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1. 执行过程

  假设WordCount的两个输入文本text1.txt和text2.txt如下。

Hello World
Bye World

Hello Hadoop
Bye Hadoop

1.1 分割

  将每个文件拆分成split分片，由于测试文件比较小，所以每个文件为一个split，并将文件按行分割形成<key，value>对，如下图所示。这一步由MapReduce自动完成，其中key值为偏移量，由MapReduce自动计算出来，包括回车所占的字符数。

1.2 Map

  将分割好的<key，value>对交给用户定义的Map方法处理，生成新的<key，value>对。处理流程为先对每一行文字按空格拆分为多个单词，每个单词出现次数设初值为1，key为某个单词，value为1，如下图所示。

1.3 Combine

  得到Map方法输出的<key，value>对后，Mapper将它们按照key值进行升序排列，并执行Combine合并过程，将key值相同的value值累加，得到Mapper的最终输出结果，并写入磁盘，如下图所示。

1.4 Reduce

  Reducer先对从Mapper接受的数据进行排序，并将key值相同的value值合并到一个list列表中，再交由用户自定义的Reduce方法进行汇总处理，得到新的<key，value>对，并作为WordCount的输出结果，存入HDFS，如下图所示。

2. 代码和结果

2.1 pom.xml中依赖配置

	<dependencies><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.11</version><scope>test</scope></dependency><dependency><groupId>org.apache.hadoop</groupId><artifactId>hadoop-common</artifactId><version>3.3.6</version><exclusions><exclusion><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.apache.hadoop</groupId><artifactId>hadoop-mapreduce-client-core</artifactId><version>3.3.6</version><type>pom</type></dependency><dependency><groupId>org.apache.hadoop</groupId><artifactId>hadoop-mapreduce-client-jobclient</artifactId><version>3.3.6</version></dependency></dependencies>

2.2 工具类util

  util.removeALL的功能是删除hdfs上的指定输出路径(如果存在的话)，而util.showResult的功能是打印wordcount的结果。

import java.net.URI;
import java.util.regex.Matcher;
import java.util.regex.Pattern;import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;public class util {public static FileSystem getFileSystem(String uri, Configuration conf) throws Exception {URI add = new URI(uri);return FileSystem.get(add, conf);}public static void removeALL(String uri, Configuration conf, String path) throws Exception {FileSystem fs = getFileSystem(uri, conf);if (fs.exists(new Path(path))) {boolean isDeleted = fs.delete(new Path(path), true);System.out.println("Delete Output Folder? " + isDeleted);}}public static void  showResult(String uri, Configuration conf, String path) throws Exception {FileSystem fs = getFileSystem(uri, conf);String regex = "part-r-";Pattern pattern = Pattern.compile(regex);if (fs.exists(new Path(path))) {FileStatus[] files = fs.listStatus(new Path(path));for (FileStatus file : files) {Matcher matcher = pattern.matcher(file.getPath().toString());if (matcher.find()) {FSDataInputStream openStream = fs.open(file.getPath());IOUtils.copyBytes(openStream, System.out, 1024);openStream.close();}}}}
}

2.3 WordCount

  正常来说，MapReduce编程都是要把代码打包成jar文件，然后用hadoop jar jar文件名 主类名称 输入路径 输出路径。下面代码中直接给出了输入和输出路径，可以直接运行。

import java.io.IOException;
import java.util.StringTokenizer;import org.apache.hadoop.conf.Configuration;
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;public class App {public static class MyMapper extends Mapper<LongWritable, Text, Text, IntWritable> {public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {System.out.println(key + " " + value);Text keyOut;IntWritable valueOut = new IntWritable(1);StringTokenizer token = new StringTokenizer(value.toString());while (token.hasMoreTokens()) {keyOut = new Text(token.nextToken());context.write(keyOut, valueOut);}}}public static class MyReducer extends Reducer<Text, IntWritable, Text, IntWritable> {public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {int sum = 0;for (IntWritable value : values) {sum += value.get();}context.write(key, new IntWritable(sum));} }public static void main(String[] args) throws Exception {Configuration conf = new Configuration();String[] myArgs = {"file:///home/developer/CodeArtsProjects/WordCount/text1.txt", "file:///home/developer/CodeArtsProjects/WordCount/text2.txt", "hdfs://localhost:9000/user/developer/wordcount/output"};util.removeALL("hdfs://localhost:9000", conf, myArgs[myArgs.length - 1]);Job job = Job.getInstance(conf, "wordcount");job.setJarByClass(App.class);job.setMapperClass(MyMapper.class);job.setReducerClass(MyReducer.class);job.setCombinerClass(MyReducer.class);job.setOutputKeyClass(Text.class);job.setOutputValueClass(IntWritable.class);for (int i = 0; i < myArgs.length - 1; i++) {FileInputFormat.addInputPath(job, new Path(myArgs[i]));}FileOutputFormat.setOutputPath(job, new Path(myArgs[myArgs.length - 1]));int res = job.waitForCompletion(true) ? 0 : 1;if (res == 0) {System.out.println("WordCount结果:");util.showResult("hdfs://localhost:9000", conf, myArgs[myArgs.length - 1]);}System.exit(res);}
}

2.4 结果

参考

吴章勇 杨强著 大数据Hadoop3.X分布式处理实战