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 }