在linux平台上执行wordcount，有官方示例，相应的jar包放在hadoop-2.0.0-cdh4.5.0\share\hadoop\mapreduce1下的hadoop-examples-2.0.0-mr1-cdh4.5.0.jar（注：本人用的是CDH4.5.0版本），我们首先需要准备好数据：
echo "Hello World Hello Hadoop" > 1.txtecho "Hello Hadoop Bye " >2.txt
然后把数据put到HDFS里：
hadoop fs -mkdir /inputhadoop fs -put /root/1.txt /inputhadoop fs -put /root/2.txt /input
再然后进入到jar所在的目录里“
cd hadoop-2.0.0-cdh4.5.0\share\hadoop\mapreduce1
执行命令：
hadoop jar hadoop-mapreduce-examples-2.0.0-cdh4.5.0.jar WordCount /input /output
其中，/output是执行结果输出目录。
到此，HelloWorld就顺利执行了，你可以用hadoop fs -cat /output/part 命令来查看结果.
接下来，我们在看看在window上的eclipse如何执行。
首先贴出代码：
public class WordCount { // mapper public static class Map extends Mapper { private static IntWritable one = new IntWritable(1); private Text word = new Text(); @Override public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String line = value.toString(); StringTokenizer token = new StringTokenizer(line); while (token.hasMoreElements()) { word.set(token.nextToken()); context.write(word, one); } }; } // reduce public static class Reduce extends Reducer { protected void reduce(Text key, Iterable 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(); System.setProperty("HADOOP_USER_NAME", "root");//这句话很重要，要不然会告你没有权限执行 Job job = new Job(conf); String[] ioArgs = new String[] { "hdfs://192.168.1.101:7001/input", "hdfs://192.168.1.101:7001/output" }; String[] otherArgs = new GenericOptionsParser(conf, ioArgs).getRemainingArgs(); job.setJarByClass(WordCount.class); FileInputFormat.addInputPath(job, new Path(otherArgs[0])); FileOutputFormat.setOutputPath(job, new Path(otherArgs[1])); job.setMapperClass(Map.class); job.setReducerClass(Reduce.class); job.setOutputKeyClass(Text.class); job.setOutputValueClass(IntWritable.class); System.exit(job.waitForCompletion(true) ? 0 : 1); }} 然后在eclipse上点执行即可，在执行时可能发现jvm内存不够，添加-Xmx1024M参数执行即可。