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<T>接口，而实际上，WritableComparable<T>接口继承了 Writable和Comparable<T>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。

上源码：

    public interface WritableComparable<T> extends Writable, Comparable<T> {
    }

public interface Writable {  

  void write(DataOutput out) throws IOException;  

  void readFields(DataInput in) throws IOException;
}

    public interface Comparable<T> {  

        public int compareTo(T o);
    }

以下是实现复合key的实例：

public class SortKey implements WritableComparable<SortKey>{  

    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);
        }
    }

    <span style="white-space:pre">    </span>//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);
            }
        }
    }  

}