Hadoop WritableComparable接口

WritableComparable接口

Writable接口大家可能都知道，它是一个实现了序列化协议的序列化对象。在Hadoop中定义一个结构化对象都要实现Writable接口，使得该结构化对象可以序列化为字节流，字节流也可以反序列化为结构化对象。那WritableComparable接口是可序列化并且可比较的接口。MapReduce中所有的key值类型都必须实现这个接口，既然是可序列化的那就必须得实现readFiels（）和write（）这两个序列化和反序列化函数，既然也是可比较的那就必须得实现compareTo（）函数，该函数即是比较和排序规则的实现。这样MR中的key值就既能可序列化又是可比较的。下面几符图是API中对WritableComparable接口的解释及其方法，还有一个实现了该接口的对象的列子：

public interface WritableComparable<T>

     extends 

     Writable, 

     Comparable<T>

    

A Writable which is alsoComparable.

WritableComparables can be compared to each other, typically via Comparators. Any type which is to be used as a key in the Hadoop Map-Reduce framework should implement this interface.

Example:

 

     public class MyWritableComparable implements WritableComparable {

       // 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 int compareTo(MyWritableComparable w) {

         int thisValue = this.value;

         int thatValue = ((IntWritable)o).value;

         return (thisValue < thatValue ? -1 : (thisValue==thatValue ? 0 : 1));

       }

     }