| /** |
| * 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.io.DataInput; |
| import java.io.DataOutput; |
| import java.io.IOException; |
| |
| import org.apache.hadoop.classification.InterfaceAudience; |
| import org.apache.hadoop.classification.InterfaceStability; |
| import org.apache.hadoop.conf.Configurable; |
| import org.apache.hadoop.conf.Configuration; |
| import org.apache.hadoop.util.ReflectionUtils; |
| |
| /** |
| * A wrapper for Writable instances. |
| * <p> |
| * When two sequence files, which have same Key type but different Value |
| * types, are mapped out to reduce, multiple Value types is not allowed. |
| * In this case, this class can help you wrap instances with different types. |
| * </p> |
| * |
| * <p> |
| * Compared with <code>ObjectWritable</code>, this class is much more effective, |
| * because <code>ObjectWritable</code> will append the class declaration as a String |
| * into the output file in every Key-Value pair. |
| * </p> |
| * |
| * <p> |
| * Generic Writable implements {@link Configurable} interface, so that it will be |
| * configured by the framework. The configuration is passed to the wrapped objects |
| * implementing {@link Configurable} interface <i>before deserialization</i>. |
| * </p> |
| * |
| * how to use it: <br> |
| * 1. Write your own class, such as GenericObject, which extends GenericWritable.<br> |
| * 2. Implements the abstract method <code>getTypes()</code>, defines |
| * the classes which will be wrapped in GenericObject in application. |
| * Attention: this classes defined in <code>getTypes()</code> method, must |
| * implement <code>Writable</code> interface. |
| * <br><br> |
| * |
| * The code looks like this: |
| * <blockquote><pre> |
| * public class GenericObject extends GenericWritable { |
| * |
| * private static Class[] CLASSES = { |
| * ClassType1.class, |
| * ClassType2.class, |
| * ClassType3.class, |
| * }; |
| * |
| * protected Class[] getTypes() { |
| * return CLASSES; |
| * } |
| * |
| * } |
| * </pre></blockquote> |
| * |
| * @since Nov 8, 2006 |
| */ |
| @InterfaceAudience.Public |
| @InterfaceStability.Stable |
| public abstract class GenericWritable implements Writable, Configurable { |
| |
| private static final byte NOT_SET = -1; |
| |
| private byte type = NOT_SET; |
| |
| private Writable instance; |
| |
| private Configuration conf = null; |
| |
| /** |
| * Set the instance that is wrapped. |
| * |
| * @param obj |
| */ |
| public void set(Writable obj) { |
| instance = obj; |
| Class<? extends Writable> instanceClazz = instance.getClass(); |
| Class<? extends Writable>[] clazzes = getTypes(); |
| for (int i = 0; i < clazzes.length; i++) { |
| Class<? extends Writable> clazz = clazzes[i]; |
| if (clazz.equals(instanceClazz)) { |
| type = (byte) i; |
| return; |
| } |
| } |
| throw new RuntimeException("The type of instance is: " |
| + instance.getClass() + ", which is NOT registered."); |
| } |
| |
| /** |
| * Return the wrapped instance. |
| */ |
| public Writable get() { |
| return instance; |
| } |
| |
| public String toString() { |
| return "GW[" + (instance != null ? ("class=" + instance.getClass().getName() + |
| ",value=" + instance.toString()) : "(null)") + "]"; |
| } |
| |
| public void readFields(DataInput in) throws IOException { |
| type = in.readByte(); |
| Class<? extends Writable> clazz = getTypes()[type & 0xff]; |
| try { |
| instance = ReflectionUtils.newInstance(clazz, conf); |
| } catch (Exception e) { |
| e.printStackTrace(); |
| throw new IOException("Cannot initialize the class: " + clazz); |
| } |
| instance.readFields(in); |
| } |
| |
| public void write(DataOutput out) throws IOException { |
| if (type == NOT_SET || instance == null) |
| throw new IOException("The GenericWritable has NOT been set correctly. type=" |
| + type + ", instance=" + instance); |
| out.writeByte(type); |
| instance.write(out); |
| } |
| |
| /** |
| * Return all classes that may be wrapped. Subclasses should implement this |
| * to return a constant array of classes. |
| */ |
| abstract protected Class<? extends Writable>[] getTypes(); |
| |
| public Configuration getConf() { |
| return conf; |
| } |
| |
| public void setConf(Configuration conf) { |
| this.conf = conf; |
| } |
| |
| } |