/** * Copyright 2005 The Apache Software Foundation * * Licensed 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.io;import java.util.HashMap;import java.io.IOException;/** Utility to permit renaming of Writable implementation classes without * invalidiating files that contain their class name. * @author Doug Cutting */public class WritableName {  private static HashMap NAME_TO_CLASS = new HashMap();  private static HashMap CLASS_TO_NAME = new HashMap();  static {                                        // define important types    WritableName.setName(NullWritable.class, "null");    WritableName.setName(LongWritable.class, "long");    WritableName.setName(UTF8.class, "UTF8");    WritableName.setName(MD5Hash.class, "MD5Hash");  }  private WritableName() {}                      // no public ctor  /** Set the name that a class should be known as to something other than the   * class name. */  public static synchronized void setName(Class writableClass, String name) {    CLASS_TO_NAME.put(writableClass, name);    NAME_TO_CLASS.put(name, writableClass);  }  /** Add an alternate name for a class. */  public static synchronized void addName(Class writableClass, String name) {    NAME_TO_CLASS.put(name, writableClass);  }  /** Return the name for a class.  Default is {@link Class#getName()}. */  public static synchronized String getName(Class writableClass) {    String name = (String)CLASS_TO_NAME.get(writableClass);    if (name != null)      return name;    return writableClass.getName();  }  /** Return the class for a name.  Default is {@link Class#forName(String)}.*/  public static synchronized Class getClass(String name) throws IOException {    Class writableClass = (Class)NAME_TO_CLASS.get(name);    if (writableClass != null)      return writableClass;    try {      return Class.forName(name);    } catch (ClassNotFoundException e) {      throw new IOException(e.toString());    }  }}