按照官方教程,http://hadoop.apache.org/docs/…,开始着手写一个 Hadoop 的 HelloWorld 程序,叫做 WordCount
首先准备好代码如下:
package org.myorg; import java.io.IOException; import java.util.*; import org.apache.hadoop.fs.Path; import org.apache.hadoop.conf.*; import org.apache.hadoop.io.*; import org.apache.hadoop.mapred.*; import org.apache.hadoop.util.*; public class WordCount { public static class Map extends MapReduceBase implements Mapper<LongWritable, Text, Text, IntWritable> { private final static IntWritable one = new IntWritable(1); private Text word = new Text(); public void map(LongWritable key, Text value, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException { String line = value.toString(); StringTokenizer tokenizer = new StringTokenizer(line); while (tokenizer.hasMoreTokens()) { word.set(tokenizer.nextToken()); output.collect(word, one); } } } public static class Reduce extends MapReduceBase implements Reducer<Text, IntWritable, Text, IntWritable> { public void reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException { int sum = 0; while (values.hasNext()) { sum += values.next().get(); } output.collect(key, new IntWritable(sum)); } } public static void main(String[] args) throws Exception { JobConf conf = new JobConf(WordCount.class); conf.setJobName("wordcount"); conf.setOutputKeyClass(Text.class); conf.setOutputValueClass(IntWritable.class); conf.setMapperClass(Map.class); conf.setCombinerClass(Reduce.class); conf.setReducerClass(Reduce.class); conf.setInputFormat(TextInputFormat.class); conf.setOutputFormat(TextOutputFormat.class); FileInputFormat.setInputPaths(conf, new Path(args[0])); FileOutputFormat.setOutputPath(conf, new Path(args[1])); JobClient.runJob(conf); } }
然后进行编译打包
$ mkdir wordcount_classes $ javac -classpath ${HADOOP_HOME}/hadoop-${HADOOP_VERSION}-core.jar -d wordcount_classes WordCount.java $ jar -cvf /usr/joe/wordcount.jar -C wordcount_classes/ .
编译完了可以看到有这三个文件
root@cluster-1:~/workspace/hadoop-1.1.2/wordcount_classes/org/myorg# ll total 20 drwxr-xr-x 2 root root 4096 2013-06-20 18:00 ./ drwxr-xr-x 3 root root 4096 2013-06-20 18:00 ../ -rw-r--r-- 1 root root 1546 2013-06-20 18:00 WordCount.class -rw-r--r-- 1 root root 1938 2013-06-20 18:00 WordCount$Map.class -rw-r--r-- 1 root root 1611 2013-06-20 18:00 WordCount$Reduce.class
打包完得到 wordcount.jar
如果之前有按照官方例程做那个 example 的话,需要先清空文件
root@cluster-1:~/workspace/hadoop-1.1.2# bin/hadoop fs -rmr input Deleted hdfs://cluster-1:9000/user/root/input root@cluster-1:~/workspace/hadoop-1.1.2# bin/hadoop fs -rmr output Deleted hdfs://cluster-1:9000/user/root/output
然后根据教程准备好输入的文件
Sample text-files as input:
$ bin/hadoop dfs -ls /usr/joe/wordcount/input/
/usr/joe/wordcount/input/file01
/usr/joe/wordcount/input/file02$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file01
Hello World Bye World$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file02
Hello Hadoop Goodbye Hadoop
然后开始运行
$ bin/hadoop jar wordcount.jar org.myorg.WordCount /user/root/input /user/root/output
得到结果
bin/hadoop fs -cat output/* Bye 1 Goodbye 1 Hadoop 2 Hello 2 World 2