[120] | 1 | /** |
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| 2 | * Licensed to the Apache Software Foundation (ASF) under one |
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| 3 | * or more contributor license agreements. See the NOTICE file |
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| 4 | * distributed with this work for additional information |
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| 5 | * regarding copyright ownership. The ASF licenses this file |
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| 6 | * to you under the Apache License, Version 2.0 (the |
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| 7 | * "License"); you may not use this file except in compliance |
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| 8 | * with the License. You may obtain a copy of the License at |
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| 9 | * |
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| 10 | * http://www.apache.org/licenses/LICENSE-2.0 |
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| 11 | * |
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| 12 | * Unless required by applicable law or agreed to in writing, software |
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| 13 | * distributed under the License is distributed on an "AS IS" BASIS, |
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| 14 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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| 15 | * See the License for the specific language governing permissions and |
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| 16 | * limitations under the License. |
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| 17 | */ |
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| 18 | |
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| 19 | package org.apache.hadoop.io; |
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| 20 | |
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| 21 | import java.io.DataInput; |
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| 22 | import java.io.DataOutput; |
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| 23 | import java.io.IOException; |
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| 24 | |
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| 25 | import org.apache.hadoop.conf.Configurable; |
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| 26 | import org.apache.hadoop.conf.Configuration; |
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| 27 | import org.apache.hadoop.util.ReflectionUtils; |
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| 28 | |
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| 29 | /** |
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| 30 | * A wrapper for Writable instances. |
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| 31 | * <p> |
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| 32 | * When two sequence files, which have same Key type but different Value |
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| 33 | * types, are mapped out to reduce, multiple Value types is not allowed. |
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| 34 | * In this case, this class can help you wrap instances with different types. |
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| 35 | * </p> |
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| 36 | * |
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| 37 | * <p> |
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| 38 | * Compared with <code>ObjectWritable</code>, this class is much more effective, |
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| 39 | * because <code>ObjectWritable</code> will append the class declaration as a String |
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| 40 | * into the output file in every Key-Value pair. |
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| 41 | * </p> |
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| 42 | * |
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| 43 | * <p> |
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| 44 | * Generic Writable implements {@link Configurable} interface, so that it will be |
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| 45 | * configured by the framework. The configuration is passed to the wrapped objects |
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| 46 | * implementing {@link Configurable} interface <i>before deserialization</i>. |
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| 47 | * </p> |
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| 48 | * |
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| 49 | * how to use it: <br> |
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| 50 | * 1. Write your own class, such as GenericObject, which extends GenericWritable.<br> |
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| 51 | * 2. Implements the abstract method <code>getTypes()</code>, defines |
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| 52 | * the classes which will be wrapped in GenericObject in application. |
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| 53 | * Attention: this classes defined in <code>getTypes()</code> method, must |
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| 54 | * implement <code>Writable</code> interface. |
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| 55 | * <br><br> |
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| 56 | * |
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| 57 | * The code looks like this: |
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| 58 | * <blockquote><pre> |
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| 59 | * public class GenericObject extends GenericWritable { |
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| 60 | * |
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| 61 | * private static Class[] CLASSES = { |
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| 62 | * ClassType1.class, |
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| 63 | * ClassType2.class, |
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| 64 | * ClassType3.class, |
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| 65 | * }; |
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| 66 | * |
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| 67 | * protected Class[] getTypes() { |
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| 68 | * return CLASSES; |
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| 69 | * } |
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| 70 | * |
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| 71 | * } |
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| 72 | * </pre></blockquote> |
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| 73 | * |
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| 74 | * @since Nov 8, 2006 |
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| 75 | */ |
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| 76 | public abstract class GenericWritable implements Writable, Configurable { |
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| 77 | |
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| 78 | private static final byte NOT_SET = -1; |
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| 79 | |
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| 80 | private byte type = NOT_SET; |
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| 81 | |
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| 82 | private Writable instance; |
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| 83 | |
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| 84 | private Configuration conf = null; |
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| 85 | |
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| 86 | /** |
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| 87 | * Set the instance that is wrapped. |
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| 88 | * |
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| 89 | * @param obj |
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| 90 | */ |
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| 91 | public void set(Writable obj) { |
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| 92 | instance = obj; |
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| 93 | Class<? extends Writable> instanceClazz = instance.getClass(); |
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| 94 | Class<? extends Writable>[] clazzes = getTypes(); |
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| 95 | for (int i = 0; i < clazzes.length; i++) { |
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| 96 | Class<? extends Writable> clazz = clazzes[i]; |
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| 97 | if (clazz.equals(instanceClazz)) { |
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| 98 | type = (byte) i; |
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| 99 | return; |
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| 100 | } |
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| 101 | } |
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| 102 | throw new RuntimeException("The type of instance is: " |
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| 103 | + instance.getClass() + ", which is NOT registered."); |
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| 104 | } |
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| 105 | |
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| 106 | /** |
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| 107 | * Return the wrapped instance. |
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| 108 | */ |
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| 109 | public Writable get() { |
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| 110 | return instance; |
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| 111 | } |
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| 112 | |
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| 113 | public String toString() { |
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| 114 | return "GW[" + (instance != null ? ("class=" + instance.getClass().getName() + |
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| 115 | ",value=" + instance.toString()) : "(null)") + "]"; |
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| 116 | } |
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| 117 | |
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| 118 | public void readFields(DataInput in) throws IOException { |
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| 119 | type = in.readByte(); |
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| 120 | Class<? extends Writable> clazz = getTypes()[type & 0xff]; |
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| 121 | try { |
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| 122 | instance = ReflectionUtils.newInstance(clazz, conf); |
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| 123 | } catch (Exception e) { |
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| 124 | e.printStackTrace(); |
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| 125 | throw new IOException("Cannot initialize the class: " + clazz); |
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| 126 | } |
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| 127 | instance.readFields(in); |
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| 128 | } |
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| 129 | |
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| 130 | public void write(DataOutput out) throws IOException { |
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| 131 | if (type == NOT_SET || instance == null) |
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| 132 | throw new IOException("The GenericWritable has NOT been set correctly. type=" |
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| 133 | + type + ", instance=" + instance); |
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| 134 | out.writeByte(type); |
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| 135 | instance.write(out); |
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| 136 | } |
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| 137 | |
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| 138 | /** |
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| 139 | * Return all classes that may be wrapped. Subclasses should implement this |
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| 140 | * to return a constant array of classes. |
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| 141 | */ |
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| 142 | abstract protected Class<? extends Writable>[] getTypes(); |
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| 143 | |
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| 144 | public Configuration getConf() { |
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| 145 | return conf; |
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| 146 | } |
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| 147 | |
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| 148 | public void setConf(Configuration conf) { |
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| 149 | this.conf = conf; |
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| 150 | } |
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| 151 | |
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| 152 | } |
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