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Configuration.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.conf;

import java.io.BufferedInputStream;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.Reader;
import java.net.URL;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.Properties;
import java.util.Set;
import java.util.StringTokenizer;
import java.util.WeakHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableUtils;
import org.apache.hadoop.util.StringUtils;
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;

/** 
 * Provides access to configuration parameters.
 *
 * <h4 id="Resources">Resources</h4>
 *
 * <p>Configurations are specified by resources. A resource contains a set of
 * name/value pairs as XML data. Each resource is named by either a 
 * <code>String</code> or by a {@link Path}. If named by a <code>String</code>, 
 * then the classpath is examined for a file with that name.  If named by a 
 * <code>Path</code>, then the local filesystem is examined directly, without 
 * referring to the classpath.
 *
 * <p>Unless explicitly turned off, Hadoop by default specifies two 
 * resources, loaded in-order from the classpath: <ol>
 * <li><tt><a href="{@docRoot}/../core-default.html">core-default.xml</a>
 * </tt>: Read-only defaults for hadoop.</li>
 * <li><tt>core-site.xml</tt>: Site-specific configuration for a given hadoop
 * installation.</li>
 * </ol>
 * Applications may add additional resources, which are loaded
 * subsequent to these resources in the order they are added.
 * 
 * <h4 id="FinalParams">Final Parameters</h4>
 *
 * <p>Configuration parameters may be declared <i>final</i>. 
 * Once a resource declares a value final, no subsequently-loaded 
 * resource can alter that value.  
 * For example, one might define a final parameter with:
 * <tt><pre>
 *  &lt;property&gt;
 *    &lt;name&gt;dfs.client.buffer.dir&lt;/name&gt;
 *    &lt;value&gt;/tmp/hadoop/dfs/client&lt;/value&gt;
 *    <b>&lt;final&gt;true&lt;/final&gt;</b>
 *  &lt;/property&gt;</pre></tt>
 *
 * Administrators typically define parameters as final in 
 * <tt>core-site.xml</tt> for values that user applications may not alter.
 *
 * <h4 id="VariableExpansion">Variable Expansion</h4>
 *
 * <p>Value strings are first processed for <i>variable expansion</i>. The
 * available properties are:<ol>
 * <li>Other properties defined in this Configuration; and, if a name is
 * undefined here,</li>
 * <li>Properties in {@link System#getProperties()}.</li>
 * </ol>
 *
 * <p>For example, if a configuration resource contains the following property
 * definitions: 
 * <tt><pre>
 *  &lt;property&gt;
 *    &lt;name&gt;basedir&lt;/name&gt;
 *    &lt;value&gt;/user/${<i>user.name</i>}&lt;/value&gt;
 *  &lt;/property&gt;
 *  
 *  &lt;property&gt;
 *    &lt;name&gt;tempdir&lt;/name&gt;
 *    &lt;value&gt;${<i>basedir</i>}/tmp&lt;/value&gt;
 *  &lt;/property&gt;</pre></tt>
 *
 * When <tt>conf.get("tempdir")</tt> is called, then <tt>${<i>basedir</i>}</tt>
 * will be resolved to another property in this Configuration, while
 * <tt>${<i>user.name</i>}</tt> would then ordinarily be resolved to the value
 * of the System property with that name.
 */
public class Configuration implements Iterable<Map.Entry<String,String>>,
                                      Writable {
  private static final Log LOG =
    LogFactory.getLog(Configuration.class);

  private boolean quietmode = true;
  
  /**
   * List of configuration resources.
   */
  private ArrayList<Object> resources = new ArrayList<Object>();

  /**
   * List of configuration parameters marked <b>final</b>. 
   */
  private Set<String> finalParameters = new HashSet<String>();
  
  private boolean loadDefaults = true;
  
  /**
   * Configurtion objects
   */
  private static final WeakHashMap<Configuration,Object> REGISTRY = 
    new WeakHashMap<Configuration,Object>();
  
  /**
   * List of default Resources. Resources are loaded in the order of the list 
   * entries
   */
  private static final CopyOnWriteArrayList<String> defaultResources =
    new CopyOnWriteArrayList<String>();
  
  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");
  }
  
  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) {
    this.loadDefaults = loadDefaults;
    if (LOG.isDebugEnabled()) {
      LOG.debug(StringUtils.stringifyException(new IOException("config()")));
    }
    synchronized(Configuration.class) {
      REGISTRY.put(this, null);
    }
  }
  
