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.DataOutput; |
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22 | import java.io.DataInput; |
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23 | import java.io.IOException; |
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24 | |
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25 | /** A base class for Writables that provides version checking. |
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26 | * |
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27 | * <p>This is useful when a class may evolve, so that instances written by the |
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28 | * old version of the class may still be processed by the new version. To |
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29 | * handle this situation, {@link #readFields(DataInput)} |
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30 | * implementations should catch {@link VersionMismatchException}. |
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31 | */ |
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32 | public abstract class VersionedWritable implements Writable { |
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33 | |
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34 | /** Return the version number of the current implementation. */ |
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35 | public abstract byte getVersion(); |
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36 | |
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37 | // javadoc from Writable |
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38 | public void write(DataOutput out) throws IOException { |
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39 | out.writeByte(getVersion()); // store version |
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40 | } |
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41 | |
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42 | // javadoc from Writable |
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43 | public void readFields(DataInput in) throws IOException { |
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44 | byte version = in.readByte(); // read version |
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45 | if (version != getVersion()) |
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46 | throw new VersionMismatchException(getVersion(), version); |
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47 | } |
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48 | |
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49 | |
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50 | } |
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