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## Using Configuration

Commons Configuration allows you to access configuration properties from a variety of different sources. No matter if they are stored in a properties file, a XML document, or a JNDI tree, they can all be accessed in the same way through the generic `Configuration` interface.

Another strength of Commons Configuration is its ability to mix configurations from heterogeneous sources and treat them like a single logic configuration. This section will introduce you to the different configurations available and will show you how to combine them.

### Configuration Sources

Currently there are quite a number of different sources of Configuration objects. But, by just using a Configuration object versus a specific type like XMLConfiguration or JNDIConfiguration, you are sheltered from the mechanics of actually retrieving the configuration values. These various sources include:

- PropertiesConfiguration Loads configuration values from a properties file.
- XMLConfiguration Takes values from an XML document.
- PropertyListConfiguration Loads values from an OpenStep .plist file. XMLPropertyListConfiguration is also available to read the XML variant used by Mac OSX.
- JNDIConfiguration Using a key in the JNDI tree, can retrieve values as configuration properties.
- BaseConfiguration An in-memory method of populating a Configuration object.
- HierarchicalConfiguration An in-memory Configuration object that is able to deal with complex structured data.
- SystemConfiguration A configuration using the system properties
- ConfigurationConverter Takes a java.util.Properties or an o.a.c.collections.ExtendedProperties and converts it to a Configuration object.

### Mixing Configuration Sources

Often you want to provide a base set of configuration values, but allow the user to easily override them for their specific environment. Well one way is to hard code the default values into your code, and have then provide a property file that overrides this. However, this is a very rigid way of doing things. Instead, with the `CompositeConfiguration` you can provide many different ways of setting up a configuration. You can either do it manually:

    CompositeConfiguration config = new CompositeConfiguration();
    config.addConfiguration(new SystemConfiguration());
    config.addConfiguration(new PropertiesConfiguration("application.properties"));

or via the `ConfigurationFactory` class:

    ConfigurationFactory factory = new ConfigurationFactory("config.xml");
    Configuration config = factory.getConfiguration();

The `config.xml` file used in the example above is a configuration descriptor, it specifies the Configuration objects to load. Here is an example of descriptor:

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

    <configuration>
      <system/>
      <properties fileName="application.properties"/>
    </configuration>

What this says is that we are loading up all system properties, as well as the properties file `application.properties`. The order of precedence is first to last. So in the above example, if a property is not found in the system properties, it'll be searched in the properties file. This allows you to set up default values in a properties file, and override them with the system properties.

### The Configuration interface

All the classes in this package that represent different kinds of configuration sources share a single interface: `Configuration`. This interface allows you to access and manipulate configuration properties in a generic way.

A major part of the methods defined in the `Configuration` interface deals with retrieving properties of different data types. All these methods take a key as an argument that points to the desired property. This is a string value whose exact meaning depends on the concrete `Configuration` implementation used. They try to find the property specified by the passed in key and convert it to their target type; this converted value will be returned. There are also overloaded variants of all methods that allow to specify a default value, which will be returned if the property cannot be found. The following data types are supported:

- BigDecimal
- BigInteger
- boolean
- byte
- double
- float
- int
- long
- short
- String

The names of these methods start with `get` followed by their data type. The `getString()` method for instance will return String values, `getInt()` will operate on integers.

Properties can have multiple values, so it is also possible to query a list containing all of the available values. This is done using the `getList()` method.

For manipulating properties or their values the following methods can be used:

`addProperty()`
  Adds a new property to the configuration. If this property already exists, another value is added to it (so it becomes a multi-valued property).
`clearProperty()`
  Removes the specified property from the configuration.
`setProperty()`
  Overwrites the value of the specified property. This is the same as removing the property and then calling `addProperty()` with the new property value.
`clear()`
  Wipes out the whole configuration