Hadoop 要使一个类能序例化， 要实现Writable接口， Writable 调用DataInput和DataOutput实现序例化。
DataOutput是JDK中IO包下的一个类， 提供了writeBoolean, writeByte, writeShort。等方法了。
这样让用户决定哪一个字段序例化， 怎么反序例化。
在org.apache.hadoop.io包下包含了大量的可序列化的组件，它们都实现了Writable接口，Writable接口提供了两个方法，write和readFields，分别用来序列化和反序列化。
/** * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you 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 * * * * 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; import org.apache.hadoop.classification.InterfaceAudience; import org.apache.hadoop.classification.InterfaceStability; /** * A serializable object which implements a simple, efficient, serialization * protocol, based on {@link DataInput} and {@link DataOutput}. * * <p>Any <code>key</code> or <code>value</code> type in the Hadoop Map-Reduce * framework implements this interface.</p> * * <p>Implementations typically implement a static <code>read(DataInput)</code> * method which constructs a new instance, calls {@link #readFields(DataInput)} * and returns the instance.</p> * * <p>Example:</p> * <p><blockquote><pre> * public class MyWritable implements Writable { * // Some data * private int counter; * private long timestamp; * * public void write(DataOutput out) throws IOException { * out.writeInt(counter); * out.writeLong(timestamp); * } * * public void readFields(DataInput in) throws IOException { * counter = in.readInt(); * timestamp = in.readLong(); * } * * public static MyWritable read(DataInput in) throws IOException { * MyWritable w = new MyWritable(); * w.readFields(in); * return w; * } * } * </pre></blockquote></p> */ @InterfaceAudience.Public @InterfaceStability.Stable public interface Writable { /** * Serialize the fields of this object to <code>out</code>. * * @param out <code>DataOuput</code> to serialize this object into. * @throws IOException */ void write(DataOutput out) throws IOException; /** * Deserialize the fields of this object from <code>in</code>. * * <p>For efficiency, implementations should attempt to re-use storage in the * existing object where possible.</p> * * @param in <code>DataInput</code> to deseriablize this object from. * @throws IOException */ void readFields(DataInput in) throws IOException; }