  /** 
   * A new configuration with the same settings cloned from another.
   * 
   * @param other the configuration from which to clone settings.
   */
  @SuppressWarnings("unchecked")
  public Configuration(Configuration other) {
    if (LOG.isDebugEnabled()) {
      LOG.debug(StringUtils.stringifyException
                (new IOException("config(config)")));
    }
   
   this.resources = (ArrayList)other.resources.clone();
   synchronized(other) {
     if (other.properties != null) {
       this.properties = (Properties)other.properties.clone();
     }

     if (other.overlay!=null) {
       this.overlay = (Properties)other.overlay.clone();
     }
   }
   
    this.finalParameters = new HashSet<String>(other.finalParameters);
    synchronized(Configuration.class) {
      REGISTRY.put(this, null);
    }
  }
  
  /**
   * Add a default resource. Resources are loaded in the order of the resources 
   * added.
   * @param name file name. File should be present in the classpath.
   */
  public static synchronized void addDefaultResource(String name) {
    if(!defaultResources.contains(name)) {
      defaultResources.add(name);
      for(Configuration conf : REGISTRY.keySet()) {
        if(conf.loadDefaults) {
          conf.reloadConfiguration();
        }
      }
    }
  }

  /**
   * Add a configuration resource. 
   * 
   * The properties of this resource will override properties of previously 
   * added resources, unless they were marked <a href="#Final">final</a>. 
   * 
   * @param name resource to be added, the classpath is examined for a file 
   *             with that name.
   */
  public void addResource(String name) {
    addResourceObject(name);
  }

  /**
   * Add a configuration resource. 
   * 
   * The properties of this resource will override properties of previously 
   * added resources, unless they were marked <a href="#Final">final</a>. 
   * 
   * @param url url of the resource to be added, the local filesystem is 
   *            examined directly to find the resource, without referring to 
   *            the classpath.
   */
  public void addResource(URL url) {
    addResourceObject(url);
  }

  /**
   * Add a configuration resource. 
   * 
   * The properties of this resource will override properties of previously 
   * added resources, unless they were marked <a href="#Final">final</a>. 
   * 
   * @param file file-path of resource to be added, the local filesystem is
   *             examined directly to find the resource, without referring to 
   *             the classpath.
   */
  public void addResource(Path file) {
    addResourceObject(file);
  }

  /**
   * Add a configuration resource. 
   * 
   * The properties of this resource will override properties of previously 
   * added resources, unless they were marked <a href="#Final">final</a>. 
   * 
   * @param in InputStream to deserialize the object from. 
   */
  public void addResource(InputStream in) {
    addResourceObject(in);
  }
  
  
  /**
   * Reload configuration from previously added resources.
   *
   * This method will clear all the configuration read from the added 
   * resources, and final parameters. This will make the resources to 
   * be read again before accessing the values. Values that are added
   * via set methods will overlay values read from the resources.
   */
  public synchronized void reloadConfiguration() {
    properties = null;                            // trigger reload
    finalParameters.clear();                      // clear site-limits
  }
  
  private synchronized void addResourceObject(Object resource) {
    resources.add(resource);                      // add to resources
    reloadConfiguration();
  }
  
  private static Pattern varPat = Pattern.compile("\\$\\{[^\\}\\$\u0020]+\\}");
  private static int MAX_SUBST = 20;

  private String substituteVars(String expr) {
    if (expr == null) {
      return null;
    }
    Matcher match = varPat.matcher("");
    String eval = expr;
    for(int s=0; s<MAX_SUBST; s++) {
      match.reset(eval);
      if (!match.find()) {
        return eval;
      }
      String var = match.group();
      var = var.substring(2, var.length()-1); // remove ${ .. }
      String val = null;
      try {
        val = System.getProperty(var);
      } catch(SecurityException se) {
        LOG.warn("Unexpected SecurityException in Configuration", se);
      }
      if (val == null) {
        val = getRaw(var);
      }
      if (val == null) {
        return eval; // return literal ${var}: var is unbound
      }
      // substitute
      eval = eval.substring(0, match.start())+val+eval.substring(match.end());
    }
    throw new IllegalStateException("Variable substitution depth too large: " 
                                    + MAX_SUBST + " " + expr);
  }
  
