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);

     }

 }