下面是四個檔案及其內容。

程式碼實現：

Mapper：

package cn.tedu.invert;

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;

 
public class InvertMapper extends Mapper<LongWritable, Text, Text, Text> {
    
    @Override
    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        // 獲取檔名
        FileSplit fileSplit = (FileSplit)context.getInputSplit();
        String pathName 
 
= fileSplit.getPath().getName();
        
        // 將檔案中的內容提取 
        String[] words = value.toString().split(" ");
        
        // 每一個單詞都對應著自己所在檔案的檔名
        for(String word:words){
            context.write(new Text(word), new Text(pathName));
        }
    }
}

Reducer：

package cn.tedu.invert;

 
import java.io.IOException;
import java.util.HashSet;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class InvertReducer extends Reducer<Text, Text, Text, Text> {

    public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        
        // 雜湊表不存重複元素，將重複的檔名去掉
        HashSet<String> set = new HashSet<>();
        for (Text text : values) {
            set.add(text.toString());
        }
        
        StringBuilder sb = new StringBuilder();
        for (String str : set) {
            sb.append(str.toString()).append(" ");
        }
        
        context.write(key, new Text(sb.toString()));
    }
}

 Driver：

package cn.tedu.invert;

import org.apache.hadoop.conf.Configuration;
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;

public class InvertDriver {

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "JobName");
        job.setJarByClass(cn.tedu.invert.InvertDriver.class);
        job.setMapperClass(InvertMapper.class);
        job.setReducerClass(InvertReducer.class);
        
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        FileInputFormat.setInputPaths(job, new Path("hdfs://192.168.74.129:9000/text/invert"));
        FileOutputFormat.setOutputPath(job, new Path("hdfs://192.168.74.129:9000/result/invert_result"));

        if (!job.waitForCompletion(true))
            return;
    }
}

結果：