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.util.HashMap; |
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22 | import java.io.IOException; |
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23 | |
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24 | import org.apache.hadoop.conf.Configuration; |
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25 | |
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26 | /** Utility to permit renaming of Writable implementation classes without |
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27 | * invalidiating files that contain their class name. |
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28 | */ |
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29 | public class WritableName { |
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30 | private static HashMap<String, Class<?>> NAME_TO_CLASS = |
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31 | new HashMap<String, Class<?>>(); |
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32 | private static HashMap<Class<?>, String> CLASS_TO_NAME = |
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33 | new HashMap<Class<?>, String>(); |
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34 | |
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35 | static { // define important types |
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36 | WritableName.setName(NullWritable.class, "null"); |
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37 | WritableName.setName(LongWritable.class, "long"); |
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38 | WritableName.setName(UTF8.class, "UTF8"); |
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39 | WritableName.setName(MD5Hash.class, "MD5Hash"); |
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40 | } |
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41 | |
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42 | private WritableName() {} // no public ctor |
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43 | |
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44 | /** Set the name that a class should be known as to something other than the |
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45 | * class name. */ |
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46 | public static synchronized void setName(Class writableClass, String name) { |
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47 | CLASS_TO_NAME.put(writableClass, name); |
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48 | NAME_TO_CLASS.put(name, writableClass); |
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49 | } |
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50 | |
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51 | /** Add an alternate name for a class. */ |
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52 | public static synchronized void addName(Class writableClass, String name) { |
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53 | NAME_TO_CLASS.put(name, writableClass); |
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54 | } |
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55 | |
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56 | /** Return the name for a class. Default is {@link Class#getName()}. */ |
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57 | public static synchronized String getName(Class writableClass) { |
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58 | String name = CLASS_TO_NAME.get(writableClass); |
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59 | if (name != null) |
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60 | return name; |
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61 | return writableClass.getName(); |
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62 | } |
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63 | |
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64 | /** Return the class for a name. Default is {@link Class#forName(String)}.*/ |
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65 | public static synchronized Class<?> getClass(String name, Configuration conf |
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66 | ) throws IOException { |
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67 | Class<?> writableClass = NAME_TO_CLASS.get(name); |
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68 | if (writableClass != null) |
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69 | return writableClass.asSubclass(Writable.class); |
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70 | try { |
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71 | return conf.getClassByName(name); |
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72 | } catch (ClassNotFoundException e) { |
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73 | IOException newE = new IOException("WritableName can't load class: " + name); |
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74 | newE.initCause(e); |
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75 | throw newE; |
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76 | } |
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77 | } |
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78 | |
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79 | } |
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