Hadoop Configuration

Configuration的主要是加载配置文件，并储存在properties中。

细节内容不重复了，主要参考Hadoop技术内幕，Hadoop源代码，以及：

http://blog.csdn.net/zhoubangtao/article/details/25977561

• 整个资源的加载流程如下：

• 过程中遇到的问题：
  • Java Io 学习：
    • http://blog.csdn.net/yczz/article/details/38761237
  • Document Builder使用：
    • http://blog.csdn.net/redarmy_chen/article/details/12912065
    • http://docs.oracle.com/javase/7/docs/api/javax/xml/parsers/DocumentBuilder.html

package com.baidu.configuration;

import java.io.IOException;
import java.io.InputStream;
import java.io.File;
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.lang.System;
import java.util.Objects;
import java.util.Properties;
import java.net.URL;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.DOMException;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.w3c.dom.Text;
import org.xml.sax.SAXException;


public class Configuration {

    private Properties properties;

    /**
     * 判断参数是否为空，并对url进行处理
     * @param builder
     * @param url
     * @return
     * @throws IOException
     * @throws SAXException
     */
    private Document parse(DocumentBuilder builder, URL url)
            throws IOException, SAXException {
        if (url == null) {
            return null;
        }
        return parse(builder, url.openStream(), url.toString());
    }

    /**
     * 判断参数是否为空，实际调用Documentbuilder parse 函数
     * @param builder
     * @param is
     * @param systemId
     * @return
     * @throws IOException
     * @throws SAXException
     */
    private Document parse(DocumentBuilder builder, InputStream is,
                           String systemId) throws IOException, SAXException {
        if (is == null) {
            return null;
        }
        try {
            return (systemId == null) ? builder.parse(is) : builder.parse(is,
                    systemId);
        } finally {
            is.close();
        }
    }


    /**
     * 获取 properties 对应key的value
     * @param key
     * @return
     */
    public String get(String key) {
        return properties.getProperty(key);
    }

    public void addResource(String source_name) {
        properties = new Properties();
        loadResource(properties, source_name);
    }

    /**
     * 通过DocumentBuilder来解析xml文件， resource 直接使用了 Object 来代替，
     * 实际就是一个字符串
     * @param properties 储存的prpoerties
     * @param resource xml文件的绝对路径
     */
    public void loadResource(Properties properties, Object resource) {
        try {
            DocumentBuilderFactory docBuilderFactory
                    = DocumentBuilderFactory.newInstance();

            //ignore all comments inside the xml file
            docBuilderFactory.setIgnoringComments(true);

            //allow includes in the xml file
            docBuilderFactory.setNamespaceAware(true);
            try {
                docBuilderFactory.setXIncludeAware(true);
            } catch (UnsupportedOperationException e) {
                System.out.println("Failed to set setXIncludeAware(true) for parser");
            }
            DocumentBuilder builder = docBuilderFactory.newDocumentBuilder();
            Document doc = null;
            Element root = null;

            if (resource instanceof String) {          // a file resource
                // Can't use FileSystem API or we get an infinite loop
                // since FileSystem uses Configuration AP
                //
                //I.  Use java.io.File instead.
                File file = new File(((String)resource)).getAbsoluteFile();
                if (file.exists()) {
                    doc = parse(builder, new BufferedInputStream(
                            new FileInputStream(file)), ((String)resource).toString());
                }
            }

            root = doc.getDocumentElement();

            NodeList props = root.getChildNodes();

            for (int i = 0; i < props.getLength(); i++) {
                Node propNode = props.item(1);

                Element prop = (Element)propNode;

                if ("configuration".equals(prop.getTagName())) {
                    loadResource(properties, prop.getTagName());
                }

                NodeList fields = prop.getChildNodes();
                String attr = null;
                String value = null;

                for (int j = 0; j < fields.getLength(); j++) {
                    Node fieldNode = fields.item(j);
                    if (!(fieldNode instanceof Element))
                        continue;
                    Element field = (Element)fieldNode;
                    if ("name".equals(field.getTagName()) && field.hasChildNodes())
                        attr = ((Text)field.getFirstChild()).getData().trim();

                    if ("value".equals(field.getTagName()) && field.hasChildNodes())
                        value = ((Text)field.getFirstChild()).getData().trim();
                }
                //System.out.println("name is " + attr);
                //System.out.println("value is " + value);
                properties.setProperty(attr, value);
            }

        } catch (IOException e) {
            e.printStackTrace();
        } catch (DOMException e) {
            e.printStackTrace();
        } catch (SAXException e) {
            e.printStackTrace();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
    }

    public static void main(String args[]) {
        //System.out.println("Hello");
        Configuration c = new Configuration();
        c.addResource("test.xml");

        String value = c.get("yarn.nodemanager.aux-services");
        System.out.println(value);
    }

}