  /**
   * Get the value of the <code>name</code> property, <code>null</code> if
   * no such property exists.
   * 
   * Values are processed for <a href="#VariableExpansion">variable expansion</a> 
   * before being returned. 
   * 
   * @param name the property name.
   * @return the value of the <code>name</code> property, 
   *         or null if no such property exists.
   */
  public String get(String name) {
    return substituteVars(getProps().getProperty(name));
  }

  /**
   * Get the value of the <code>name</code> property, without doing
   * <a href="#VariableExpansion">variable expansion</a>.
   * 
   * @param name the property name.
   * @return the value of the <code>name</code> property, 
   *         or null if no such property exists.
   */
  public String getRaw(String name) {
    return getProps().getProperty(name);
  }

  /** 
   * Set the <code>value</code> of the <code>name</code> property.
   * 
   * @param name property name.
   * @param value property value.
   */
  public void set(String name, String value) {
    getOverlay().setProperty(name, value);
    getProps().setProperty(name, value);
  }
  
  /**
   * Sets a property if it is currently unset.
   * @param name the property name
   * @param value the new value
   */
  public void setIfUnset(String name, String value) {
    if (get(name) == null) {
      set(name, value);
    }
  }
  
  private synchronized Properties getOverlay() {
    if (overlay==null){
      overlay=new Properties();
    }
    return overlay;
  }

  /** 
   * Get the value of the <code>name</code> property. If no such property 
   * exists, then <code>defaultValue</code> is returned.
   * 
   * @param name property name.
   * @param defaultValue default value.
   * @return property value, or <code>defaultValue</code> if the property 
   *         doesn't exist.                    
   */
  public String get(String name, String defaultValue) {
    return substituteVars(getProps().getProperty(name, defaultValue));
  }
    
  /** 
   * Get the value of the <code>name</code> property as an <code>int</code>.
   *   
   * If no such property exists, or if the specified value is not a valid
   * <code>int</code>, then <code>defaultValue</code> is returned.
   * 
   * @param name property name.
   * @param defaultValue default value.
   * @return property value as an <code>int</code>, 
   *         or <code>defaultValue</code>. 
   */
  public int getInt(String name, int defaultValue) {
    String valueString = get(name);
    if (valueString == null)
      return defaultValue;
    try {
      String hexString = getHexDigits(valueString);
      if (hexString != null) {
        return Integer.parseInt(hexString, 16);
      }
      return Integer.parseInt(valueString);
    } catch (NumberFormatException e) {
      return defaultValue;
    }
  }

  /** 
   * Set the value of the <code>name</code> property to an <code>int</code>.
   * 
   * @param name property name.
   * @param value <code>int</code> value of the property.
   */
  public void setInt(String name, int value) {
    set(name, Integer.toString(value));
  }


  /** 
   * Get the value of the <code>name</code> property as a <code>long</code>.  
   * If no such property is specified, or if the specified value is not a valid
   * <code>long</code>, then <code>defaultValue</code> is returned.
   * 
   * @param name property name.
   * @param defaultValue default value.
   * @return property value as a <code>long</code>, 
   *         or <code>defaultValue</code>. 
   */
  public long getLong(String name, long defaultValue) {
    String valueString = get(name);
    if (valueString == null)
      return defaultValue;
    try {
      String hexString = getHexDigits(valueString);
      if (hexString != null) {
        return Long.parseLong(hexString, 16);
      }
      return Long.parseLong(valueString);
    } catch (NumberFormatException e) {
      return defaultValue;
    }
  }

  private String getHexDigits(String value) {
    boolean negative = false;
    String str = value;
    String hexString = null;
    if (value.startsWith("-")) {
      negative = true;
      str = value.substring(1);
    }
    if (str.startsWith("0x") || str.startsWith("0X")) {
      hexString = str.substring(2);
      if (negative) {
        hexString = "-" + hexString;
      }
      return hexString;
    }
    return null;
  }
  
  /** 
   * Set the value of the <code>name</code> property to a <code>long</code>.
   * 
   * @param name property name.
   * @param value <code>long</code> value of the property.
   */
  public void setLong(String name, long value) {
    set(name, Long.toString(value));
  }

