MapReduce 分组GroupingComparator

public class GroupingComparator extends WritableComparator {    //传入作为key的bean的class类型，以及制定需要让框架做反射获取实例对象    protected ItemidGroupingComparator() {        super(OrderBean.class, true);    }    @Override    public int compare(WritableComparable a, WritableComparable b) {        OrderBean abean = (OrderBean) a;        OrderBean bbean = (OrderBean) b;        //比较两个bean时，指定只比较bean中的orderid        return abean.getItemid().compareTo(bbean.getItemid());    }}

//在此设置自定义的Groupingcomparator类                        job.setGroupingComparatorClass(ItemidGroupingComparator.class);//在此设置自定义的partitioner类job.setPartitionerClass(ItemIdPartitioner.class);job.setNumReduceTasks(2);