MapReduce实现倒序排序的两种方法

版权声明：本文为博主原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接和本声明。

本文链接： https://blog.csdn.net/qq262593421/article/details/102306800

目录

一、外部比较器实现

        二、自定义序列化比较器实现

---

一、外部比较器实现

需要注意，这里的比较是对MapReduce中key类型的比较，并且key的类型为InWritable类型。

1、自定义比较器继承WritableComparator类

package com.gxwz.utils;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;

/**
 * TODO 	 IntWritable类的倒叙排序
 * @author   administrator
 * @Date	 2019年10月7日 	 
 */
public class IntWritableComparator extends WritableComparator {

	/*
	 * 重写构造方法，定义比较类 IntWritable
	 */
	public IntWritableComparator() {
		super(IntWritable.class, true);
	}
	/*
	 * 重写compare方法，自定义比较规则
	 */
	@Override
	public int compare(WritableComparable a, WritableComparable b) {
		//向下转型
		IntWritable ia = (IntWritable) a;
		IntWritable ib = (IntWritable) b;
		return ib.compareTo(ia);
	}
}

2、job设置比较器

//Job job = Job.getInstance(conf, "DescSort");
job.setSortComparatorClass(IntWritableComparator.class);

二、自定义序列化比较器实现

1、自定义序列化类，实现WritableComparable接口

package com.gxwz.entity;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.hadoop.io.WritableComparable;
/**
 * TODO 各地区年收入信息
 * @author   com
 * @Date	 2019年10月3日 	 
 */
public class AnnualIncome implements WritableComparable<AnnualIncome>{

	private String workerId = "";	//id
	private String area = "";		//地区
	private String date = "";		//时间
	private String amount = "";		//收入
	public String getWorkerId() {
		return workerId;
	}
	public void setWorkerId(String workerId) {
		this.workerId = workerId;
	}
	public String getArea() {
		return area;
	}
	public void setArea(String area) {
		this.area = area;
	}
	public String getDate() {
		return date;
	}
	public void setDate(String date) {
		this.date = date;
	}
	public String getAmount() {
		return amount;
	}
	public void setAmount(String amount) {
		this.amount = amount;
	}
	
	@Override
	public String toString() {
		return workerId + "\t" + area + "\t" + date + "\t" + amount;
	}
	@Override
	public void write(DataOutput out) throws IOException {
		out.writeUTF(workerId);
		out.writeUTF(area);
		out.writeUTF(date);
		out.writeUTF(amount);
	}
	@Override
	public void readFields(DataInput in) throws IOException {
		this.workerId = in.readUTF();
		this.area = in.readUTF();
		this.date = in.readUTF();
		this.amount = in.readUTF();
	}
	@Override
	public int compareTo(AnnualIncome o) {
		return this.workerId.compareTo(o.workerId);
	}
}