  /** 
   * Get the value of the <code>name</code> property as a <code>float</code>.  
   * If no such property is specified, or if the specified value is not a valid
   * <code>float</code>, then <code>defaultValue</code> is returned.
   * 
   * @param name property name.
   * @param defaultValue default value.
   * @return property value as a <code>float</code>, 
   *         or <code>defaultValue</code>. 
   */
  public float getFloat(String name, float defaultValue) {
    String valueString = get(name);
    if (valueString == null)
      return defaultValue;
    try {
      return Float.parseFloat(valueString);
    } catch (NumberFormatException e) {
      return defaultValue;
    }
  }
  /**
   * Set the value of the <code>name</code> property to a <code>float</code>.
   * 
   * @param name property name.
   * @param value property value.
   */
  public void setFloat(String name, float value) {
    set(name,Float.toString(value));
  }
 
  /** 
   * Get the value of the <code>name</code> property as a <code>boolean</code>.  
   * If no such property is specified, or if the specified value is not a valid
   * <code>boolean</code>, then <code>defaultValue</code> is returned.
   * 
   * @param name property name.
   * @param defaultValue default value.
   * @return property value as a <code>boolean</code>, 
   *         or <code>defaultValue</code>. 
   */
  public boolean getBoolean(String name, boolean defaultValue) {
    String valueString = get(name);
    if ("true".equals(valueString))
      return true;
    else if ("false".equals(valueString))
      return false;
    else return defaultValue;
  }

  /** 
   * Set the value of the <code>name</code> property to a <code>boolean</code>.
   * 
   * @param name property name.
   * @param value <code>boolean</code> value of the property.
   */
  public void setBoolean(String name, boolean value) {
    set(name, Boolean.toString(value));
  }

  /**
   * Set the given property, if it is currently unset.
   * @param name property name
   * @param value new value
   */
  public void setBooleanIfUnset(String name, boolean value) {
    setIfUnset(name, Boolean.toString(value));
  }

  /**
   * A class that represents a set of positive integer ranges. It parses 
   * strings of the form: "2-3,5,7-" where ranges are separated by comma and 
   * the lower/upper bounds are separated by dash. Either the lower or upper 
   * bound may be omitted meaning all values up to or over. So the string 
   * above means 2, 3, 5, and 7, 8, 9, ...
   */
  public static class IntegerRanges {
    private static class Range {
      int start;
      int end;
    }

    List<Range> ranges = new ArrayList<Range>();
    
    public IntegerRanges() {
    }
    
    public IntegerRanges(String newValue) {
      StringTokenizer itr = new StringTokenizer(newValue, ",");
      while (itr.hasMoreTokens()) {
        String rng = itr.nextToken().trim();
        String[] parts = rng.split("-", 3);
        if (parts.length < 1 || parts.length > 2) {
          throw new IllegalArgumentException("integer range badly formed: " + 
                                             rng);
        }
        Range r = new Range();
        r.start = convertToInt(parts[0], 0);
        if (parts.length == 2) {
          r.end = convertToInt(parts[1], Integer.MAX_VALUE);
        } else {
          r.end = r.start;
        }
        if (r.start > r.end) {
          throw new IllegalArgumentException("IntegerRange from " + r.start + 
                                             " to " + r.end + " is invalid");
        }
        ranges.add(r);
      }
    }

    /**
     * Convert a string to an int treating empty strings as the default value.
     * @param value the string value
     * @param defaultValue the value for if the string is empty
     * @return the desired integer
     */
    private static int convertToInt(String value, int defaultValue) {
      String trim = value.trim();
      if (trim.length() == 0) {
        return defaultValue;
      }
      return Integer.parseInt(trim);
    }

    /**
     * Is the given value in the set of ranges
     * @param value the value to check
     * @return is the value in the ranges?
     */
    public boolean isIncluded(int value) {
      for(Range r: ranges) {
        if (r.start <= value && value <= r.end) {
          return true;
        }
      }
      return false;
    }
    
    @Override
    public String toString() {
      StringBuffer result = new StringBuffer();
      boolean first = true;
      for(Range r: ranges) {
        if (first) {
          first = false;
        } else {
          result.append(',');
        }
        result.append(r.start);
        result.append('-');
        result.append(r.end);
      }
      return result.toString();
    }
  }

  /**
   * Parse the given attribute as a set of integer ranges
   * @param name the attribute name
   * @param defaultValue the default value if it is not set
   * @return a new set of ranges from the configured value
   */
  public IntegerRanges getRange(String name, String defaultValue) {
    return new IntegerRanges(get(name, defaultValue));
  }

