Hadoop WritableComparable接口

WritableComparable接口

Writable接口大家可能都知道，它是一個實現了序列化協議的序列化對象。在Hadoop中定義一個結構化對象都要實現Writable接口，使得該結構化對象可以序列化為字節流，字節流也可以反序列化為結構化對象。那WritableComparable接口是可序列化並且可比較的接口。MapReduce中所有的key值類型都必須實現這個接口，既然是可序列化的那就必須得實現readFiels（）和write（）這兩個序列化和反序列化函數，既然也是可比較的那就必須得實現compareTo（）函數，該函數即是比較和排序規則的實現。這樣MR中的key值就既能可序列化又是可比較的。下面幾符圖是API中對WritableComparable接口的解釋及其方法，還有一個實現了該接口的對象的列子：

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));
       }
     }