This section explains how a
ConfigurationFactory
object is setup that provides access
to a collection of different configuration sources.
When a single configuration file (e.g. a properties file) is the only
source of configuration data it is very simple to
load it using the specific configuration class that deals with
the corresponding format (e.g. PropertiesConfiguration
for properties files or XMLConfiguration
for XML files). But because
we think that later other sources will be added (otherwise
this example section would be too silly) we will use a
ConfigurationFactory
object to load it.
ConfigurationFactory
allows to combine
multiple configuration sources. The properties defined in these
sources can then be accessed as if they were defined in a
single configuration file. To make use of this we have to
create a XML file which tells the factory from which sources
the properties are to be collected. The following listing shows
the content of this file:
<?xml version="1.0" encoding="ISO-8859-1" ?> <configuration> <properties fileName="usergui.properties"/> </configuration>
Definition files for ConfigurationFactory
are
normal XML files. The root element must be named
configuration
. It can contain different sub
elements that specify 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 for
ConfigurationFactory
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.
Now we have to create a ConfigurationFactory
object and let it read this definition file. This is quite simple:
Just create a new instance and set the name of the definition
file with the setConfigurationFileName()
method.
ConfigurationFactory factory = new ConfigurationFactory(); URL configURL = new File("config.xml").toURL(); factory.setConfigurationFileName(configURL.toString()); Configuration config = factory.getConfiguration();
As this code fragment shows the file name passed to the factory can be a full URL. This is also the recommended way of specifying the file because it provides the greatest flexibility and a consistent way of handling relative file names found in the definition file.
Here we assumed the configuration definition file to be located in the current directory. It is also possible (and probably a better approach) to load the file from the class path. This could be done as follows:
ConfigurationFactory factory = new ConfigurationFactory(); URL configURL = getClass().getResource("/config.xml"); factory.setConfigurationURL(configURL); Configuration config = factory.getConfiguration();
Whatever way we used to load the configuration factory, we
should now have a Configuration
object that was
returned by the factory's getConfiguration()
method. This object is actually an instance of the
CompositeConfiguration
class, a specific implementation
of the Configuration
interface that is able to
deal with multiple contained configuration objects. Of course
this class provides all the getter methods defined in the
Configuration
interface, so for accessing a
string property for instance we would use the getString()
method:
String backColor = config.getString("color.background");
Using ConfigurationFactory
to collect configuration
sources does not make much sense if there is only one source to be
loaded. So let's add another one! This time we will embedd a XML file.
Many applications use the popular XML format for storing configuration information. So it is no wonder that Configuration also supports this type of configuration sources. In general each XML document can be used to define configuration settings. We start here with a rather simple one:
<?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 ConfigurationFactory
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 problem 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 easy be realized by saving the
user's settings in a 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 need not be aware whether a property was overriden by
the user. The ConfigurationFactory
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).
The example above with two configuration sources - one for user settings and one with default values - raises an interesting question: What will happen 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 ConfigurationFactory
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 will still work because
a complete set of configuration properties is defined in the
second file.
ConfigurationFactory
supports such optional
configuration sources. For this purpose in the definition of a
(file based) configuration source the 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" 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, ConfigurationFactory
will
not throw an exception, but only write a warning message to its
logger. Note that the optional
attribute is absent
for the second properties file. Thus it is mandatory, and the
getConfiguration()
method of
ConfigurationFactory
would throw an exception if it
could not be found.
In an earlier section about the configuration definition file for
ConfigurationFactory
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 times 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 XMLConfiguration. 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 would be 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.
ConfigurationFactory
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 include 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" at="tables"/> </additional> </configuration>
Compared to the older versions of this file a couple of changes has 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 sake of clearity it is recommended to use them
when 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. The only difference is an additionally supported
at
attribute that specifies the position in the logic union
configuration where the included properties are to be added. In this
example we set the at
attribute of the second element to
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 the ConfigurationFactory
will allow access to three
database tables. A call of config.getString("tables.table(2).name");
will result 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).
We have seen how to write configuration definition files for
including properties and XML files. This section deals with other
options that can be specified in such a definition file and that
are evaluated by ConfigurationFactory
.
From time to time the question is raised whether there is a document type definition that exactly defines the structure of a configuration definition file. Frankly, the answer is no. This is because for a future version of Commons Configuration it is planed to make the configuration definition files extensible, i.e. allow developers to register their own tags and corresponding implementations of the Configuration interface.
In the current version the set of supported XML elements is fixed. Below is a list of all supported tags and a description of each:
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
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
.
JNDIConfiguration
class. The prefix
attribute can be used to
select a subset of the JNDI tree.
plist
element allows to embedd 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.
SystemConfiguration
is added to the resulting configuration allowing access to
system properties.
All of these elements can occur in a configuration definition file in arbitrary number and order. The following listing shows an example file using many of these tags.
<?xml version="1.0" encoding="ISO-8859-1" ?> <configuration> <system/> <jndi prefix="java:comp/env"/> <properties fileName="test.properties"/> <xml fileName="test.xml"/> <properties fileName="test.properties.xml"/> </configuration>
Many specialized configuration classes support initialization
properties that influence the behavior of their instances. For
example for file based configurations the encoding of the files
to load can be specified using the setEncoding()
method, or an XMLConfiguration
can be told to
perform validation by calling the setValidating()
method. How can such properties be set in a configuration definition
file?
Fortunately this is easy possible. For each XML element in a
configuration definition file additional attributes can be
specified that correspond to (bean) setter methods defined in the
associated configuration class. To derive the name of an attribute
from a setter method to be called, just drop the prefix "set" and
make the first letter lower case. So for instance the attribute
that invokes the setEncoding()
method would be
encoding
. The following example shows how a XML
document with a certain encoding and enabled validation can be
loaded:
<?xml version="1.0" encoding="ISO-8859-1" ?> <configuration> <xml fileName="test.xml" encoding="UTF-8" validating="true"/> </configuration>
Using this mechanism many properties of configuration classes can
be set when they are used together with ConfigurationFactory
.
To find out, which attributes are supported by a specific XML
element, refer to the list in the previous section that explains,
which configuration classes are used for which tags. In the JavaDoc
of the corresponding class you can find the setter methods you can
address by attributes.