  /** 
   * Get the comma delimited values of the <code>name</code> property as 
   * a collection of <code>String</code>s.  
   * If no such property is specified then empty collection is returned.
   * <p>
   * This is an optimized version of {@link #getStrings(String)}
   * 
   * @param name property name.
   * @return property value as a collection of <code>String</code>s. 
   */
  public Collection<String> getStringCollection(String name) {
    String valueString = get(name);
    return StringUtils.getStringCollection(valueString);
  }

  /** 
   * Get the comma delimited values of the <code>name</code> property as 
   * an array of <code>String</code>s.  
   * If no such property is specified then <code>null</code> is returned.
   * 
   * @param name property name.
   * @return property value as an array of <code>String</code>s, 
   *         or <code>null</code>. 
   */
  public String[] getStrings(String name) {
    String valueString = get(name);
    return StringUtils.getStrings(valueString);
  }

  /** 
   * Get the comma delimited values of the <code>name</code> property as 
   * an array of <code>String</code>s.  
   * If no such property is specified then default value is returned.
   * 
   * @param name property name.
   * @param defaultValue The default value
   * @return property value as an array of <code>String</code>s, 
   *         or default value. 
   */
  public String[] getStrings(String name, String... defaultValue) {
    String valueString = get(name);
    if (valueString == null) {
      return defaultValue;
    } else {
      return StringUtils.getStrings(valueString);
    }
  }

  /** 
   * Set the array of string values for the <code>name</code> property as 
   * as comma delimited values.  
   * 
   * @param name property name.
   * @param values The values
   */
  public void setStrings(String name, String... values) {
    set(name, StringUtils.arrayToString(values));
  }
 
  /**
   * Load a class by name.
   * 
   * @param name the class name.
   * @return the class object.
   * @throws ClassNotFoundException if the class is not found.
   */
  public Class<?> getClassByName(String name) throws ClassNotFoundException {
    return Class.forName(name, true, classLoader);
  }

  /** 
   * Get the value of the <code>name</code> property
   * as an array of <code>Class</code>.
   * The value of the property specifies a list of comma separated class names.  
   * If no such property is specified, then <code>defaultValue</code> is 
   * returned.
   * 
   * @param name the property name.
   * @param defaultValue default value.
   * @return property value as a <code>Class[]</code>, 
   *         or <code>defaultValue</code>. 
   */
  public Class<?>[] getClasses(String name, Class<?> ... defaultValue) {
    String[] classnames = getStrings(name);
    if (classnames == null)
      return defaultValue;
    try {
      Class<?>[] classes = new Class<?>[classnames.length];
      for(int i = 0; i < classnames.length; i++) {
        classes[i] = getClassByName(classnames[i]);
      }
      return classes;
    } catch (ClassNotFoundException e) {
      throw new RuntimeException(e);
    }
  }

  /** 
   * Get the value of the <code>name</code> property as a <code>Class</code>.  
   * If no such property is specified, then <code>defaultValue</code> is 
   * returned.
   * 
   * @param name the class name.
   * @param defaultValue default value.
   * @return property value as a <code>Class</code>, 
   *         or <code>defaultValue</code>. 
   */
  public Class<?> getClass(String name, Class<?> defaultValue) {
    String valueString = get(name);
    if (valueString == null)
      return defaultValue;
    try {
      return getClassByName(valueString);
    } catch (ClassNotFoundException e) {
      throw new RuntimeException(e);
    }
  }

  /** 
   * Get the value of the <code>name</code> property as a <code>Class</code>
   * implementing the interface specified by <code>xface</code>.
   *   
   * If no such property is specified, then <code>defaultValue</code> is 
   * returned.
   * 
   * An exception is thrown if the returned class does not implement the named
   * interface. 
   * 
   * @param name the class name.
   * @param defaultValue default value.
   * @param xface the interface implemented by the named class.
   * @return property value as a <code>Class</code>, 
   *         or <code>defaultValue</code>.
   */
  public <U> Class<? extends U> getClass(String name, 
                                         Class<? extends U> defaultValue, 
                                         Class<U> xface) {
    try {
      Class<?> theClass = getClass(name, defaultValue);
      if (theClass != null && !xface.isAssignableFrom(theClass))
        throw new RuntimeException(theClass+" not "+xface.getName());
      else if (theClass != null)
        return theClass.asSubclass(xface);
      else
        return null;
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

