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