本文主要是介绍MapReduce(五)：表关联，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
给出：
Tom Lucy
Tom Jack
Jone Lucy
Jone Jack
Lucy Mary
Lucy Ben
Jack Alice
Jack Jesse
输出：
Tom Alice
Tom Jesse
Jone Alice
Jone Jesse
Tom Mary
Tom Ben
Jone Mary
Jone Ben
源代码如下：
import java.io.IOException;import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
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 SingleTable_Join {public static class STMapper extends Mapper<Object,Text,Text,Text>{private Text k1=new Text();private Text v1=new Text();private Text k2=new Text();private Text v2=new Text();@Overrideprotected void map(Object key, Text value,Context context)throws IOException, InterruptedException {String line=value.toString();String[]str=line.split(" ");String a=str[0];String b=str[1];String relationType=new String();//建立左表relationType="1";k1.set(b);v1.set(relationType+" "+a);context.write(k1, v1);//建立右表relationType="2";k2.set(a);v2.set(relationType+" "+b);context.write(k2, v2);}}public static class STReducer extends Reducer<Text,Text,Text,Text>{private Text k=new Text();private Text v=new Text();@Overrideprotected void reduce(Text key, Iterable<Text> values,Context context)throws IOException, InterruptedException {String[]grandChild=new String[10];String[]grandParent=new String[10];int grandChildNum=0;int grandParentNum=0;for(Text t:values){String str=t.toString();String[] s=str.split(" ");if(s.length==0)continue;if(s[0].compareTo("1")==0)grandChild[grandChildNum++]=s[1];elsegrandParent[grandParentNum++]=s[1];}if(grandChildNum!=0&&grandParentNum!=0){for(int i=0;i<grandChildNum;i++){k.set(grandChild[i]);for(int j=0;j<grandParentNum;j++){v.set(grandParent[j]);context.write(k,v);}}}}}public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {Configuration conf=new Configuration();Job job=Job.getInstance(conf,"SingleTable_Join");job.setJarByClass(SingleTable_Join.class);job.setMapperClass(STMapper.class);job.setMapOutputKeyClass(Text.class);job.setMapOutputValueClass(Text.class);job.setReducerClass(STReducer.class);job.setOutputKeyClass(Text.class);job.setOutputValueClass(Text.class);FileInputFormat.setInputPaths(job, new Path(args[0]));FileOutputFormat.setOutputPath(job, new Path(args[1]));System.exit(job.waitForCompletion(true)? 0:1);}}
注：多表关联思路也一样！
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