MapReduce---连接操作--map端连接

在项目开发中，要实现两个“表”的join操作，其中一个表数据量小，一个表很大，这种场景在实际中非常常见，比如“订单日志” join “产品信息”采用map端连接

 原理：适用于大表 + 小表(载入内存)。

map之前执行，加载文件到内存，形成map

可以大大提高join操作的并发度，加快处理速度

1、JoinMapper

package hadoop.join.map;

import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;

/**
 * Mapper
 */
public class JoinMapper extends Mapper<LongWritable,Text ,Text,NullWritable>{
	private Map<String,String> customers  ;

	/**
	 * map之前执行，加载文件到内存，形成map
	 */
	protected void setup(Context context) throws IOException, InterruptedException {
		//加载customers.txt
		customers = new HashMap<String, String>();
		String path = context.getConfiguration().get("customers.path") ;
		FSDataInputStream in = FileSystem.get(context.getConfiguration()).open(new Path(path));
		BufferedReader br = new BufferedReader(new InputStreamReader(in)) ;
		String line = null ;
		while((line = br.readLine()) != null){
			String[] arr = line.split(",");
			customers.put(arr[0] , line) ;
		}
     }

2.App

package hadoop.join.map;

import com.it18zhang.hadoop.lean.key.DataLeanMapper1;
import com.it18zhang.hadoop.lean.key.DataLeanMapper2;
import com.it18zhang.hadoop.lean.key.DataLeanReducer1;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.KeyValueTextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

/**
 * join:map端连接
 */
public class App {
	public static void main(String[] args) throws Exception {
		args = new String[]{"d:/java/mr/join/orders.txt", "d:/java/mr/out", "d:/java/mr/join/customers.txt" } ;
		Configuration conf = new Configuration();
		conf.set("customers.path",args[2]);

		FileSystem fs = FileSystem.get(conf);
		if(fs.exists(new Path(args[1]))){
			fs.delete(new Path(args[1]),true);
		}

		Job job = Job.getInstance(conf);

		job.setJobName("join-map");
		job.setJarByClass(App.class);

		job.setMapperClass(JoinMapper.class);

		//添加输入路径
		FileInputFormat.addInputPath(job,new Path(args[0]));
		//设置输出路径
		FileOutputFormat.setOutputPath(job,new Path(args[1]));

		//设置mapreduce输出
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(NullWritable.class);

		job.setNumReduceTasks(0);

		//第一个阶段(job)
		job.waitForCompletion(true) ;
	}
}