WritableName.java
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上传日期:2018-01-08
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文件大小:3k
- /**
- * 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.io;
- import java.util.HashMap;
- import java.io.IOException;
- import org.apache.hadoop.conf.Configuration;
- /** Utility to permit renaming of Writable implementation classes without
- * invalidiating files that contain their class name.
- */
- public class WritableName {
- private static HashMap<String, Class<?>> NAME_TO_CLASS =
- new HashMap<String, Class<?>>();
- private static HashMap<Class<?>, String> CLASS_TO_NAME =
- new HashMap<Class<?>, String>();
- 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 = 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, Configuration conf
- ) throws IOException {
- Class<?> writableClass = NAME_TO_CLASS.get(name);
- if (writableClass != null)
- return writableClass.asSubclass(Writable.class);
- try {
- return conf.getClassByName(name);
- } catch (ClassNotFoundException e) {
- IOException newE = new IOException("WritableName can't load class: " + name);
- newE.initCause(e);
- throw newE;
- }
- }
- }