hadoop wordcount V2.0

学习hadoop的mapreduce，最经典的例子就是wordcount 。然而在hadoop官网上还有一个wordcount v2.0，今天花了半个下午时间研究了一下，感觉有点心得。现把源码放上，并加以注释，以便日后回顾之用。解读可能有不妥之处，欢迎指正。

源码来源：http://hadoop.apache.org/docs/r2.6.4/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html#Example:_WordCount_v2.0

<pre name="code" class="java">package wc2;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.net.URI;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
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.mapreduce.Counter;
import org.apache.hadoop.util.GenericOptionsParser;
import org.apache.hadoop.util.StringUtils;

public class WordCount2 {

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

static enum CountersEnum { INPUT_WORDS }

private final static IntWritable one = new IntWritable(1);
private Text word = new Text();

private boolean caseSensitive;
private Set<String> patternsToSkip = new HashSet<String>();

private Configuration conf;
private BufferedReader fis;

/*context:
* 它是mapper的一个内部类，
* 简单的说是为了在map或是reduce任务中跟踪task的状态，
* 很自然的MapContext就是记录了map执行的上下文，
* 在mapper类中，这个context可以存储一些job conf的信息，
* 比如运行时参数等，我们可以在map函数中处理这个信息
*/

/*URI:
* URL的拓展集，暂时理解为文件所在路径
*/
@Override
public void setup(Context context) throws IOException,
InterruptedException {
conf = context.getConfiguration();
caseSensitive = conf.getBoolean("wordcount.case.sensitive", true);
if (conf.getBoolean("wordcount.skip.patterns", true)) {
URI[] patternsURIs = Job.getInstance(conf).getCacheFiles();
for (URI patternsURI : patternsURIs) {
Path patternsPath = new Path(patternsURI.getPath());
String patternsFileName = patternsPath.getName().toString();
parseSkipFile(patternsFileName);
}
}
}

private void parseSkipFile(String fileName) {
try {
fis = new BufferedReader(new FileReader(fileName));
String pattern = null;
while ((pattern = fis.readLine()) != null) {
patternsToSkip.add(pattern);
}
} catch (IOException ioe) {
System.err.println("Caught exception while parsing the cached file '"
+ StringUtils.stringifyException(ioe));
}
}

@Override
public void map(Object key, Text value, Context context
) throws IOException, InterruptedException {
String line = (caseSensitive) ?
value.toString() : value.toString().toLowerCase();
for (String pattern : patternsToSkip) {
line = line.replaceAll(pattern, "");
}
StringTokenizer itr = new StringTokenizer(line);
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
Counter counter = context.getCounter(CountersEnum.class.getName(),
CountersEnum.INPUT_WORDS.toString());
counter.increment(1);
}
}
}

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);
}
}
/*
* GenericOptionsParser：解释hadoop命令中[genericOptions]部分的一个类
* hadoop命令的一般格式：bin/hadoop command [genericOptions] [commandOptions]
* */
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
GenericOptionsParser optionParser = new GenericOptionsParser(conf, args);
String[] remainingArgs = optionParser.getRemainingArgs();
if (!(remainingArgs.length != 2 || remainingArgs.length != 4)) {
System.err.println("Usage: wordcount <in> <out> [-skip skipPatternFile]");
System.exit(2);
}
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount2.class);
job.setMapperClass(TokenizerMapper.class);
job.setCombinerClass(IntSumReducer.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);

List<String> otherArgs = new ArrayList<String>();
for (int i=0; i < remainingArgs.length; ++i) {
if ("-skip".equals(remainingArgs[i])) {
job.addCacheFile(new Path(remainingArgs[++i]).toUri());
job.getConfiguration().setBoolean("wordcount.skip.patterns", true); //新增属性“wordcount.skip.patterns”,设置为true
} else {
otherArgs.add(remainingArgs[i]);
}
}
FileInputFormat.addInputPath(job, new Path(otherArgs.get(0)));
FileOutputFormat.setOutputPath(job, new Path(otherArgs.get(1)));

System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}