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
019package org.apache.hadoop.io;
020
021import java.io.DataOutput;
022import java.io.DataInput;
023import java.io.IOException;
024
025import org.apache.hadoop.classification.InterfaceAudience;
026import 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
037public 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  public void write(DataOutput out) throws IOException {
044    out.writeByte(getVersion());                  // store version
045  }
046
047  // javadoc from Writable
048  public void readFields(DataInput in) throws IOException {
049    byte version = in.readByte();                 // read version
050    if (version != getVersion())
051      throw new VersionMismatchException(getVersion(), version);
052  }
053
054    
055}