GroupingComparator分组(辅助分组)和二次排序

一、辅助排序：(GroupingComparator分组)

　　在Reduce端对key进行分组。应用于：在接受的key为bean对象时，想让一个或几个字段相同(全部字段比较不相同)的key进入到同一个reduce方法时，可以采用分组排序。

二、举例说明

　1、需求　

　　(1)统计同一品牌下，卖最贵的手机型号

　　(2)希望输出信息（品牌名、手机型号名、价格）

　　　　

xiaomi    小米10    1999    8    2020-07-10
huawei    华为P10    2999    7    2020-07-08
meizu    魅族E3660    1999    10    2020-07-09
xiaomi    小米9    1699    30    2020-07-09
xiaomi    小米8    1299    40    2020-07-11
xiaomi    小米10    1999    20    2020-07-12
xiaomi    小米9    1699    6    2020-07-13
meizu    魅族5300    2999    7    2020-07-14
meizu    魅族8    1899    8    2020-07-11
meizu    魅族e    1099    15    2020-07-06
huawei    华为P30    3999    18    2020-07-12
huawei    华为P20    2999    80    2020-07-01
huawei    华为P10    1999    60    2020-07-03
xiaomi    小米10    1999    8    2020-07-12
huawei    华为P10    2999    7    2020-07-18
meizu    魅族E3660    1999    40    2020-07-19
xiaomi    小米9    1699    30    2020-07-29
xiaomi    小米8    1299    41    2020-07-21
xiaomi    小米10    1999    70    2020-07-23
xiaomi    小米9    1699    6    2020-07-30
meizu    魅族5300    2999    7    2020-07-22
meizu    魅族8    1899    50    2020-07-16
meizu    魅族e    1099    55    2020-07-19
huawei    华为P30    3999    18    2020-07-25
huawei    华为P20    2999    80    2020-07-04
huawei    华为P10    1999    90    2020-07-03

　2、PhoneBean.java

　　　　

package com.jh.work02;


import org.apache.hadoop.io.WritableComparable;

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

public class PhoneBean implements WritableComparable<PhoneBean> {
    private String phoneName;   // 手机品牌
    private String phoneVersion;    // 手机型号
    private Long phoneMoney;    // 手机单价

    public PhoneBean() {
        super();
    }

    @Override
    public String toString() {
        return phoneName + "	" + phoneVersion + "	" + phoneMoney;
    }

    public String getPhoneVersion() {
        return phoneVersion;
    }

    public void setPhoneVersion(String phoneVersion) {
        this.phoneVersion = phoneVersion;
    }

    public Long getPhoneMoney() {
        return phoneMoney;
    }

    public void setPhoneMoney(Long phoneMoney) {
        this.phoneMoney = phoneMoney;
    }

    public String getPhoneName() {
        return phoneName;
    }

    public void setPhoneName(String phoneName) {
        this.phoneName = phoneName;
    }

    // 序列化
    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(phoneName);
        out.writeUTF(phoneVersion);
        out.writeLong(phoneMoney);
    }

    // 反序列化
    @Override
    public void readFields(DataInput in) throws IOException {
        phoneName = in.readUTF();
        phoneVersion = in.readUTF();
        phoneMoney = in.readLong();
    }

    // 排序
    @Override
    public int compareTo(PhoneBean o) {
        /**
         * 说明：
         * compareTo方法被称为自然比较方法,利用当前对象和传入的目标对象进行比较;
         * 若是当前对象比目标对象大，则返回1，那么当前对象会排在目标对象的后面　　　　　　
         * 若是当前对象比目标对象小，则返回-1，那么当前对象会排在目标对象的后面
         * 若是两个对象相等，则返回0
         */
        // 先根据手机品牌排序，相同的挨着放
        int result = this.getPhoneName().compareTo(o.getPhoneName());
        if (result == 0){
            // 手机品牌相同时，再根据手机单价倒序排序
            return o.getPhoneMoney().compareTo(this.getPhoneMoney());
        }else{
            return result;
        }
    }
}

　3、PhoneMapper.java

　　　　

package com.jh.work02;

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;

public class PhoneMapper extends Mapper<LongWritable,Text,PhoneBean,NullWritable> {
    private PhoneBean bean = new PhoneBean();

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        // 获取文本每行内容
        String line = value.toString();
        // 根据分隔符切割
        String[] split = line.split("	");

        // 赋值
        bean.setPhoneName(split[0]);
        bean.setPhoneVersion(split[1]);
        bean.setPhoneMoney(Long.parseLong(split[2]));

        context.write(bean,NullWritable.get());
    }
}

　4、PhoneGroupCompartor.java

　　　　

package com.jh.work02;

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

public class PhoneGroupCompartor extends WritableComparator {

    /*
        创建一个构造将比较对象的类传给父类，便于反序列化,
        如果不提前声明空对象，在GroupingComparator调用时会抛出空指针异常
     */
    protected PhoneGroupCompartor() {
        super(PhoneBean.class,true);
    }

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

        // 根据手机品牌分组
        return abean.getPhoneName().compareTo(bbean.getPhoneName());
    }
}

　5、PhoneReducer.java

　　　　

package com.jh.work02;

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

import java.io.IOException;

public class PhoneReducer extends Reducer<PhoneBean,NullWritable,PhoneBean,NullWritable> {
    private PhoneBean bean = new PhoneBean();

    @Override
    protected void reduce(PhoneBean key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
        // 输出每组第一个，也就是最贵的的那个
        context.write(key,values.iterator().next());
    }
}

　6、PhoneDriverWork02.java

　　　　

package com.jh.work02;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
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;

public class PhoneDriverWork02 {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        //1.获取job对象和配置文件对象
        Configuration configuration = new Configuration();
        Job job = Job.getInstance(configuration);

        //2.添加jar的路径
        job.setJarByClass(PhoneDriverWork02.class);

        //3.设置mapper类和reducer类
        job.setMapperClass(PhoneMapper.class);
        job.setReducerClass(PhoneReducer.class);

        //4.设置mapper类输出的数据类型
        job.setMapOutputKeyClass(PhoneBean.class);
        job.setMapOutputValueClass(NullWritable.class);

        //5.设置reducer类输出的数据类型
        job.setOutputKeyClass(PhoneBean.class);
        job.setOutputValueClass(NullWritable.class);

        // 设置辅助分组的类
        job.setGroupingComparatorClass(PhoneGroupCompartor.class);

        //设置文件的输入出路径
        FileInputFormat.setInputPaths(job,new Path(args[0]));
        FileOutputFormat.setOutputPath(job,new Path(args[1]));

        //提交任务
        boolean result = job.waitForCompletion(true);

        //成功返回0，失败返回1
        System.exit(result ? 0:1);
    }
}

　7、输出文件为

　　

　8、如果想统计同一品牌下，价格前两名的手机型号，只需修改PhoneReducer.java

　　　　

package com.jh.work02;

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

import java.io.IOException;

public class PhoneReducer extends Reducer<PhoneBean,NullWritable,PhoneBean,NullWritable> {
    private PhoneBean bean = new PhoneBean();

    @Override
    protected void reduce(PhoneBean key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
        // 输出每组第一个，也就是最贵的的那个
        for (int i = 0; i < 2; i++) {
            if (values.iterator().hasNext()) {
                values.iterator().next();
                context.write(key, NullWritable.get());
            }
        }
    }
}