Hadoop--Map/Reduce实现多表链接

MR实现多表连接的原理和单表连接时一样的，甚至比单表连接还要简单。

在map阶段只需要根据文件的名称区分左表还是右表。使用关联的字段作为key2。

在reduce中对values中的值分别存储到一个左表list和右表list中。对左表list和右表list进行一个笛卡尔积完事。

import java.io.*;
import java.util.*;

import org.apache.hadoop.io.*;
import org.apache.hadoop.util.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapreduce.*;
import org.apache.hadoop.mapreduce.lib.input.*;
import org.apache.hadoop.mapreduce.lib.output.*;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.util.Tool;
public class MTjoin extends Configured implements Tool {
    /*
     * 多表链接，与单表链接思路类似。将关联列作为map的key值，用数字区分左表和右表。在Reduce阶段对两个表进行笛卡尔积
     * */
    public static class Map extends Mapper<LongWritable,Text,Text,Text>{
        public void map(LongWritable key,Text value,Context context)throws IOException,InterruptedException{
            String line=value.toString();
            int linelen=line.length();
            //去除文件首行
            if(line.indexOf("factoryname")==-1&&line.indexOf("addressID")==-1)
            {
                //处理factory数据
                if(line.charAt(linelen-2)==' ')
                {
                    String facstr="1"+line.substring(0, linelen-2);
                    String addrestr=String.valueOf(line.charAt(linelen-1));
                    context.write(new Text(addrestr), new Text(facstr));
                }else{
                    String addreidstr=String.valueOf(line.charAt(0));
                    String addrenastr="2"+line.substring(1);
                    context.write(new Text(addreidstr), new Text(addrenastr));
                }
                                
            }
        }
        
    }
    
    public static class Reduce extends Reducer<Text,Text,Text,Text>{
        public void reduce(Text key,Iterable<Text> values,Context context)throws IOException, InterruptedException{
            ArrayList<String> facarr=new ArrayList<String>();
            ArrayList<String> addarr=new ArrayList<String>();
            for(Text var:values){
                if(var.toString().charAt(0)=='1')
                {
                    facarr.add(var.toString().substring(1));
                }else if(var.toString().charAt(0)=='2')
                {
                    addarr.add(var.toString().substring(1));
                }
                
            }
            if(facarr.size()!=0&&addarr.size()!=0)
            {
                for(int i=0;i<facarr.size();i++)
                {
                    context.write(new Text(facarr.get(i)), new Text(addarr.get(0)));
                }
                
            }
        }
    }
    @Override
    public int run(String[] args) throws Exception {
        // TODO Auto-generated method stub
        Configuration conf=new Configuration();
        Job job=new Job(conf,"MTjoin");
        job.setJarByClass(MTjoin.class);
        
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        
        job.setMapperClass(Map.class);
        job.setReducerClass(Reduce.class);
        
        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);
        
        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        
        boolean success=job.waitForCompletion(true);
        return success?0:1;
    }
    public static void main(String[] args)throws Exception{
        int ret=ToolRunner.run(new MTjoin(), args);
        System.exit(ret);
    }

}