Hello，大家好！博主上篇讲解了合并，这篇要讲的是辅助排序。如何讲解这个章节呢？首先先对什么是合并进行解释，然后通过案例进行证明。

一. GroupingComparator分组的简介

什么是GroupingComparator分组（辅助排序）？   对Reduce阶段的数据根据某一个或几个字段进行分组。

分组排序的步骤：

• 1. 自定义类继承WritableComparator
• 2. 重写compare()方法

@Override
public int compare(WritableComparable a, WritableComparable b) {
		// 比较的业务逻辑
		return result;
}

• 3. 创建一个构造将比较对象的类传给父类

protected OrderGroupingComparator() {
		super(OrderBean.class, true);
}

二. 根据案例分析

2.1 需求

• 1. 有如下订单数据

现在需要求出每一个订单中最贵的商品。

• 2. 输入的数据

• 3. 期望输出数据

1	222.8
2	722.4
3	232.8

2.2 需求分析

（1）利用“订单id和成交金额”作为key，可以将Map阶段读取到的所有订单数据按照id升序排序，如果id相同再按照金额降序排序，发送到Reduce。 （2）在Reduce端利用groupingComparator将订单id相同的kv聚合成组，然后取第一个即是该订单中最贵商品，如下图所示所示。

2.3 代码实现

1. 定义订单信息OrderBean类

package com.buwenbuhuo.groupingcomparator;

import org.apache.hadoop.io.WritableComparable;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
/**
 * @author 卜温不火
 * @create 2020-04-24 23:43
 * com.buwenbuhuo.groupingcomparator - the name of the target package where the new class or interface will be created.
 * mapreduce0422 - the name of the current project.
 */
public class OrderBean implements WritableComparable<OrderBean> {

    private String orderId;
    private String productId;
    private double price;

    @Override
    public String toString() {
        return orderId + "\t" + productId + "\t" + price;
    }

    public String getOrderId() {
        return orderId;
    }

    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }

    public String getProductId() {
        return productId;
    }

    public void setProductId(String productId) {
        this.productId = productId;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public int compareTo(OrderBean o) {

        int compare = this.orderId.compareTo(o.orderId);

        if (compare == 0) {
            return Double.compare(o.price, this.price);
        } else {
            return compare;
        }
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(orderId);
        out.writeUTF(productId);
        out.writeDouble(price);
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        this.orderId = in.readUTF();
        this.productId = in.readUTF();
        this.price = in.readDouble();
    }
}

2. 编写OrderSortMapper类

package com.buwenbuhuo.groupingcomparator;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;
/**
 * @author 卜温不火
 * @create 2020-04-24 23:43
 * com.buwenbuhuo.groupingcomparator - the name of the target package where the new class or interface will be created.
 * mapreduce0422 - the name of the current project.
 */
public class OrderMapper extends Mapper<LongWritable, Text, OrderBean, NullWritable> {


    private OrderBean orderBean = new OrderBean();

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String[] fields = value.toString().split("\t");
        orderBean.setOrderId(fields[0]);
        orderBean.setProductId(fields[1]);
        orderBean.setPrice(Double.parseDouble(fields[2]));
        context.write(orderBean, NullWritable.get());
    }
}

3. 编写OrderSortGroupingComparator类

package com.buwenbuhuo.groupingcomparator;

import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
/**
 * @author 卜温不火
 * @create 2020-04-24 23:43
 * com.buwenbuhuo.groupingcomparator - the name of the target package where the new class or interface will be created.
 * mapreduce0422 - the name of the current project.
 */
public class OrderComparator extends WritableComparator {

    protected OrderComparator() {
        super(OrderBean.class, true);
    }

    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        OrderBean oa = (OrderBean) a;
        OrderBean ob = (OrderBean) b;

        return oa.getOrderId().compareTo(ob.getOrderId());
    }
}

4. 编写OrderSortReducer类

package com.buwenbuhuo.groupingcomparator;

import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;
import java.util.Iterator;
/**
 * @author 卜温不火
 * @create 2020-04-24 23:43
 * com.buwenbuhuo.groupingcomparator - the name of the target package where the new class or interface will be created.
 * mapreduce0422 - the name of the current project.
 */
public class OrderReducer extends Reducer<OrderBean, NullWritable, OrderBean, NullWritable> {

    @Override
    protected void reduce(OrderBean key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
        Iterator<NullWritable> iterator = values.iterator();
        for (int i = 0; i < 2; i++) {
            if (iterator.hasNext()) {
                context.write(key, iterator.next());
            }
        }
    }
}

5. 编写OrderSortDriver类

package com.buwenbuhuo.groupingcomparator;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;
/**
 * @author 卜温不火
 * @create 2020-04-24 23:43
 * com.buwenbuhuo.groupingcomparator - the name of the target package where the new class or interface will be created.
 * mapreduce0422 - the name of the current project.
 */
public class OrderDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        Job job = Job.getInstance(new Configuration());

        job.setJarByClass(OrderDriver.class);

        job.setMapperClass(OrderMapper.class);
        job.setReducerClass(OrderReducer.class);

        job.setMapOutputKeyClass(OrderBean.class);
        job.setMapOutputValueClass(NullWritable.class);

        job.setGroupingComparatorClass(OrderComparator.class);

        job.setOutputKeyClass(OrderBean.class);
        job.setOutputValueClass(NullWritable.class);

        FileInputFormat.setInputPaths(job, new Path("d:\\input"));
        FileOutputFormat.setOutputPath(job, new Path("d:\\output"));

        boolean b = job.waitForCompletion(true);
        System.exit(b ? 0 : 1);
    }
}

2.4 运行与结果实现

• 1. 运行

• 2. 结果与对比