Hadoop学习七：Hadoop-Hdfs源码 conf包

Hadoop学习七：Hadoop-Hdfs源码 conf包

博客分类： 
• Hadoop

 

一.conf包下四个类

 

二.详细描述

1.  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。
2. 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。
3. 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。
4. 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);

1.  第一段代码：按照三里的12步先静态代码块，再初始化。
2. 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。
3. 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。