hadoop复合键排序使用方法

在Hadoop中处理复杂业务时，需要用到复合键，复合不同于单纯的继承Writable接口，而是继承了WritableComparable接口，而实际上，WritableComparable接口继承了Writable和Comparable接口，如果只需要使用某一个类作为传值对象而不是作为key，继承Writable接口即可。
public interface WritableComparable extends Writable, Comparable {
}

public interface Writable {

void write(DataOutput out) throws IOException;

void readFields(DataInput in) throws IOException;
}

public interface Comparable {

public int compareTo(T o);  

}
以下是实现复合key的实例，亲测，可用
[java] view plain copy
print?
public class SortKey implements WritableComparable{

private Text name;  private IntWritable right;  public SortKey() {      set(new Text(), new IntWritable());  }  public SortKey(Text name, IntWritable right) {      set(name, right);  }  private void set(Text name,IntWritable right){      this.name = name;      this.right = right;  }  /**  * @return the name  */  public Text getName() {      return name;  }  /**  * @param name the name to set  */  public void setName(Text name) {      this.name = name;  }  /**  * @return the right  */  public IntWritable getRight() {      return right;  }  /**  * @param right the right to set  */  public void setRight(IntWritable right) {      this.right = right;  }  @Override  public void write(DataOutput out) throws IOException {      name.write(out);      right.write(out);  }  @Override  public void readFields(DataInput in) throws IOException {      name.readFields(in);      right.readFields(in);  }  @Override  public int compareTo(SortKey o) {      int cmp = name.compareTo(o.name);      if(cmp != 0){          return cmp;      }else{          return right.compareTo(o.right);      }  }  

//到目前为止，你只能将其作为key来使用，但是如果你需要按照key的某一个值来排序，以下是重点
static{
WritableComparator.define(SortKey.class, new Comparator());
}

public static class Comparator extends WritableComparator{      private static final Text.Comparator TEXT_COMPARATOR = new Text.Comparator();      protected Comparator() {          super(SortKey.class);      }      /* (non-Javadoc)      * @see org.apache.hadoop.io.WritableComparator#compare(byte[], int, int, byte[], int, int)      */      @Override      public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {          try{              int firstL1 = WritableUtils.decodeVIntSize(b1[s1]) + readVInt(b1, s1);              int firstL2 = WritableUtils.decodeVIntSize(b2[s2]) + readVInt(b2, s2);              return TEXT_COMPARATOR.compare(b1, s1, firstL1, b2, s2, firstL2);          }catch(Exception e){              throw new IllegalArgumentException(e);          }      }     }  

}