Hadoop中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

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

WritableComparable：

WritableComparable<T> extends Writable, Comparable<T>

public interface Comparable<T> {
    public int 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;