通过前面两篇博客的学习，我们学习了Hadoop的伪分布式部署和完全分布式部署。这一篇文章就来介绍一下Hadoop的第一个程序WordCount。以及在运行代码的过程中遇到的问题。

笔者开发环境：

Linux: CentOS 6.6(Final) x64

Windows: Win7 64位

JDK: java version "1.7.0_75"
OpenJDK Runtime Environment (rhel-2.5.4.0.el6_6-x86_64 u75-b13)
OpenJDK 64-Bit Server VM (build 24.75-b04, mixed mode)

SSH: OpenSSH_5.3p1, OpenSSL 1.0.1e-fips 11 Feb 2013

Hadoop: hadoop-1.2.1

Eclipse: Release 4.2.0

这边使用Linux和Window两个系统来开发的目的在于，我的Cygwin还没安装好。。。而Linux上的Hadoop安装和部署很方便，又感觉Winows上Eclipse比Linux上的Eclipse用着舒服。

下面的代码和一些遇到的问题，以及对于这上问题的解决方案。

主代码(WordCount.java)：

public class WordCount {

	public static class TokenizerMapper

	extends Mapper<Object, Text, Text, IntWritable> {

		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());
			while (itr.hasMoreTokens()) {
				word.set(itr.nextToken());
				context.write(word, one);
			}
		}
	}

	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;
			for (IntWritable val : values) {
				sum += val.get();
			}
			result.set(sum);
			context.write(key, result);
		}
	}

	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> <out>");
			System.exit(2);
		}

		Job job = new Job(conf, "word count");
		job.setJarByClass(WordCount1.class);
		job.setMapperClass(TokenizerMapper.class);
		job.setCombinerClass(IntSumReducer.class);
		job.setReducerClass(IntSumReducer.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
		FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
		System.exit(job.waitForCompletion(true) ? 0 : 1);
	}
}

运行过程及说明：

1.将我们在Windows上开发的Java程序打成jar包，上传到Linux上。

2.执行命令java -jar wordcount1.jar /home/moon/coding/tmp/wordcount /home/moon/coding/tmp/wordres