Hadoop中Writable和WritableComparable区别
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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;
- void readFields(DataInput in) throws IOException;
也就是读数据和写数据的方式
2> WritableComparable:
- WritableComparable<T> extends Writable, Comparable<T>
- publicinterface Comparable<T> {
- publicint compareTo(T o);
- }
也就是比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;
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