在 IDEA中运行 WordCount

一、新建一个maven项目

二、pom.xml 中内容

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>1</groupId>
    <artifactId>1</artifactId>
    <version>1.0-SNAPSHOT</version>

    <repositories>
        <repository>
            <id>apache</id>
            <url>http://maven.apache.org</url>
        </repository>
    </repositories>

    <dependencies>
        <!--<dependency>-->
            <!--<groupId>org.apache.hadoop</groupId>-->
            <!--<artifactId>hadoop-core</artifactId>-->
            <!--<version>2.7.2</version>-->
        <!--</dependency>-->
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-common</artifactId>
            <version>2.7.2</version>
        </dependency>

        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-hdfs</artifactId>
            <version>2.7.2</version>
        </dependency>
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-client</artifactId>
            <version>2.7.2</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-dependency-plugin</artifactId>
                <configuration>
                    <excludeTransitive>false</excludeTransitive>
                    <stripVersion>true</stripVersion>
                    <outputDirectory>./lib</outputDirectory>
                </configuration>

            </plugin>
        </plugins>
    </build>
</project>

三、准备数据文件

注意点：因为Windows当前用户是 Administrator ，所以需要在 hdfs://master:8020/user/ 目录下创建文件夹 Administrator ，以后进行本地测试都使用此文件夹。

文件夹创建好之后，还需要给与写的权限。此处直接给最大权限。

su hdfs
hdfs dfs -mkdir -p /user/Administrator/input
hdfs dfs -chmod -R 777 /user/Administrator
hdfs dfs -put ./wordCountData.txt /user/Administrator/input
exit

四、创建 WordCount.java 文件
注意点： 因为是在 Windows 上提交 mapreduce 任务，需要在 conf 中设置下面内容。
　　conf.set("mapreduce.app-submission.cross-platform", "true"); // 跨平台，保证在 Windows 下可以提交 mr job

否则报错：/bin/bash: line 0: fg: no job control

package com.zjc.mr;

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.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 WordCount {

    public static class TokenizerMapper extends Mapper<Object, Text, Text, IntWritable> {

        // 下面的IntWritable 跟 Text 类是hadoop内部类，相当于 java 中的 int 与 String
        // MapReduce 程序中互相传递的是这种类型的参数
        private final static IntWritable one = new IntWritable(1);
        private Text word = new Text();

        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
            StringTokenizer itr = new StringTokenizer(value.toString());//java 自带的字符串分割函数
            while (itr.hasMoreTokens()) {
                word.set(itr.nextToken());
                context.write(word, one);
                /*
                 *eg map output:
                 *        hello  1
                 *        word   1
                 *        hello  1
                 *        hadoop 1
                 */
            }
        }
    }

    /*
     * Reduce 输入：
     * key:       hello
     * value:  [1,1]
     *
     *     Hadoop负责将Map产生的<key,value>处理成{具有相同key的value集合}，传给Reducer
        输入：<key,(listof values)>
        输出：<key,value>
        reduce函数（必须是这个名字）的参数，（输入key,输入具有相同key的value集合，Context）其中，
        输入的key,value必须类型与map的输出<key,value>相同，这一点适用于map，reduce类及函数
     *
     */
    public static class IntSumReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
        private IntWritable result = new IntWritable();

        public void reduce(Text key, Iterable<IntWritable> values, Context context)
                throws IOException, InterruptedException {
            int sum = 0;
            System.out.println("-----------------------------------------");
            System.out.println("key: "+key);
            for (IntWritable val : values) {
                System.out.println("val: "+val);
                sum += val.get();
            }
            result.set(sum);
            System.out.println("result: "+result.toString());
            context.write(key, result);
        }
    }

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        conf.set("mapreduce.app-submission.cross-platform", "true");  // 跨平台，保证在 Windows 下可以提交 mr job
        Job job = Job.getInstance(conf, "word count"); // 任务名
        job.setJarByClass(WordCount.class); // 指定Class
        job.setMapperClass(TokenizerMapper.class); // 指定 Mapper Class
        job.setCombinerClass(IntSumReducer.class); // 指定 Combiner Class，与 reduce 计算逻辑一样
        job.setReducerClass(IntSumReducer.class); // 指定Reucer Class
        job.setOutputKeyClass(Text.class); // 指定输出的KEY的格式
        job.setOutputValueClass(IntWritable.class); // 指定输出的VALUE的格式
        job.setNumReduceTasks(1); //设置Reducer 个数默认1
        // Mapper<Object, Text, Text, IntWritable> 输出格式必须与继承类的后两个输出类型一致
        String args_0 = "hdfs://master:8020/user/Administrator/input";
        String args_1 = "hdfs://master:8020/user/Administrator/output";
        FileInputFormat.addInputPath(job, new Path(args_0)); // 输入路径
        FileOutputFormat.setOutputPath(job, new Path(args_1)); // 输出路径
        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}
// 每次运行都需要先删除hdfs中，上一次执行生成的 output 文件夹。  hdfs dfs -rm -R /user/Administrator/output

五、查看结果