Configuration简析

        以下分析基于Hadoop-0.19.2

        我们在写job的时候经常会写这么一句

JobConf conf = new JobConf(***.class)

        显而易见，这是加载Hadoop的配置。虽然整个过程简单说无非就是读取配置文件，但还是来简单分析一下内部是如何进行处理的。看一下JobConf这个类的继承关系

public class JobConf extends Configuration {
    public JobConf(Class exampleClass) {
        setJarByClass(exampleClass);
  }
………
}

        我们一般调用的就是上面的这个构造函数，由于没有指明父构造器的类型，所以在new JobConf的时候会调用父类的默认构造器。那么下面来看一下Configuration这个类。

public class Configuration implements Iterable<Map.Entry<String,String>>, Writable {
  private static final Log LOG = LogFactory.getLog(Configuration.class);

  //从参数名
  private boolean quietmode = true;
  
  //配置列表
  private ArrayList<Object> resources = new ArrayList<Object>();

  //最终配置列表
  private Set<String> finalParameters = new HashSet<String>();

  //
  private Properties properties;
  private Properties overlay;
  private ClassLoader classLoader;
  {
    classLoader = Thread.currentThread().getContextClassLoader();
    if (classLoader == null) {
      classLoader = Configuration.class.getClassLoader();
    }
  }
  
  /** A new configuration. */
  public Configuration() {
    this(true);
  }

  /** A new configuration where the behavior of reading from the default 
   * resources can be turned off.
   * 
   * If the parameter {@code loadDefaults} is false, the new instance
   * will not load resources from the default files. 
   * @param loadDefaults specifies whether to load from the default files
   */
  public Configuration(boolean loadDefaults) {
    if (LOG.isDebugEnabled()) {
      LOG.debug(StringUtils.stringifyException(new IOException("config()")));
    }
    if (loadDefaults) {
      resources.add("hadoop-default.xml");
      resources.add("hadoop-site.xml");
    }
  }

暂停……待续