要求

输出的样式

三张表

score.csv

student.csv

subject.csv

创建三个类

StudentSc

getset方法

插入getset方法，可用javabean插件一键生成

实现类

 public StudentSc(String stuName, String subName, Integer scScore, String flag) {this.stuName = stuName;this.subName = subName;this.scScore = scScore;}@Overridepublic int compareTo(nj.zb.kb21.demo5.StudentScore o) {return 0;}@Overridepublic void write(DataOutput dataOutput) throws IOException {dataOutput.writeUTF(stuName);dataOutput.writeUTF(subName);dataOutput.writeInt(scScore);}

MapJoinDriver

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;import java.io.IOException;public class MapJoinDriver {public static void main(String[] args) throws IOException, InterruptedException, ClassNotFoundException {Configuration configuration = new Configuration();Job job = Job.getInstance(configuration);job.setJarByClass(MapJoinDriver.class);job.setMapperClass(MapJoinMapper.class);job.setMapOutputKeyClass(StudentSc.class);job.setMapOutputValueClass(NullWritable.class);Path inPath = new Path("D:\\kb21\\myworkspace\\njzb\\hadoopexec\\in\\demo6\\score.csv");FileInputFormat.setInputPaths(job,inPath);Path outPath = new Path("D:\\kb21\\myworkspace\\njzb\\hadoopexec\\out7");FileSystem fs = FileSystem.get(outPath.toUri(), configuration);if (fs.exists(outPath)){fs.delete(outPath,true);}FileOutputFormat.setOutputPath(job,outPath);//设置Reduce阶段的任务数量job.setNumReduceTasks(0);//配置Map阶段的缓存，尽量使用小文件做缓存，如果文件太大，会引起OOM（内存溢出）Path cachePath = new Path("D:\\kb21\\myworkspace\\njzb\\hadoopexec\\in\\demo6\\student.csv");job.addCacheFile(cachePath.toUri());Path cachePath2 = new Path("D:\\kb21\\myworkspace\\njzb\\hadoopexec\\in\\demo6\\subject.csv");job.addCacheFile(cachePath2.toUri());boolean result = job.waitForCompletion(true);System.out.println(result);}
}

用mapjoin不需要reduce

MapJoinMapper

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URI;
import java.util.HashMap;
import java.util.Map;public class MapJoinMapper extends Mapper<LongWritable, Text, StudentSc, NullWritable> {Map<Integer, StudentSc> studentScMap = new HashMap<Integer, StudentSc>();Map<Integer, StudentSc> studentScMap2 = new HashMap<Integer, StudentSc>();@Overrideprotected void setup(Mapper<LongWritable, Text, StudentSc, NullWritable>.Context context) throws IOException, InterruptedException {URI[] cacheFiles = context.getCacheFiles();for (URI uri : cacheFiles) {String currentFileName = new Path(uri).getName();if (currentFileName.startsWith("student")) {String path = uri.getPath();BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(path)));String line;while ((line = br.readLine()) != null) {String[] fields = line.split(",");StudentSc studentSc = new StudentSc(fields[1],"",0,"");studentScMap.put(Integer.parseInt(fields[0]), studentSc);
> 这里按照要求将student的名字添加到studentScMap表中}br.close();}if (currentFileName.startsWith("subject")) {String path = uri.getPath();BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(path)));String line;while ((line = br.readLine()) != null) {String[] fields = line.split(",");StudentSc studentSc = new StudentSc("",fields[1],0,"");studentScMap2.put(Integer.parseInt(fields[0]), studentSc);
>这里按照要求将subject的科目名字添加到studentScMap2表中}br.close();}}}@Overrideprotected void map(LongWritable key, Text value, Mapper<LongWritable, Text, StudentSc, NullWritable>.Context context) throws IOException, InterruptedException {String[] scFields = value.toString().split(",");//这个集合读取的是driver中的inpath的表 scoreStudentSc currentStudent = studentScMap.get(Integer.parseInt(scFields[0]));StudentSc currentStudent2 = studentScMap2.get(Integer.parseInt(scFields[1]));StudentSc studentScs = new StudentSc();studentScs.setStuName(currentStudent.getStuName());studentScs.setFlag("0");//flag不重要，是我上一个项目多写的，懒得删studentScs.setSubName(currentStudent2.getSubName());studentScs.setScScore(Integer.parseInt(scFields[2]));context.write(studentScs, NullWritable.get());}
}

运行结果