  /** 
   * Set the value of the <code>name</code> property to the name of a 
   * <code>theClass</code> implementing the given interface <code>xface</code>.
   * 
   * An exception is thrown if <code>theClass</code> does not implement the 
   * interface <code>xface</code>. 
   * 
   * @param name property name.
   * @param theClass property value.
   * @param xface the interface implemented by the named class.
   */
  public void setClass(String name, Class<?> theClass, Class<?> xface) {
    if (!xface.isAssignableFrom(theClass))
      throw new RuntimeException(theClass+" not "+xface.getName());
    set(name, theClass.getName());
  }

  /** 
   * Get a local file under a directory named by <i>dirsProp</i> with
   * the given <i>path</i>.  If <i>dirsProp</i> contains multiple directories,
   * then one is chosen based on <i>path</i>'s hash code.  If the selected
   * directory does not exist, an attempt is made to create it.
   * 
   * @param dirsProp directory in which to locate the file.
   * @param path file-path.
   * @return local file under the directory with the given path.
   */
  public Path getLocalPath(String dirsProp, String path)
    throws IOException {
    String[] dirs = getStrings(dirsProp);
    int hashCode = path.hashCode();
    FileSystem fs = FileSystem.getLocal(this);
    for (int i = 0; i < dirs.length; i++) {  // try each local dir
      int index = (hashCode+i & Integer.MAX_VALUE) % dirs.length;
      Path file = new Path(dirs[index], path);
      Path dir = file.getParent();
      if (fs.mkdirs(dir) || fs.exists(dir)) {
        return file;
      }
    }
    LOG.warn("Could not make " + path + 
             " in local directories from " + dirsProp);
    for(int i=0; i < dirs.length; i++) {
      int index = (hashCode+i & Integer.MAX_VALUE) % dirs.length;
      LOG.warn(dirsProp + "[" + index + "]=" + dirs[index]);
    }
    throw new IOException("No valid local directories in property: "+dirsProp);
  }

  /** 
   * Get a local file name under a directory named in <i>dirsProp</i> with
   * the given <i>path</i>.  If <i>dirsProp</i> contains multiple directories,
   * then one is chosen based on <i>path</i>'s hash code.  If the selected
   * directory does not exist, an attempt is made to create it.
   * 
   * @param dirsProp directory in which to locate the file.
   * @param path file-path.
   * @return local file under the directory with the given path.
   */
  public File getFile(String dirsProp, String path)
    throws IOException {
    String[] dirs = getStrings(dirsProp);
    int hashCode = path.hashCode();
    for (int i = 0; i < dirs.length; i++) {  // try each local dir
      int index = (hashCode+i & Integer.MAX_VALUE) % dirs.length;
      File file = new File(dirs[index], path);
      File dir = file.getParentFile();
      if (dir.exists() || dir.mkdirs()) {
        return file;
      }
    }
    throw new IOException("No valid local directories in property: "+dirsProp);
  }

  /** 
   * Get the {@link URL} for the named resource.
   * 
   * @param name resource name.
   * @return the url for the named resource.
   */
  public URL getResource(String name) {
    return classLoader.getResource(name);
  }
  
  /** 
   * Get an input stream attached to the configuration resource with the
   * given <code>name</code>.
   * 
   * @param name configuration resource name.
   * @return an input stream attached to the resource.
   */
  public InputStream getConfResourceAsInputStream(String name) {
    try {
      URL url= getResource(name);

      if (url == null) {
        LOG.info(name + " not found");
        return null;
      } else {
        LOG.info("found resource " + name + " at " + url);
      }

      return url.openStream();
    } catch (Exception e) {
      return null;
    }
  }

  /** 
   * Get a {@link Reader} attached to the configuration resource with the
   * given <code>name</code>.
   * 
   * @param name configuration resource name.
   * @return a reader attached to the resource.
   */
  public Reader getConfResourceAsReader(String name) {
    try {
      URL url= getResource(name);

      if (url == null) {
        LOG.info(name + " not found");
        return null;
      } else {
        LOG.info("found resource " + name + " at " + url);
      }

      return new InputStreamReader(url.openStream());
    } catch (Exception e) {
      return null;
    }
  }

  private synchronized Properties getProps() {
    if (properties == null) {
      properties = new Properties();
      loadResources(properties, resources, quietmode);
      if (overlay!= null)
        properties.putAll(overlay);
    }
    return properties;
  }

