One of the strength of Commons Configuration is its ability to mix configurations from heterogeneous sources, this section will introduce you to the different configurations available and will show you how to combine them.
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:
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
you can provide many different ways of setting up a configuration. You can either do it
CompositeConfiguration config = new CompositeConfiguration(); config.addConfiguration(new SystemConfiguration()); config.addConfiguration(new PropertiesConfiguration("application.properties"));
or via the
ConfigurationFactory factory = new ConfigurationFactory("config.xml"); Configuration config = factory.getConfiguration();
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
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.
Configuration is done by taking the configuration descriptor XML file and parsing the individual configurations. Make sure to include the various dependencies required for each type of configuration!
This configuration file is very simple. You just need to specify the path to the property file.
The configuration is very similar to the classic properties file. However, the xml file must be in a specific format. Currently there is no DTD.
<baseElement> <element>value</element> <element2> <subelement> <subsubelement>I'm complex!</subsubelement> </subelement> </element2> <test> <short>8</short> </test> </baseElement>
In the above example, the root element is ignored. So to get the value "8", you would
request from your Configuration object the key "
test.short". The root
element can be called anything.
This configuration is very useful for setting environment specific settings like mail
servers! The prefix tells the
ConfigurationFactory what the root will be
to look up your configuration settings.
<env-entry> <env-entry-name>smtp</env-entry-name> <env-entry-value>127.0.0.1</env-entry-value> <env-entry-type>java.lang.String</env-entry-type> </env-entry> <env-entry> <env-entry-name>test/short</env-entry-name> <env-entry-value>80</env-entry-value> <env-entry-type>java.lang.Short</env-entry-type> </env-entry>
Note! If you have a property called "
test.short" with spaces
in it, then it will be translated as the key "
test/short". Therefore, you
should NOT use spaces in the name of properties that are loaded from JNDI! If you do want
to use them, then make sure to convert in your
web.xml the "." characters to
"/" characters, like in the
test.short example above.