MapReduce的基本内容介绍（附代码）

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 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);
 }
 }
 }
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);
 }
 }
}