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## Utility classes and Tips and Tricks

In this section some utility classes will be introduced that can be used to make handling of configuration objects easier. These classes already provide solutions for some often occurring problems. We will list these problems in no specific order and show how they can be solved with classes provided by Commons Configuration.

### Copy a configuration

Often it is required to copy the data of one `Configuration` object into another one. For this purpose the `AbstractConfiguration` class (which serves as the base class for most of the configuration implementations shipped with this library) provides two methods implementing a basic copy operation:

- `append()` takes the configuration to be copied as argument and adds all of its properties to the current configuration.
- `copy()` is very similar to `append()`. The difference is that properties that already exist in the target configuration are replaced by the properties of the source configuration.

These methods work fine if the target configuration is not a hierarchical configuration. If a hierarchical configuration is to be copied into another one, the methods are not able to handle the hierarchical structure; so the resulting configuration will contain all of the properties of the source configuration, but the specific parent-child relations will probably be lost. If a hierarchical configuration needs to be copied, there are the following options:

- The `clone()` method can be used to create a copy of a hierarchical configuration. This also works for non-hierarchical configurations. Most of the configuration implementations provided by Commons Configurations support cloning. The `cloneConfiguration()` method of `ConfigurationUtils` can be used for creating a copy of an arbitrary `Configuration` object. This method checks whether the passed in configuration implements the `Cloneable` interface and, if so, invokes its `clone()` method.
- Most hierarchical configurations have a constructor, which takes another hierarchical configuration as argument. This constructor copies the content of the specified configuration into the newly created object.

### Converting a flat configuration into a hierarchical one

Hierarchical configurations provide some enhanced features that are not available for "flat" configurations. For instance they support more sophisticated query facilities. Because of that it may be sometimes useful to transform an ordinary configuration into a hierarchical one. The following code fragment shows how this can be done:

    // Create a flat configuration
    PropertiesConfiguration flatConfig = new PropertiesConfiguration();
    flatConfig.load(...);
    HierarchicalConfiguration hc =
      ConfigurationUtils.convertToHierarchical(flatConfig);

The `convertToHierarchical()` method of `ConfigurationUtils` checks whether the passed in object is already a hierarchical configuration. If this is the case, it is returned unchanged. Otherwise a new `HierarchicalConfiguration` object is created, and the properties of the source configuration are copied into it.

### Converting between properties and configurations

When working with the JDK the `java.util.Properties` class is typically used for storing configuration data. If Commons Configuration is to be integrated in such an application, there may be the requirement of converting from `Properties` objects to `Configuration` objects and vice versa. For this purpose an utility class can be used: `ConfigurationConverter`.

Usage of this class is pretty simple. It provides some static utility methods that perform different conversions. Below you can see some examples. In this fragment we assume that we have a method `processConfiguration()` that is called from older parts of an application that are not aware of the Commons Configuration API. So they pass in a `Properties` object and expect one as return value. Inside the method a temporary `Configuration` object is created and used.

    /**
     * Does some processing of properties.
     * @param props the source properties
     * @return the processed properties
     */
    Properties processConfiguration(Properties props)
    {
        // Create a configuration for the properties for easy access
        Configuration config = ConfigurationConverter.getConfiguration(props);

        // Now use the Configuration API for manipulating the configuration data
        ...

        // Return a Properties object with the results
        return ConfigurationConverter.getProperties(config);
    }

Please refer to the Javadocs of `ConfigurationConverter` to learn more about the available conversion methods and their limitations.

### Interpolation of all variables

Another issue with the integration of Commons Configuration with native Java applications can be variables: Configuration implementations are able to detect variables like `${myReference}` or `${sys:java.version}` in the values of their properties and substitute them by their current values (see the section Variable Interpolation for more details). External components probably do not know how to handle such placeholders when processing configuration files written by Commons Configuration.

`AbstractConfiguration` provides the method `interpolatedConfiguration()`. This method creates a clone of the current configuration and then performs interpolation on all of its properties. So the result of this method is a configuration object with basically the same content as the original configuration, but with all variables replaced by their actual values (as far as this was possible). The following code fragment shows how a `PropertiesConfiguration` object can be saved in a way that the resulting properties file does not contain any variables:

    // Load a properties file (which may contain variables)
    PropertiesConfiguration config = new PropertiesConfiguration("config.properties");

    // Perform interpolation on all variables
    PropertiesConfiguration extConfig =
        (PropertiesConfiguration) config.interpolatedConfiguration();

    // Save the interpolated configuration (no more variables)
    extConfig.save("external_config.properties");

### Handling of runtime exceptions

Section Error listeners introduces a way of dealing with runtime exceptions that can occur on accessing configuration properties by registering an event listener. If you do not want to provide a special error handler, but only need to propagate the exception that caused the error event, you can make use of a convenience method of the `ConfigurationUtils` class: `enableRuntimeExceptions()` registers a special error listener at the passed in configuration that will throw a `ConfigurationRuntimeException` exception for each received error event. The following code fragment shows an example of using this method:

    JNDIConfiguration config = new JNDIConfiguration();
    ConfigurationUtils.enableRuntimeExceptions(config);

    // This may now throw a ConfigurationRuntimeException
    String value = config.getString("myKey");

`enableRuntimeExceptions()` can be called for all `Configuration` implementations that are derived from `EventSource` (which is the case for almost all configuration classes provided by this library). Of course the affected implementation must support the mechanism of error events, otherwise the registered listener will not be triggered. In Error listeners more information can be found.