# 示例

• A:C,F,H,M,E
  B:F,H,E,X,C
  C:B,F,D,E
  D:M,H,B,D

Map阶段

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

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.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 com._51doit.AnLie.FlowMapper;
import com._51doit.AnLie.FlowReducer;
import com.sun.jersey.api.ConflictException;

public class Friends {
   static class FriendsMapper extends Mapper<LongWritable, Text, Text, Text>
   {
     Text k= new Text();
     Text v= new Text();
      @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Text, Text>.Context context)
            throws IOException, InterruptedException {
        String line = value.toString();
        String[] sp = line.split(":");
        String user = sp[0];//value
        v.set(user);
        String fs = sp[1];
        String[] f = fs.split(",");//
        for (String st : f) {//key
            k.set(st);
            context.write(k, v);//输出为b,a c,a等
        }
    } 
   }
   static class FriendsReducer extends Reducer<Text, Text, Text, NullWritable>
   {
       @Override
    protected void reduce(Text key, Iterable<Text> itera, Reducer<Text, Text, Text,NullWritable>.Context context)
            throws IOException, InterruptedException {
           //Text都是一个一个的,我们要把他们的共同用户放置一个list集合里
           List<String> list = new ArrayList<>();
        for (Text text : itera) {
            //Text是同一个对象,但是toString不是同一个对象
            list.add(text.toString());
        }
        Collections.sort(list);
        for (int i = 0; i < list.size()-1; i++) {
            for (int j = i+1; j < list.size(); j++) {
                String k=list.get(i)+"和"+list.get(j)+"的共同好友是:"+key.toString();
                context.write(new Text(k), NullWritable.get());
            }
        }
    }
   }
   public static void main(String[] args) throws Exception {
     //配置
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        // 设置map和reduce类  调用类中自定义的map reduce方法的业务逻辑
        job.setMapperClass(FriendsMapper.class);
        job.setReducerClass(FriendsReducer.class);
        //Mapper的输出格式
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);
        //Reducer的输出格式
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(NullWritable.class);
        //输出文件
        FileInputFormat.setInputPaths(job, new Path("D:\\www"));
        //输入文件
        FileOutputFormat.setOutputPath(job, new Path("D:\\rr"));
        //提交任务
        job.waitForCompletion(true);
}
}

第一阶段的结果：
C和D的共同好友是:B A和B的共同好友是:C C和D的共同好友是:D A和B的共同好友是:E A和C的共同好友是:E
B和C的共同好友是:E A和B的共同好友是:F A和C的共同好友是:F B和C的共同好友是:F A和B的共同好友是:H
A和D的共同好友是:H B和D的共同好友是:H A和D的共同好友是:M

Reduce阶段

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
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 Friends2 {
   static class FriendsMapper extends Mapper<LongWritable, Text, Text, Text>
   {
     Text k= new Text();
     Text v= new Text(); 
      @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Text, Text>.Context context)
            throws IOException, InterruptedException {
        String line = value.toString();
        String[] sp = line.split(":");
        k.set(sp[0]);
        v.set(sp[1]);
        context.write(k, v);
   }
   }
   static class FriendsReducer extends Reducer<Text, Text, Text, Text>
   {    Text v=new Text();
       @Override
    protected void reduce(Text key, Iterable<Text> itera, Reducer<Text, Text, Text,Text>.Context context)
            throws IOException, InterruptedException {
           
          StringBuilder b = new StringBuilder();
        for (Text text : itera) {
            b.append(text.toString()+"");
        }
       v.set(b.toString().trim());
       context.write(key, v);
   }
   }
   public static void main(String[] args) throws Exception {
     //配置
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        // 设置map和reduce类  调用类中自定义的map reduce方法的业务逻辑
        job.setMapperClass(FriendsMapper.class);
        job.setReducerClass(FriendsReducer.class);
        //Mapper的输出格式
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);
        //Reducer的输出格式
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        //输出文件
        FileInputFormat.setInputPaths(job, new Path("D:\\rr"));
        //输入文件
        FileOutputFormat.setOutputPath(job, new Path("D:\\vvv"));
        //提交任务
        job.waitForCompletion(true);
}
}

第二阶段结果：
A和B的共同好友是 FCHE A和C的共同好友是 EF A和D的共同好友是 MH B和C的共同好友是 EF
B和D的共同好友是 H C和D的共同好友是 DB

MapReduce的两个阶段分为治之的思想，奠定了大数据离线批处理的si'xin思想。有兴趣的，可以找一下Goole的那3篇关于离线处理大数据思想的论文。