一、簡(jiǎn)介

求TopK是算法中最常使用到的，現(xiàn)在使用Mapreduce在海量數(shù)據(jù)中統(tǒng)計(jì)數(shù)據(jù)的求TopK。

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二、例子

（1）實(shí)例描述
給出三個(gè)文件，每個(gè)文件中都存儲(chǔ)了若干個(gè)數(shù)值，求所有數(shù)值中的求Top 5。

樣例輸入： ???????????????????????????????????????????
1）file1： ?

1 2 3 7 9 -99 2

2）file2： ?

11 2 23 17 9 199 22

3）file3： ?

21 12 3 17 2 39 12

?期望輸出：

199 39 23 22 21

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（2）問題分析
實(shí)現(xiàn)統(tǒng)計(jì)海量數(shù)據(jù)的求TopK，不能將所有的數(shù)據(jù)加載到內(nèi)存，計(jì)算只能使用類似外部排序的方式，加載一部分?jǐn)?shù)據(jù)統(tǒng)計(jì)求TopK，接著加載另一部分進(jìn)行統(tǒng)計(jì)TopK。

（3）實(shí)現(xiàn)步驟

1）Map過程?
????首先使用默認(rèn)的TextInputFormat類對(duì)輸入文件進(jìn)行處理，得到文本中每行的偏移量及其內(nèi)容。顯然，Map過程首先必須分析輸入的<key,value>對(duì)，得到數(shù)值，然后在mapper中統(tǒng)計(jì)單個(gè)分塊的求TopK。

2）Reduce過程?
????經(jīng)過map方法處理后，Reduce過程將獲取每個(gè)mapper的求TopK進(jìn)行統(tǒng)計(jì)，分行統(tǒng)計(jì)出總的TopK。

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（3）關(guān)鍵代碼

package com.mk.mapreduce;import org.apache.commons.lang.StringUtils; 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.NullWritable; 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.net.URI; import java.util.ArrayList; import java.util.Collections; import java.util.List;public class TopK {public static class TopKMapper extends Mapper<LongWritable, Text, IntWritable, NullWritable> {private List<Integer> top5 = new ArrayList<>(5);@Overrideprotected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {if (StringUtils.isBlank(value.toString())) {System.out.println("空白行");return;}Integer v = Integer.valueOf(value.toString().trim());if(top5.size()<5){top5.add(v);}else{Integer min = Collections.min(top5);if (min < v) {top5.remove(min);top5.add(v);}}}@Overrideprotected void cleanup(Context context) throws IOException, InterruptedException {for (Integer v : top5)context.write(new IntWritable(v), NullWritable.get());}}public static class TopKReducer extends Reducer< IntWritable, NullWritable,IntWritable, NullWritable> {private List<Integer> top5 = new ArrayList<>(5);@Overrideprotected void reduce(IntWritable key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {Integer v = key.get();if(top5.size()<5){top5.add(v);}else{Integer min = Collections.min(top5);if (min < v) {top5.remove(min);top5.add(v);}}}@Overrideprotected void cleanup(Context context) throws IOException, InterruptedException {top5.sort((a,b)->b-a);for (Integer v : top5)context.write(new IntWritable(v), NullWritable.get());}}public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {String uri = "hdfs://192.168.150.128:9000";String input = "/topk/input";String output = "/topk/output";Configuration conf = new Configuration();if (System.getProperty("os.name").toLowerCase().contains("win"))conf.set("mapreduce.app-submission.cross-platform", "true");FileSystem fileSystem = FileSystem.get(URI.create(uri), conf);Path path = new Path(output);fileSystem.delete(path, true);Job job = new Job(conf, "TopK");job.setJar("./out/artifacts/hadoop_test_jar/hadoop-test.jar");job.setJarByClass(TopK.class);job.setMapperClass(TopKMapper.class);job.setReducerClass(TopKReducer.class);job.setMapOutputKeyClass(IntWritable.class);job.setMapOutputValueClass(NullWritable.class);job.setOutputKeyClass(IntWritable.class);job.setOutputValueClass(NullWritable.class);FileInputFormat.addInputPaths(job, uri + input);FileOutputFormat.setOutputPath(job, new Path(uri + output));boolean ret = job.waitForCompletion(true);System.out.println(job.getJobName() + "-----" + ret);} }

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