mapreduce (六) MapReduce实现去重 NullWritable的使用

习题来源：http://www.cnblogs.com/xia520pi/archive/2012/06/04/2534533.html
file1

2012-3-1 a

2012-3-2 b

2012-3-3 c

2012-3-4 d

2012-3-5 a

2012-3-6 b

2012-3-7 c

2012-3-3 c

file2

2012-3-1 b

2012-3-2 a

2012-3-3 b

2012-3-4 d

2012-3-5 a

2012-3-6 c

2012-3-7 d

2012-3-3 c





import java.io.IOException;



import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.fs.Path;

import org.apache.hadoop.io.NullWritable;

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.input.TextInputFormat;

import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;



public class MyDedup {



    public static class LineNullMapper extends Mapper<Object, Text, Text, NullWritable>{

        public void map(Object key, Text value, Context context) throws IOException, InterruptedException{

            context.write(value, NullWritable.get());

        }

    }

    

    public static class SortReducer extends Reducer<Text, NullWritable, Text, NullWritable>{

        public void reduce(Text key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException{

            context.write(key, NullWritable.get());

        }

    }
   

如果把Iterable<NullWritable> values 替换为 NullWritable values 如果是不用Iterable迭代器的话，则不进行分组么？
结果是：只排序了并没有完成去重
2012-3-1 a

2012-3-1 b

2012-3-2 a

2012-3-2 b

2012-3-3 b

2012-3-3 c

2012-3-3 c

2012-3-3 c

2012-3-4 d

2012-3-4 d

2012-3-5 a

2012-3-5 a

2012-3-6 b

2012-3-6 c

2012-3-7 c

2012-3-7 d

 

public static void main(String[] args) throws Exception {

        

        String dir_in = "hdfs://localhost:9000/in_dedup";

        String dir_out = "hdfs://localhost:9000/out_dedup";



        Path in = new Path(dir_in);

        Path out = new Path(dir_out);

        

        Configuration conf = new Configuration();

        Job sortJob = new Job(conf, "my_dedup");



        sortJob.setJarByClass(MyDedup.class);



        sortJob.setInputFormatClass(TextInputFormat.class);

        sortJob.setMapperClass(LineNullMapper.class);

        sortJob.setCombinerClass(SortReducer.class);

        //countJob.setPartitionerClass(HashPartitioner.class);

        sortJob.setMapOutputKeyClass(Text.class);

        sortJob.setMapOutputValueClass(NullWritable.class);



        FileInputFormat.addInputPath(sortJob, in);



        sortJob.setReducerClass(SortReducer.class);

        // countJob.setNumReduceTasks(1);

        sortJob.setOutputKeyClass(Text.class);

        sortJob.setOutputValueClass(NullWritable.class);

        //countJob.setOutputFormatClass(SequenceFileOutputFormat.class);



        FileOutputFormat.setOutputPath(sortJob, out);



        sortJob.waitForCompletion(true);



    }



}

运行结果：
2012-3-1 a

2012-3-1 b

2012-3-2 a

2012-3-2 b

2012-3-3 b

2012-3-3 c

2012-3-4 d

2012-3-5 a

2012-3-6 b

2012-3-6 c

2012-3-7 c

2012-3-7 d