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 }