在近期的Hadoop的学习中,在学习mapreduce时遇到问题:让求所给数据的top10,们我们指导mapreduce中是有默认的排列机制的,是按照key的升序从大到小排列的

然而top10问题的求解需要按照降序排列。在网上找了很长时间才得以解决,解决方法如下:

 

自定义一个比较器,这个比较器要继承WritableComparator类,代码如下:

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.WritableComparator;

public  class DescSort extends WritableComparator{

     public DescSort() {
         super(LongWritable.class,true);//注册排序组件
    }
     @Override
    public int compare(byte[] arg0, int arg1, int arg2, byte[] arg3,
            int arg4, int arg5) {
        return -super.compare(arg0, arg1, arg2, arg3, arg4, arg5);//注意使用负号来完成降序
    }

     @Override
    public int compare(Object a, Object b) {

        return   -super.compare(a, b);//注意使用负号来完成降序
    }
}

在主函数中要执行时要声明该比较器的类的名称,代码如下:

package Sort;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.RawComparator;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class SortRunner {

    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
         Configuration conf = new Configuration();
         conf.set("fs.defaultFS","hdfs://192.168.252.200:9000");
          Job job = Job.getInstance(conf);
          job.setJarByClass(SortRunner.class);
          
          job.setSortComparatorClass(DescSort.class);
          job.setMapperClass(SortMapper.class);
          job.setReducerClass(SortReducer.class);
      
          job.setMapOutputKeyClass(LongWritable.class);
          job.setMapOutputValueClass(NullWritable.class);
          
          job.setOutputKeyClass(LongWritable.class);
          job.setOutputValueClass(NullWritable.class);
          
          //输入输出的路径
          
          
           FileInputFormat.setInputPaths(job,new Path("/sort/srcdata/"));
         FileOutputFormat.setOutputPath(job, new Path("/sort/output3"));
          System.exit(job.waitForCompletion(true)?0:1);
        
    }
}

注:红色部分便是声明比较器

这样就可以实现降序输出了。

网上与很多按照自定义类类型的排序的输出,在这里便不进行详细介绍,望采纳!!!!