MapReduce 分組GroupingComparator

public class GroupingComparator extends WritableComparator {

    //傳入作爲key的bean的class類型，以及制定需要讓框架做反射獲取實例對象
    protected ItemidGroupingComparator() {
        super(OrderBean.class, true);
    }


    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        OrderBean abean = (OrderBean) a;
        OrderBean bbean = (OrderBean) b;

        //比較兩個bean時，指定只比較bean中的orderid
        return abean.getItemid().compareTo(bbean.getItemid());

    }

}

//在此設置自定義的Groupingcomparator類                        job.setGroupingComparatorClass(ItemidGroupingComparator.class);

//在此設置自定義的partitioner類
job.setPartitionerClass(ItemIdPartitioner.class);

job.setNumReduceTasks(2);