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;
    }


    @Override
    public int compareTo(nj.zb.kb21.demo5.StudentScore o) {
        return 0;
    }

    @Override
    public 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>();

    @Override
    protected 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();
            }
        }
    }

    @Override
    protected 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 score

        StudentSc 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