. To make use of this mechanism for the custom configuration class used in this example, a specialized provider class has to be created (before BaseConfigurationBuilderProvider was instantiated directly):

package com.foo;

public class MyConfigurationBuilderProvider extends BaseConfigurationBuilderProvider
{
    public MyConfigurationBuilderProvider()
    {
        super("org.apache.commons.configuration2.builder.FileBasedConfigurationBuilder",
            "org.apache.commons.configuration2.builder.ReloadingFileBasedConfigurationBuilder",
            "com.foo.MyConfig",
            Collections.singleton(MyConfigParams.class.getName()));
    }
}

Now this class can be referenced in the configuration definition file.

<configuration>
  <header>
    <providers>
      <provider config-tag="myconfig"
                config-class="com.foo.MyConfigurationBuilderProvider"/>
    </providers>
  </header>
  <additional>
    <xml fileName="settings.xml"/>
    <myconfig fileName="special.cfg" throwExceptionOnMissing="false"/>
  </additional>
</configuration>

Now this file can be processed by a default CombinedConfigurationBuilder instance. No special configuration related to builder providers is necessary any more:

Parameters params = new Parameters();
CombinedConfigurationBuilder builder = new CombinedConfigurationBuilder()
    .configure(params.fileBased().setFileName("definition.xml"));
CombinedConfiguration config = builder.getConfiguration();

Builder Configuration Related to Combined Configurations

The special parameters object for CombinedConfigurationBuilder has already been mentioned and used within examples before. It can be obtained from the combined() method of a Parameters instance. Its type is CombinedBuilderParameters.

This type supports the settings common to all configurations. Additional settings specific to CombinedConfigurationBuilder are defined by the CombinedBuilderProperties interface. This includes

  • the configuration builder for obtaining the definition configuration
  • the parameters object for the configuration builder for obtaining the definition configuration
  • a base path for file-based configuration sources; sources with a relative file name are searched in this path.
  • information about custom configuration builder providers
  • default handlers for initialization parameters of configuration sources. They work in the same way as global default initialization parameters for configuration builders, but only impact the configuration sources defined in the current definition file. For instance, it is possible to define default settings for all XML configuration sources. These are applied for every XML source defined in the definition file unless they are overridden there. See below for an example.
  • controlling the inheritance of builder parameters to child configuration builders (see below).

Specifying default settings for the configuration sources to be included is a convenient feature. Often the configuration files to be included share similar conventions, and thus can be read using similar parameters. Rather than repeating these settings in the configuration definition file for each configuration source, there are ways to define default settings.

One option is to use a mechanism similar to default initialization parameters: The parameters object for a combined configuration builder allows defining default parameter objects for specific types of configuration sources. In the following example all XML configuration sources are configured to make use of a specific list delimiter handler:

DefaultListDelimiterHandler listDelimiterHandler = new DefaultListDelimiterHandler(',');
Parameters params = new Parameters();
XMLBuilderParameters xmlParams = params.xml()
    .setListDelimiterHandler(listDelimiterHandler);
XMLBuilderParameters definitionParams = params.xml()
    .setFile(new File("configDefinition.xml"));
CombinedBuilderParameters combinedParameters = params.combined()
    .setDefinitionBuilderParameters(definitionParams)
    .registerChildDefaultsHandler(XMLBuilderProperties.class,
        new CopyObjectDefaultHandler(xmlParams));
builder.configure(combinedParameters);
CombinedConfiguration config = builder.getConfiguration();

Here the xmlParams object defines default settings for all XML configuration sources to be included. Note how it is used to register a defaults handler using the registerChildDefaultsHandler() method of the parameters object for the combined builder. This mechanism of default parameters is very flexible because it allows setting different options for different types of configuration sources. However, as the example demonstrates, the initialization of the builder becomes complex; multiple parameter objects have to be dealt with.

A more lite-weight alternative is the ability of a combined configuration builder to inherit its parameters to the child configurations created by it. This feature is enabled by default and it works as follows: Whenever a parameter object for a child configuration source is created, it is first populated with compatible parameters set for the combined builder. Then the specific settings as defined by the configuration definition file are applied. So the settings defined for the combined builder as a whole serve as a kind of default parameters. With this mechanism the following code can be used to set a special list delimiter handler for all child configurations and a special expression engine for all hierarchical child configurations:

Parameters params = new Parameters();
DefaultExpressionEngineSymbols symbols = new DefaultExpressionEngineSymbols.Builder(
    DefaultExpressionEngineSymbols.DEFAULT_SYMBOLS)
    .setPropertyDelimiter("/").create();
DefaultExpressionEngine engine = new DefaultExpressionEngine(symbols);
DefaultListDelimiterHandler listDelimiterHandler = new DefaultListDelimiterHandler(',');
XMLBuilderParameters xmlParams = params.xml()
    .setExpressionEngine(engine)
    .setListDelimiterHandler(listDelimiterHandler)
    .setFile(new File("configDefinition.xml"));
builder.configure(xmlParams);
CombinedConfiguration config = builder.getConfiguration();

So here the builder is configured with an XML parameters object which defines some default settings and also the file to be loaded. The default settings are applied automatically to child configuration sources where applicable. This is often a natural approach; for many use cases it makes sense that settings defined for the parent combined builder are also applied to child configuration sources. Only if child configuration sources follow different conventions - maybe some files from a legacy system need to be integrated that use a different list delimiter character -, inheritance does not help. But in such cases it is often possible to use settings inheritance by default and override specific settings in affected child configuration sources; of course, the settings defined for a configuration source in the configuration definition file take precedence over inherited settings. The inheritance mechanism can also be turned off completely by invoking CombinedBuilderParameters.setInheritSettings() with a value of false.