Developing a MapReduce Application - Hadoop: The Definitive Guide - page 166

Configuration conf = new Configuration ();
conf . addResource ( "configuration-1.xml" );
assertThat ( conf . get ( "color" ), is ( "yellow" ));
assertThat ( conf . getInt ( "size" , 0 ), is ( 10 ));
assertThat ( conf . get ( "breadth" , "wide" ), is ( "wide" ));
There are a couple of things to note: type information is not stored in the XML file; in-
stead, properties can be interpreted as a given type when they are read. Also, the get()
methods allow you to specify a default value, which is used if the property is not defined
in the XML file, as in the case of breadth here.
Combining Resources
Things get interesting when more than one resource is used to define a Configura-
tion . This is used in Hadoop to separate out the default properties for the system,
defined internally in a file called core-default.xml , from the site-specific overrides in core-
site.xml . The file in Example 6-2 defines the size and weight properties.
Example 6-2. A second configuration file, configuration-2.xml
<?xml version="1.0"?>
<configuration>
<property>
<name> size </name>
<value> 12 </value>
</property>
<property>
<name> weight </name>
<value> light </value>
</property>
</configuration>
Resources are added to a Configuration in order:
Configuration conf = new Configuration ();
conf . addResource ( "configuration-1.xml" );
conf . addResource ( "configuration-2.xml" );
Properties defined in resources that are added later override the earlier definitions. So the
size property takes its value from the second configuration file, configuration-2.xml :
assertThat ( conf . getInt ( "size" , 0 ), is ( 12 ));