  /**
   * Return the number of keys in the configuration.
   *
   * @return number of keys in the configuration.
   */
  public int size() {
    return getProps().size();
  }

  /**
   * Clears all keys from the configuration.
   */
  public void clear() {
    getProps().clear();
    getOverlay().clear();
  }

  /**
   * Get an {@link Iterator} to go through the list of <code>String</code> 
   * key-value pairs in the configuration.
   * 
   * @return an iterator over the entries.
   */
  public Iterator<Map.Entry<String, String>> iterator() {
    // Get a copy of just the string to string pairs. After the old object
    // methods that allow non-strings to be put into configurations are removed,
    // we could replace properties with a Map<String,String> and get rid of this
    // code.
    Map<String,String> result = new HashMap<String,String>();
    for(Map.Entry<Object,Object> item: getProps().entrySet()) {
      if (item.getKey() instanceof String && 
          item.getValue() instanceof String) {
        result.put((String) item.getKey(), (String) item.getValue());
      }
    }
    return result.entrySet().iterator();
  }

  private void loadResources(Properties properties,
                             ArrayList resources,
                             boolean quiet) {
    if(loadDefaults) {
      for (String resource : defaultResources) {
        loadResource(properties, resource, quiet);
      }
    
      //support the hadoop-site.xml as a deprecated case
      if(getResource("hadoop-site.xml")!=null) {
        loadResource(properties, "hadoop-site.xml", quiet);
      }
    }
    
    for (Object resource : resources) {
      loadResource(properties, resource, quiet);
    }
  }

  private void loadResource(Properties properties, Object name, boolean quiet) {
    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) {
        LOG.error("Failed to set setXIncludeAware(true) for parser "
                + docBuilderFactory
                + ":" + e,
                e);
      }
      DocumentBuilder builder = docBuilderFactory.newDocumentBuilder();
      Document doc = null;
      Element root = null;

      if (name instanceof URL) {                  // an URL resource
        URL url = (URL)name;
        if (url != null) {
          if (!quiet) {
            LOG.info("parsing " + url);
          }
          doc = builder.parse(url.toString());
        }
      } else if (name instanceof String) {        // a CLASSPATH resource
        URL url = getResource((String)name);
        if (url != null) {
          if (!quiet) {
            LOG.info("parsing " + url);
          }
          doc = builder.parse(url.toString());
        }
      } else if (name instanceof Path) {          // a file resource
        // Can't use FileSystem API or we get an infinite loop
        // since FileSystem uses Configuration API.  Use java.io.File instead.
        File file = new File(((Path)name).toUri().getPath())
          .getAbsoluteFile();
        if (file.exists()) {
          if (!quiet) {
            LOG.info("parsing " + file);
          }
          InputStream in = new BufferedInputStream(new FileInputStream(file));
          try {
            doc = builder.parse(in);
          } finally {
            in.close();
          }
        }
      } else if (name instanceof InputStream) {
        try {
          doc = builder.parse((InputStream)name);
        } finally {
          ((InputStream)name).close();
        }
      } else if (name instanceof Element) {
        root = (Element)name;
      }

      if (doc == null && root == null) {
        if (quiet)
          return;
        throw new RuntimeException(name + " not found");
      }

      if (root == null) {
        root = doc.getDocumentElement();
      }
      if (!"configuration".equals(root.getTagName()))
        LOG.fatal("bad conf file: top-level element not <configuration>");
      NodeList props = root.getChildNodes();
      for (int i = 0; i < props.getLength(); i++) {
        Node propNode = props.item(i);
        if (!(propNode instanceof Element))
          continue;
        Element prop = (Element)propNode;
        if ("configuration".equals(prop.getTagName())) {
          loadResource(properties, prop, quiet);
          continue;
        }
        if (!"property".equals(prop.getTagName()))
          LOG.warn("bad conf file: element not <property>");
        NodeList fields = prop.getChildNodes();
        String attr = null;
        String value = null;
        boolean finalParameter = false;
        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();
          if ("final".equals(field.getTagName()) && field.hasChildNodes())
            finalParameter = "true".equals(((Text)field.getFirstChild()).getData());
        }
        
