Hadoop 中的Configuration类
Configiration类：
javadoc中是这么说的：
Provides access to configuration parameters.
Resources（xml文件，先加载default xml，而后，加载自定义的xml）
Configurations are specified by resources. A resource contains a set of name/value pairs as XML data. Each resource is named by either a String or by a Path. If named by a String, then the classpath is examined for a file with that name. If named by a Path, then the local filesystem is examined directly, without referring to the classpath.
Unless explicitly turned off, Hadoop by default specifies two resources, loaded in-order from the classpath:
core-default.xml : Read-only defaults for hadoop.
core-site.xml: Site-specific configuration for a given hadoop installation.
Applications may add additional resources, which are loaded subsequent to these resources in the order they are added.
Final Parameters（如果定义属性为final则后来加载的key-value不能覆盖）
Configuration parameters may be declared final. Once a resource declares a value final, no subsequently-loaded resource can alter that value. For example, one might define a final parameter with:
<property>
<name>dfs.client.buffer.dir</name>
<value>/tmp/hadoop/dfs/client</value>
<final>true</final>
</property>
Administrators typically define parameters as final in core-site.xml for values that user applications may not alter.
Variable Expansion（可以用${}格式，使用前面的属性作为变量）
Value strings are first processed for variable expansion. The available properties are:
Other properties defined in this Configuration; and, if a name is undefined here,
Properties in System.getProperties().
For example, if a configuration resource contains the following property definitions:
<property>
<name>basedir</name>
<value>/user/${user.name}</value>
</property>
<property>
<name>tempdir</name>
<value>${basedir}/tmp</value>
</property>
When conf.get("tempdir") is called, then ${basedir} will be resolved to another property in this Configuration, while ${user.name} would then ordinarily be resolved to the value of the System property with that name.
我想说，我看完我看源码的一点感悟：
1、每一个类当需要configuration类来配置自己时候，只需要extends Configured，这样我们就可以使用Configuration来配置自己了。需要单独的配置来时只需要，在使用configuration之前单独把自己的配置文件load进来就行，就像sqoop的配置文件sqoop-site.xml：
static { Configuration.addDefaultResource("sqoop-site.xml"); }