一、简介

最大值是统计中最常使用到的，现在使用Mapreduce在海量数据中统计数据的最大值。

 

二、例子

（1）实例描述
给出三个文件，每个文件中都存储了若干个数值，求所有数值中的最大值。

样例输入：                                            
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

期望输出：

199

（2）问题分析
实现统计海量数据的最大值，不能将所有的数据加载到内存，计算只能使用类似外部排序的方式，加载一部分数据统计最大值，接着加载另一部分进行统计。

（3）实现步骤

1）Map过程 
    首先使用默认的TextInputFormat类对输入文件进行处理，得到文本中每行的偏移量及其内容。显然，Map过程首先必须分析输入的<key,value>对，得到数值，然后在mapper中统计单个分块的最大值。

2）Reduce过程 
    经过map方法处理后，Reduce过程将获取每个mapper的最大值进行统计，分行统计出最大值。

 

（3）关键代码

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;public class MaxValue {public static class MaxValueMapper extends Mapper<LongWritable, Text, IntWritable, NullWritable> {private Integer maxValue = null;@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(maxValue==null||maxValue < v){maxValue = v;}}@Overrideprotected void cleanup(Context context) throws IOException, InterruptedException {if (maxValue != null)context.write( new IntWritable(maxValue), NullWritable.get());}}public static class MaxValueReducer extends Reducer< IntWritable, NullWritable,IntWritable, NullWritable> {private Integer maxValue = null;@Overrideprotected void reduce(IntWritable key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {Integer v = key.get();if(maxValue==null||maxValue < v){maxValue = v;}}@Overrideprotected void cleanup(Context context) throws IOException, InterruptedException {if (maxValue != null)context.write( new IntWritable(maxValue), NullWritable.get());}}public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {String uri = "hdfs://192.168.150.128:9000";String input = "/maxValue/input";String output = "/maxValue/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, "MaxValue");job.setJar("./out/artifacts/hadoop_test_jar/hadoop-test.jar");job.setJarByClass(MaxValue.class);job.setMapperClass(MaxValueMapper.class);job.setReducerClass(MaxValueReducer.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);}
}