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/**
029 * A serializable object which implements a simple, efficient, serialization 
030 * protocol, based on {@link DataInput} and {@link DataOutput}.
031 *
032 * <p>Any <code>key</code> or <code>value</code> type in the Hadoop Map-Reduce
033 * framework implements this interface.</p>
034 * 
035 * <p>Implementations typically implement a static <code>read(DataInput)</code>
036 * method which constructs a new instance, calls {@link #readFields(DataInput)} 
037 * and returns the instance.</p>
038 * 
039 * <p>Example:</p>
040 * <p><blockquote><pre>
041 *     public class MyWritable implements Writable {
042 *       // Some data     
043 *       private int counter;
044 *       private long timestamp;
045 *       
046 *       public void write(DataOutput out) throws IOException {
047 *         out.writeInt(counter);
048 *         out.writeLong(timestamp);
049 *       }
050 *       
051 *       public void readFields(DataInput in) throws IOException {
052 *         counter = in.readInt();
053 *         timestamp = in.readLong();
054 *       }
055 *       
056 *       public static MyWritable read(DataInput in) throws IOException {
057 *         MyWritable w = new MyWritable();
058 *         w.readFields(in);
059 *         return w;
060 *       }
061 *     }
062 * </pre></blockquote></p>
063 */
064@InterfaceAudience.Public
065@InterfaceStability.Stable
066public interface Writable {
067  /** 
068   * Serialize the fields of this object to <code>out</code>.
069   * 
070   * @param out <code>DataOuput</code> to serialize this object into.
071   * @throws IOException
072   */
073  void write(DataOutput out) throws IOException;
074
075  /** 
076   * Deserialize the fields of this object from <code>in</code>.  
077   * 
078   * <p>For efficiency, implementations should attempt to re-use storage in the 
079   * existing object where possible.</p>
080   * 
081   * @param in <code>DataInput</code> to deseriablize this object from.
082   * @throws IOException
083   */
084  void readFields(DataInput in) throws IOException;
085}