Hadoop 系列（三）Top N

一：流程分析

Top N简介

        关系数据库中经常有Top n数据查询的大部分是以下四种需求

        1.直接min或者max就可以取得最大或者最小的数据  （top 1）

        2.升级一点就再加上一个groupby取一个分组内的最大值，最小值（分组内的top1）

        3.top 10需求，使用order函数取一个前10

        4.分组内的top 10需求，使用window 函数生成一个虚拟列，虚拟列取< 11的数据就可以

        相同的我们在mapreduce中也可能需要实现这种需求：

        1.key取相同的值，value取最大值，或者最小值就可以。（优化一点的就是在map阶段就聚合部分的数据，不然容易数据倾斜,其实就是Combiner，但是没有做过helloworld，自己先试试）

        2.key取groupby的值，value取最大值，最小值。（优化方案：map阶段取出来组内的最大最小值）

        3.key取相同的值，value取一个前10

        4.key取groupby的值，value取一个前10

我们可以把这四种全部都实现一下：有一点需要谨记：所有的map函数和reduce函数都不是只执行一次的

代码

数据：

2020040112 1
2020040113 3
2020040114 4
2020040115 5
2020040116 6
2020040117 7
2020040118 8
2020040119 9
2020040312 1
2020040313 3
2020040314 4
2020040315 5
2020040316 6
2020040317 7
2020040318 8
2020040319 9
2020040412 1
2020040413 3
2020040414 4
2020040415 5
2020040416 6
2020040417 7
2020040418 8
2020040419 9

代码1 输出最高温度和最低温度：

package org.example;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;

class WordcountMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    private int max = 0;
    private int min = 0;
    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String line[] = value.toString().split(" ");
        if(Integer.valueOf(line[1]) > max){
            max = Integer.valueOf(line[1]);
        }
        if(Integer.valueOf(line[1]) < min){
            min = Integer.valueOf(line[1]);
        }
    }

    @Override
    protected void cleanup(Context context) throws IOException, InterruptedException {
        context.write(new Text("min"),new IntWritable(min));
        context.write(new Text("max"),new IntWritable(max));
    }
}
class WordcountReducer extends Reducer<Text,IntWritable,Text,IntWritable> {
    private int max = 0;
    private int min = 0;
    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        for (IntWritable value : values) {
            if(value.get() > max){
                max = value.get();
            }
            if(value.get() < min){
                min = value.get();
            }
        }
    }

    @Override
    protected void cleanup(Context context) throws IOException, InterruptedException {
        context.write(new Text("min"),new IntWritable(min));
        context.write(new Text("max"),new IntWritable(max));
    }
}
public class WordcountDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {

        Configuration conf = new Configuration();
        conf.set("fs.defaultFS", "file:///");
        FileSystem fs= FileSystem.get(conf);
        String outputPath = "/software/java/data/output/";
        if(fs.exists(new Path(outputPath))) fs.delete(new Path(outputPath),true);

        Job job = Job.getInstance(conf);
        job.setJarByClass(WordcountDriver.class);
        job.setMapperClass(WordcountMapper.class);
        job.setReducerClass(WordcountReducer.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

        FileInputFormat.setInputPaths(job, new Path("/software/java/data/input/"));
        FileOutputFormat.setOutputPath(job, new Path(outputPath));

        //将job配置的参数，以及job所用的java类所在的jar包提交给yarn去运行
        //job.submit();
        boolean res = job.waitForCompletion(true);
    }

}




代码2 分组内输出最高温度和最低温度：




package org.example;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

class WordcountMapper extends Mapper<LongWritable, Text, Text, Text> {
    private Map<String,String> minmaxMap = new HashMap<String,String>();
    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String line[] = value.toString().split("\ ");
        String date = line[0].substring(0,line[0].length()-2);
        int temperature = Integer.parseInt(line[1]);
        if(minmaxMap.containsKey(date)){

            int max = Integer.parseInt(minmaxMap.get(date).split("\:")[0]);
            int min = Integer.parseInt(minmaxMap.get(date).split("\:")[1]);

            if(temperature > max){
                minmaxMap.put(date,temperature+":"+min);
            }
            if(temperature < min){
                minmaxMap.put(date,max+":"+temperature);
            }
        }
        else{
            minmaxMap.put(date,temperature+":"+temperature);
        }
    }

    @Override
    protected void cleanup(Context context) throws IOException, InterruptedException {
        for (Map.Entry<String, String> dateTemperature :minmaxMap.entrySet()) {
            System.out.println("map"+dateTemperature.getKey() + "|"+dateTemperature.getValue());
            context.write(new Text(dateTemperature.getKey()),new Text(dateTemperature.getValue()));
        }
    }
}
class WordcountReducer extends Reducer<Text,Text,Text,Text> {
    private Map<String,String> minmaxMap = new HashMap<String,String>();

    @Override
    protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        for (Text value:values ) {
            String date = key.toString();
            if(minmaxMap.containsKey(date)){

                int existMax = Integer.parseInt(minmaxMap.get(date).split("\:")[0]);
                int existMin = Integer.parseInt(minmaxMap.get(date).split("\:")[1]);
                int max = Integer.parseInt(value.toString().split("\:")[0]);
                int min = Integer.parseInt(value.toString().split("\:")[1]);
                int finalMax = existMax > max ? existMax:max;
                int finalMin = existMin < min ? existMin:min;
                minmaxMap.put(date,finalMax+":"+finalMin);
            }
            else{
                minmaxMap.put(date,value.toString());
            }
        }
    }

    @Override
    protected void cleanup(Context context) throws IOException, InterruptedException {
        for (Map.Entry<String, String> dateTemperature :minmaxMap.entrySet()) {
            System.out.println("reduce"+dateTemperature.getKey() + "|"+dateTemperature.getValue());
            context.write(new Text(dateTemperature.getKey()),new Text(dateTemperature.getValue()));
        }
    }
}
public class WordcountDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {

        Configuration conf = new Configuration();
        conf.set("fs.defaultFS", "file:///");
        FileSystem fs= FileSystem.get(conf);
        String outputPath = "/software/java/data/output/";
        if(fs.exists(new Path(outputPath))) fs.delete(new Path(outputPath),true);

        Job job = Job.getInstance(conf);
        job.setJarByClass(WordcountDriver.class);
        job.setMapperClass(WordcountMapper.class);
        job.setReducerClass(WordcountReducer.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);


        FileInputFormat.setInputPaths(job, new Path("/software/java/data/input/"));
        FileOutputFormat.setOutputPath(job, new Path(outputPath));

        //将job配置的参数，以及job所用的java类所在的jar包提交给yarn去运行
        //job.submit();
        boolean res = job.waitForCompletion(true);
    }

}




       代码3和代码4就不写了，因为差不多。





        

	      

	    

	  

	  

	    
      
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          原文地址：https://www.cnblogs.com/wuxiaolong4/p/12733518.html