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## Properties files

Properties files are a popular mean of configuring applications. Of course Commons Configuration supports this format and enhances significantly the basic `java.util.Properties` class. This section introduces the features of the `PropertiesConfiguration` class. Note that `PropertiesConfiguration` is a very typical example for an implementation of the `Configuration` interface and many of the features described in this section (e.g. list handling or interpolation) are supported by other configuration classes as well. This is because most configuration implementations that ship with Commons Configuration are derived from the common base class `AbstractConfiguration`, which implementes this features.

### Using PropertiesConfiguration

Let's start with a simple properties file named `usergui.properties` with the following content:

    # Properties definining the GUI
    colors.background = #FFFFFF
    colors.foreground = #000080

    window.width = 500
    window.height = 300

To load this file, you'll write:

    Configuration config = new PropertiesConfiguration("usergui.properties");

If you do not specify an absolute path, the file will be searched automatically in the following locations:

- in the current directory
- in the user home directory
- in the classpath

Instead of using a constructor that takes a file name you can also invoke one of the `load()` methods. There are several overloaded variants that allow you to load properties from various sources. More information about loading properties files (and file-based configurations in general) can be found in the section about File-based Configurations.

After the properties file was loaded you can access its content through the methods of the `Configuration` interface, e.g.

    String backColor = config.getString("colors.background");
    Dimension size = new Dimension(config.getInt("window.width"),
      config.getInt("window.height"));

### Includes

If a property is named "`include`" and the value of that property is the name of a file on the disk, that file will be included into the configuration. Here is an example:

    # usergui.properties

    include = colors.properties
    include = sizes.properties

    # colors.properties

    colors.background = #FFFFFF

### Lists and arrays

Commons Configuration has the ability to return easily a list of values, for example if your file contains a list of comma separated values:

    # chart colors
    colors.pie = #FF0000, #00FF00, #0000FF

You don't have to split the value manually, you can retrieve an array or a `java.util.List` directly with:

    String[] colors = config.getStringArray("colors.pie");
    List colorList = config.getList("colors.pie");

Alternatively, you can specify a list of values in your properties file by using the same key on several lines:

    # chart colors
    colors.pie = #FF0000;
    colors.pie = #00FF00;
    colors.pie = #0000FF;

The `addProperty()` and `setProperty()` methods also implement special list handling. The property value that is passed to these methods can be a list or an array resulting in a property with multiple values. If the property value is a string, it is checked whether it contains the list delimiter character. If this is the case, the string is splitted, and its single parts are added one by one. The list delimiter character is the comma by default. It is also applied to the properties when the configuration file is loaded (that's the reason why the example above with the comma separated list of chart colors works). By using the `setListDelimiter()` method you can set it to a different character. Here are some examples:

    // Change the list delimiter character to a slash
    config.setListDelimiter('/');
    // Now add some properties
    config.addProperty("greeting", "Hello, how are you?");
    config.addProperty("colors.pie",
      new String[] { "#FF0000", "#00FF00", "#0000FF" });
    config.addProperty("colors.graph", "#808080/#00FFCC/#6422FF");

    // Access data
    String salut = config.getString("greeting");
    List colPie = config.getList("colors.pie");
    String[] colGraph = config.getStringArray("colors.graph");

    String firstPieColor = config.getString("colors.pie");

In this example the list delimiter character is changed from a comma to a slash. Because of this the `greeting` property won't be splitted, but remains a single string. The string passed as value for the `colors.graph` property in opposite contains the new delimiter character and thus will result in a property with three values.

Of interest is also the last line of the example fragment. Here the `getString()` method is called for a property that has multiple values. This call will return the first value of the list.

If you want to change the list delimiter character for all configuration objects, you can use the static `setDefaultListDelimiter()` method of `AbstractConfiguration`. It is also possible to disable splitting of string properties at all for a Configuration instance by calling its `setDelimiterParsingDisabled()` method with a value of true.

Note: The list handling and string splitting facilities described in this section are not specific to `PropertiesConfiguration`, but are also supported by other configuration classes.

### Variable Interpolation

If you are familiar with Ant or Maven, you have most certainly already encountered the variables (like `${token}`) that are automatically expanded when the configuration file is loaded. Commons Configuration supports this feature as well, here is an example:

    application.name = Killer App
    application.version = 1.6.2

    application.title = ${application.name} ${application.version}

If you now retrieve the value for the `application.title` property, the result will be `Killer App 1.6.2`. As for list handling, variable interpolation is a feature supported by other configuration classes as well.

### Saving

To save your configuration, just call the `save()` method:

    PropertiesConfiguration config = new PropertiesConfiguration("usergui.properties");
    config.setProperty("colors.background", "#000000);
    config.save();

You can also save a copy of the configuration to another file:

    PropertiesConfiguration config = new PropertiesConfiguration("usergui.properties");
    config.setProperty("colors.background", "#000000);
    config.save("usergui.backup.properties);

More information about saving properties files (and file-based configurations in general) can be found in the section about File-based Configurations.

### Special Characters

If you need a special character in a property like a line feed, a tabulation or an unicode character, you can specify it with the same escaped notation used for Java Strings. The list separator ("," by default), can also be escaped:

    key = This \n string \t contains \, escaped \\ characters \u0020

### Layout Objects

Each `PropertiesConfiguration` object is associated with a Layout object, an instance of the class `PropertiesConfigurationLayout`. This layout object is responsible for preserving most of the structure of loaded configuration files. This means that things like comments or blanc lines in a saved properties file will closely resemble the original properties file (the algorithm is not 100 percent perfect, but for most use cases it should be sufficient).

Normally a developer does not have to deal with these layout objects. However there are some methods that might be of interest in certain use cases. For instance `PropertiesConfigurationLayout` defines methods for obtaining and setting the comment for a property key. A header comment for the whole properties file is also supported. If the values of multi-valued properties should always be written on a single line rather than adding a new property definition for each value (which would be incompatible with `java.util.Properties`) the `setForceSingleLine()` method can be used.