MapReduce（5）----序列化

一、序列化介绍

二、Writable和WritableComparable

Hadoop的key和value的传递序列化需要涉及两个重要的接口Writable和WritableComparable

1、Writable：

void write(DataOutput out) throws IOException;   //读数据的方式
void readFields(DataInput in) throws IOException; //写数据的方式

2、WritableComparable：

WritableComparable<T> extends Writable, Comparable<T> 

publicinterface Comparable<T> { 
    publicint compareTo(T o); 
} 

比Writable多了一个compareTo方法，这个的用途是是为了确定是不是相同的key，因此得出如下结论：

hadoop为Key的数据类型必须实现WritableComparable，而Value的数据类型只需要实现Writable即可，能做Key的一定可以做Value，能做Value的未必能做Key.

常用的WritableComparable实现如下：

org.apache.hadoop.io.NullWritable; 
org.apache.hadoop.io.BooleanWritable; 
org.apache.hadoop.io.BytesWritable; 
org.apache.hadoop.io.DoubleWritable; 
org.apache.hadoop.io.FloatWritable; 
org.apache.hadoop.io.IntWritable; 
org.apache.hadoop.io.LongWritable; 
org.apache.hadoop.io.MD5Hash; 
org.apache.hadoop.io.Text; 
org.apache.hadoop.io.UTF8; 
org.apache.hadoop.io.VIntWritable; 
org.apache.hadoop.io.VLongWritable; 

常用的Writable实现如下(除了上述的)：

org.apache.hadoop.io.TwoDArrayWritable; 
org.apache.hadoop.io.SortedMapWritable; 
org.apache.hadoop.io.ObjectWritable; 
org.apache.hadoop.io.MapWritable; 
org.apache.hadoop.io.ArrayWritable;

 

 

 

参考文献：

https://www.cnblogs.com/likai198981/archive/2013/06/01/3112531.html