        // Ignore this parameter if it has already been marked as 'final'
        if (attr != null && value != null) {
          if (!finalParameters.contains(attr)) {
            properties.setProperty(attr, value);
            if (finalParameter)
              finalParameters.add(attr);
          } else {
            LOG.warn(name+":a attempt to override final parameter: "+attr
                     +";  Ignoring.");
          }
        }
      }
        
    } catch (IOException e) {
      LOG.fatal("error parsing conf file: " + e);
      throw new RuntimeException(e);
    } catch (DOMException e) {
      LOG.fatal("error parsing conf file: " + e);
      throw new RuntimeException(e);
    } catch (SAXException e) {
      LOG.fatal("error parsing conf file: " + e);
      throw new RuntimeException(e);
    } catch (ParserConfigurationException e) {
      LOG.fatal("error parsing conf file: " + e);
      throw new RuntimeException(e);
    }
  }

  /** 
   * Write out the non-default properties in this configuration to the give
   * {@link OutputStream}.
   * 
   * @param out the output stream to write to.
   */
  public void writeXml(OutputStream out) throws IOException {
    Properties properties = getProps();
    try {
      Document doc =
        DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();
      Element conf = doc.createElement("configuration");
      doc.appendChild(conf);
      conf.appendChild(doc.createTextNode("\n"));
      for (Enumeration e = properties.keys(); e.hasMoreElements();) {
        String name = (String)e.nextElement();
        Object object = properties.get(name);
        String value = null;
        if (object instanceof String) {
          value = (String) object;
        }else {
          continue;
        }
        Element propNode = doc.createElement("property");
        conf.appendChild(propNode);
      
        Element nameNode = doc.createElement("name");
        nameNode.appendChild(doc.createTextNode(name));
        propNode.appendChild(nameNode);
      
        Element valueNode = doc.createElement("value");
        valueNode.appendChild(doc.createTextNode(value));
        propNode.appendChild(valueNode);

        conf.appendChild(doc.createTextNode("\n"));
      }
    
      DOMSource source = new DOMSource(doc);
      StreamResult result = new StreamResult(out);
      TransformerFactory transFactory = TransformerFactory.newInstance();
      Transformer transformer = transFactory.newTransformer();
      transformer.transform(source, result);
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

  /**
   * Get the {@link ClassLoader} for this job.
   * 
   * @return the correct class loader.
   */
  public ClassLoader getClassLoader() {
    return classLoader;
  }
  
  /**
   * Set the class loader that will be used to load the various objects.
   * 
   * @param classLoader the new class loader.
   */
  public void setClassLoader(ClassLoader classLoader) {
    this.classLoader = classLoader;
  }
  
  @Override
  public String toString() {
    StringBuffer sb = new StringBuffer();
    sb.append("Configuration: ");
    if(loadDefaults) {
      toString(defaultResources, sb);
      if(resources.size()>0) {
        sb.append(", ");
      }
    }
    toString(resources, sb);
    return sb.toString();
  }

  private void toString(List resources, StringBuffer sb) {
    ListIterator i = resources.listIterator();
    while (i.hasNext()) {
      if (i.nextIndex() != 0) {
        sb.append(", ");
      }
      sb.append(i.next());
    }
  }

  /** 
   * Set the quietness-mode. 
   * 
   * In the quiet-mode, error and informational messages might not be logged.
   * 
   * @param quietmode <code>true</code> to set quiet-mode on, <code>false</code>
   *              to turn it off.
   */
  public synchronized void setQuietMode(boolean quietmode) {
    this.quietmode = quietmode;
  }

  /** For debugging.  List non-default properties to the terminal and exit. */
  public static void main(String[] args) throws Exception {
    new Configuration().writeXml(System.out);
  }

  @Override
  public void readFields(DataInput in) throws IOException {
    clear();
    int size = WritableUtils.readVInt(in);
    for(int i=0; i < size; ++i) {
      set(org.apache.hadoop.io.Text.readString(in), 
          org.apache.hadoop.io.Text.readString(in));
    }
  }

  //@Override
  public void write(DataOutput out) throws IOException {
    Properties props = getProps();
    WritableUtils.writeVInt(out, props.size());
    for(Map.Entry<Object, Object> item: props.entrySet()) {
      org.apache.hadoop.io.Text.writeString(out, (String) item.getKey());
      org.apache.hadoop.io.Text.writeString(out, (String) item.getValue());
    }
  }

}

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