hadoop2.0 MapReduce编程（java客户端）

1  通用开发步骤

       创建java工程

       引入相应的hadoop相关jar

               share/hadoop/mapreduce下面的全部jar
               share/hadoop/common/hadoop-common-2.7.1.jar
               share/hadoop/common/lib全部jar

      编写mapreduce程序

      导出为jar包

      将jar包拷贝到hadoop集群环境上

2  wordcount案例

用pig客户端将文件上传到HDFS

grunt> copyFromLocal /usr/local/input.txt /input
grunt> cat /input
hello
world
hadoop
hbase
hive
hadoop

import java.io.IOException;
import java.util.StringTokenizer;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.examples.WordCount.IntSumReducer;
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 static class WordCountMapper 
		extends Mapper<Object, Text, Text, IntWritable>{

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

		@Override
		protected 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()){
				word.set(itr.nextToken());
				context.write(word, one);
			}
		}

	}

	public static class WordCountReduce
		extends Reducer<Text, IntWritable, Text, IntWritable>{

		private IntWritable result = new IntWritable();

		@Override
		protected void reduce(Text key, Iterable<IntWritable> values,
				Reducer<Text, IntWritable, Text, IntWritable>.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.out.println("Usage:wordcount <in> <out>");
			System.exit(2);
		}
		Job job = new Job(conf,"wodr count");
		job.setJarByClass(WordCount.class);
		job.setMapperClass(WordCountMapper.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);
	}
}

运行并查看结果

root@localhost hadoop-2.7.1]# bin/hadoop jar /usr/local/wordcount.jar WordCount /input /output
grunt> cat /output
hadoop	2
hbase	1
hello	1
hive	1
world	1