Hadoop学习七：Hadoop-Hdfs源码 conf包

stcaac

一.conf包下四个类

 
二.详细描述

•  Configurable接口：Something that may be configured with a {@link Configuration}.，很绕口，简单理解为Hdfs系统配置文件的接口。
  
• Configured：Base class for things that may be configured with a {@link Configuration}，Hdfs系统配置文件的抽象类。持有Configuration对象的应用。
  
• Configured：配置文件资源管理类。用DOM解析xml配置文件；默认加载资源core-default.xml，core-site.xml；只有在get(key)时才去加载资源文件。
  
• ConfServlet：A servlet to print out the running configuration data。
    @Override
  public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  String format = request.getParameter("format");
  if (null == format) {
  format = "xml";
  }
  if ("xml".equals(format)) {
  response.setContentType("text/xml; charset=utf-8");
  } else if ("json".equals(format)) {
  response.setContentType("application/json; charset=utf-8");
  }
  Writer out = response.getWriter();
  try {
  out.write(getServletContext().getAttribute("hadoop.conf");)
  } catch (BadFormatException bfe) {
  response.sendError(HttpServletResponse.SC_BAD_REQUEST, bfe.getMessage());
  }
  out.close();
  }
  

三.Configuration代码 

//1.静态代码块，把默认资源放到CopyOnWriteArrayList<String> defaultResources里。
static{
//print deprecation warning if hadoop-site.xml is found in classpath
ClassLoader cL = Thread.currentThread().getContextClassLoader();
if (cL == null) {
cL = Configuration.class.getClassLoader();
}
if(cL.getResource("hadoop-site.xml")!=null) {
LOG.warn("DEPRECATED: hadoop-site.xml found in the classpath. " +
"Usage of hadoop-site.xml is deprecated. Instead use core-site.xml, "
+ "mapred-site.xml and hdfs-site.xml to override properties of " +
"core-default.xml, mapred-default.xml and hdfs-default.xml " +
"respectively");
}
addDefaultResource("core-default.xml");
addDefaultResource("core-site.xml");
}
public static synchronized void addDefaultResource(String name) {
if(!defaultResources.contains(name)) {
defaultResources.add(name);
for(Configuration conf : REGISTRY.keySet()) {
if(conf.loadDefaults) {
conf.reloadConfiguration();
}
}
}
}
//2.初始化代码块，把该实例放入WeakHashMap<Configuration,Object>里。
public Configuration(boolean loadDefaults) {
this.loadDefaults = loadDefaults;
updatingResource = new HashMap<String, String>();
synchronized(Configuration.class) {
REGISTRY.put(this, null);
}
}
//3.第一次get(key)时，才开始解析资源，保存到Properties properties里。
public String get(String name) {
return substituteVars(getProps().getProperty(name));
}
private synchronized Properties getProps() {
if (properties == null) {
properties = new Properties(); //这个时候才初始化properties
loadResources(properties, resources, quietmode);
if (overlay!= null) {
properties.putAll(overlay);
for (Map.Entry<Object,Object> item: overlay.entrySet()) {
updatingResource.put((String) item.getKey(), UNKNOWN_RESOURCE);
}
}
}
return properties;
}
private void loadResource(Properties properties, Object name, boolean quiet) {
//DOM解析资源，保存到properties里。
//需要注意的时，解析的是代码运行环境下的core-default.xml，core-site.xml。
}
四.Configuration使用例子 

Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(URI, conf);

•  第一段代码：按照三里的12步先静态代码块，再初始化。
  
• 第二段代码，创建FileSystem时，会调用conf.getBoolean(key)方法获取所需参数，此时第一次加载资源，后续还会调用conf.get(key)，直接从properties取就行了。
  
• 若上述两段代码在eclipse运行，必须在eclipse classpath下配置core-default.xml，core-site.xml。