/** * Copyright 2005 The Apache Software Foundation * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *     http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package org.apache.hadoop.io;import java.io.DataOutput;import java.io.DataInput;import java.io.IOException;/** A base class for Writables that provides version checking. * * <p>This is useful when a class may evolve, so that instances written by the * old version of the class may still be processed by the new version.  To * handle this situation, {@link #readFields(DataInput)} * implementations should catch {@link VersionMismatchException}. * * @author Doug Cutting */public abstract class VersionedWritable implements Writable {  /** Return the version number of the current implementation. */  public abstract byte getVersion();      // javadoc from Writable  public void write(DataOutput out) throws IOException {    out.writeByte(getVersion());                  // store version  }  // javadoc from Writable  public void readFields(DataInput in) throws IOException {    byte version = in.readByte();                 // read version    if (version != getVersion())      throw new VersionMismatchException(getVersion(),version);  }    }