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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