hadoop中的Writable分析

杨叔叔

　　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。

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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;
}