本文以Hadoop2.7.3单机模式环境下的WordCount实例来介绍2.7.3版本中如何编辑自己的MapReduce程序

实例中至少有3个jar包：

• $HADOOP_HOME/share/hadoop/common/hadoop-common-2.7.3.jar
• $HADOOP_HOME/share/hadoop/mapreduce/hadoop-mapreduce-client-core-2.7.3.jar
• $HADOOP_HOME/share/hadoop/common/lib/commons-cli-1.2.jar

通过命令hadoop classpath可以得到运行Hadoop程序所需的全部classpath信息

编译，打包Hadoop MapReduce程序

可以将Hadoop的classpath信息添加到CLASSPATH变量中，在/etc/profile中增加如下几行：

export HADOOP_HOME=/usr/local/hadoop
export CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath):$CLASSPATH

执行source /etc/profile使变量生效，接着就可以通过javac命令编辑WordCount.java

1.首先建立一个工作目录：

mkdir WordCount
cd WordCount

将WordCount源码放置该目录下，进行编译

javac WordCount.java

接着把.class文件打包成jar，才能在Hadoop中运行

jar -cvf WordCount.jar ./WordCount*.class

打包完成后，创建几个输入文件来测试一下：

mkdir input
echo "today is a good day" > ./input/test1
echo "today i am happy,so how about the other day" > ./input/test2

如果读者的Hadoop是单机模式，请跳过此步骤。如果读者的Hadoop环境已经配置成伪分布，那么需要执行以下命令，小编的是伪分布式
保证已经开启Hadoop服务

start-all.sh，相当于执行start-dfs.sh and start-yarn.sh

可以到hadoop安装目录下开启hadoop服务

# 把本地文件上传到伪分布式HDFS上
/usr/local/hadoop/bin/hadoop fs -put ./input input

开始执行，命令行输入

/usr/local/hadoop/bin/hadoop fs -put ./input input

伪分布正确的执行结果如下

如果读者已经将Hadoop的bin目录添加到环境变量，那么每一句命令的/usr/local/hadoop/bin/都可以不写

WordCount.java源码

import java.io.IOException;
import java.util.Iterator;
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;
import org.apache.hadoop.util.GenericOptionsParser;
 
public class WordCount {
    public WordCount() {
    }
 
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        String[] otherArgs = (new GenericOptionsParser(conf, args)).getRemainingArgs();
        if(otherArgs.length < 2) {
            System.err.println("Usage: wordcount <in> [<in>...] <out>");
            System.exit(2);
        }
 
        Job job = Job.getInstance(conf, "word count");
        job.setJarByClass(WordCount.class);
        job.setMapperClass(WordCount.TokenizerMapper.class);
        job.setCombinerClass(WordCount.IntSumReducer.class);
        job.setReducerClass(WordCount.IntSumReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
 
        for(int i = 0; i < otherArgs.length - 1; ++i) {
            FileInputFormat.addInputPath(job, new Path(otherArgs[i]));
        }
 
        FileOutputFormat.setOutputPath(job, new Path(otherArgs[otherArgs.length - 1]));
        System.exit(job.waitForCompletion(true)?0:1);
    }
 
    public static class IntSumReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
        private IntWritable result = new IntWritable();
 
        public IntSumReducer() {
        }
 
        public void reduce(Text key, Iterable<IntWritable> values, Reducer<Text, IntWritable, Text, IntWritable>.Context context) throws IOException, InterruptedException {
            int sum = 0;
 
            IntWritable val;
            for(Iterator i$ = values.iterator(); i$.hasNext(); sum += val.get()) {
                val = (IntWritable)i$.next();
            }
 
            this.result.set(sum);
            context.write(key, this.result);
        }
    }
 
    public static class TokenizerMapper extends Mapper<Object, Text, Text, IntWritable> {
        private static final IntWritable one = new IntWritable(1);
        private Text word = new Text();
 
        public TokenizerMapper() {
        }
 
        public void map(Object key, Text value, Mapper<Object, Text, Text, IntWritable>.Context context) throws IOException, InterruptedException {
            StringTokenizer itr = new StringTokenizer(value.toString());
 
            while(itr.hasMoreTokens()) {
                this.word.set(itr.nextToken());
                context.write(this.word, one);
            }
 
        }
    }
}

作者：秦景坤
GitHub：https://github.com/Roc-J
CSDN博客：http://blog.csdn.net/qjk19940101