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    /**
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
066    public 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    }