使用MapperJoin的方法进行多表查询

customers表：

orders表：

CustomerOrder类：

//mapperjoin推荐使用，但是只适用于两张表格有一张小表格的情况，假设costumer是小文件
public class CustomerOrder implements Writable {
    private String userId="";
    private String userName="";
    private String age="";
    private String orderId="";
    private String goodId="";
    private String buyNum="";
   
    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(userId);
        out.writeUTF(userName);
        out.writeUTF(age);
        out.writeUTF(orderId);
        out.writeUTF(goodId);
        out.writeUTF(buyNum);
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        this.userId = in.readUTF();
        this.userName = in.readUTF();
        this.age = in.readUTF();
        this.orderId = in.readUTF();
        this.goodId = in.readUTF();
        this.buyNum = in.readUTF();
    }
    省略geter,seter,tostring,构造器

MapJoinMapper:

import org.apache.commons.beanutils.BeanUtils;
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.FileReader;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.net.URI;
import java.util.HashMap;
import java.util.Map;

public class MapJoinMapper extends Mapper<LongWritable, Text,CustomerOrder, NullWritable> {
    Map<String,CustomerOrder> custs = new HashMap<>();//存customer表的map

    //只运行前执行一次
    @Override
    protected void setup(Context context) throws IOException, InterruptedException {
        //获取服务器节点缓存文件
        URI[] files = context.getCacheFiles();
        //判断是否为空
        if (null!=files&&files.length>0){
            String path = files[0].getPath();
            BufferedReader br = new BufferedReader(new FileReader(path));
            String line ="";
            while ((line=br.readLine())!=null){
                String[] infos = line.split(",");
                CustomerOrder co = new CustomerOrder();
                co.setUserId(infos[0]);
                co.setUserName(infos[1]);
                co.setAge(infos[2]);
                //用户userid作为map的key，其它属性填充的customer对象作为值value
                custs.put(co.getUserId(),co);
            }
        }
    }

    //每解析一行数据运行一次
    @Override
    protected void map(LongWritable key, Text value,Context context) throws IOException, InterruptedException {
        String[] infos = value.toString().split(",");//orderid,userid,goodid,buynunm
        //找到订单对应的用户
        CustomerOrder co = custs.get("1");
        //复制订单用户信息对象
        CustomerOrder cpCo = new CustomerOrder();
        try {
            BeanUtils.copyProperties(cpCo,co);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        //把订单信息填充到对应的用户订单信息表中
        cpCo.setOrderId(infos[0]);
        cpCo.setGoodId(infos[2]);
        cpCo.setBuyNum(infos[3]);
        //输出
        context.write(cpCo,NullWritable.get());
    }
}

MapJoinJob：

import org.apache.hadoop.conf.Configuration;
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;
import java.net.URI;
import java.net.URISyntaxException;

public class MapJoinJob {
    public static void main(String[] args) throws IOException, URISyntaxException, InterruptedException, ClassNotFoundException {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        job.setJarByClass(MapJoinJob.class);
        Path path = new Path("file:///e:/temp1/output");
        if (path.getFileSystem(conf).exists(path)){
            path.getFileSystem(conf).delete(path,true);
        }
        //设置缓存文件的路径
        job.addCacheFile(new URI("file:///e:/temp1/read1/customers1.csv"));
        
        //设置输入输出文件
   FileInputFormat.setInputPaths(job,"file:///e:/temp1/read1/orders.csv");
        FileOutputFormat.setOutputPath(job,path);
        //设置mapper类
        job.setMapperClass(MapJoinMapper.class);
        job.setMapOutputKeyClass(CustomerOrder.class);
        job.setMapOutputValueClass(NullWritable.class);

        job.setNumReduceTasks(0);
        job.waitForCompletion(true);
    }
}

注意：

1. mapperjoin推荐使用，但是只适用于两张表格有一张小表格的情况
2. 可以MapJoin省略Reducer，但是没有省略Map仅使用Reducer的
3. 一般遇到输出是空文件夹的情况，最好使用DeBug的方法，将断点打在每个方法的入口处，先检查方法是否进入，再根据具体情况分析。
4. 创建的文件编码十分重要，最好能使用notepad打开查看，否则Windows环境下的编码十分容易造成空输出问题的出现。例如下面两张图，第一张图输出空文件夹，第二张图正常输出。