001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    
019    package org.apache.hadoop.io;
020    
021    import java.io.DataOutput;
022    import java.io.DataInput;
023    import java.io.IOException;
024    
025    import org.apache.hadoop.classification.InterfaceAudience;
026    import org.apache.hadoop.classification.InterfaceStability;
027    
028    /** A base class for Writables that provides version checking.
029     *
030     * <p>This is useful when a class may evolve, so that instances written by the
031     * old version of the class may still be processed by the new version.  To
032     * handle this situation, {@link #readFields(DataInput)}
033     * implementations should catch {@link VersionMismatchException}.
034     */
035    @InterfaceAudience.Public
036    @InterfaceStability.Stable
037    public abstract class VersionedWritable implements Writable {
038    
039      /** Return the version number of the current implementation. */
040      public abstract byte getVersion();
041        
042      // javadoc from Writable
043      @Override
044      public void write(DataOutput out) throws IOException {
045        out.writeByte(getVersion());                  // store version
046      }
047    
048      // javadoc from Writable
049      @Override
050      public void readFields(DataInput in) throws IOException {
051        byte version = in.readByte();                 // read version
052        if (version != getVersion())
053          throw new VersionMismatchException(getVersion(), version);
054      }
055    
056        
057    }