[hadoop](1) MapReduce:ChainMapper

前言

本章主要讲述的是对于hadoop生态系统中，MapReduce写的ChainMapper的学习。MapReduce是hadoop集群数据处理的默认框架。而对于数据集中所有的数据必然有一些不友好的数据，我们需要将其丢弃。我们称之为数据的预处理。所以我们需要将预处理模块与数据处理逻辑分开，以便以后可以复用数据预处理模块。以下是一个mapper的通用模式：

• 丢弃无用的已损坏的数据
• 处理有效数据，提取感兴趣的字段
• 针对这些字段，输出我们感兴趣的数据

准备工作

数据集：ufo-60000条记录，这个数据集有一系列包含下列字段的UFO目击事件记录组成，每条记录的字段都是以tab键分割，文件名为ufo.tsv，这里就不提供下载连接了

• sighting date：UFO目击事件发生时间
• Recorded date：报告目击事件的时间
• Location：目击事件发生的地点
• Shape：UFO形状
• Duration：目击事件持续时间
• Dexcription：目击事件的大致描述

例子： 

19950915 19950915 Redmond, WA 6 min. Young man w/ 2 co-workers witness tiny, distinctly white round disc drifting slowly toward NE. Flew in dir. 90 deg. to winds.

ChainMapper介绍

全限定名： org.apache.hadoop.mapred.lib.ChainMapper 

作用：顺序的执行多个mapper，并且最后一个mapper的输出会传递给reducer。

ChainMapper的使用

题目：通过使用 ChainMapper 类验证数据集的记录是否有效，即判断每条记录是否都可以划分为6个字符串

• 上传ufo.tsv到hadoop

hadoop dfs -put ufo.tsv ufo.tsv

• 编写 UFORecordValidationMapper.java 

import java.io.IOException;

import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.mapred.lib.*;

public class UFORecordValidationMapper extends MapReduceBase implements Mapper<LongWritable, Text, LongWritable, Text> {
    public void map(LongWritable key, Text value, OutputCollector<LongWritable, Text> output, Reporter reporter) throws IOException {
        String line = value.toString();
        if(validate(line)) {
            output.collect(key, value);
        }
    }
    
    private boolean validate(String str) {
        String[] parts = str.split("	");
        if(parts.length != 6) {
            return false;
        }
        return true;
    }
}

•  编写 UFOLocation.java 

import java.io.IOException;
import java.util.Iterator;
import java.util.regex.*;

import org.apache.hadoop.conf.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.mapred.lib.*;

public class UFOLocation {
    public static class MapClass extends MapReduceBase implements Mapper<LongWritable, Text, Text, LongWritable> {
        private final static LongWritable one = new LongWritable(1);
        private static Pattern locationPattern = Pattern.compile("[a-zA-Z]{2}[^a-zA-Z]*$");

        public void map(LongWritable key, Text value, OutputCollector<Text, LongWritable> output, Reporter reporter) throws IOException {
            String line = value.toString();
            String[] fields = line.split("	");
            String location = fields[2].trim();
            if(location.length() >= 2) {
                Matcher matcher = locationPattern.matcher(location);
                if(matcher.find()) {
                    int start = matcher.start();
                    String state = location.substring(start, start + 2);
                    output.collect(new Text(state.toUpperCase()), one);
                }
            }
        }
    }

    public static void main(String...args) throws Exception {
        Configuration config = new Configuration();
        JobConf conf = new JobConf(config, UFOLocation.class);
        conf.setJobName("UFOLocation");
        conf.setOutputKeyClass(Text.class);
        conf.setOutputValueClass(LongWritable.class);

        JobConf mapconf1 = new JobConf(false);
        ChainMapper.addMapper(conf, UFORecordValidationMapper.class, LongWritable.class, Text.class, LongWritable.class, Text.class, true, mapconf1);
        JobConf mapconf2 = new JobConf(false);
        ChainMapper.addMapper(conf, MapClass.class, LongWritable.class, Text.class, Text.class, LongWritable.class, true, mapconf2);
        conf.setMapperClass(ChainMapper.class);
        conf.setCombinerClass(LongSumReducer.class);
        conf.setReducerClass(LongSumReducer.class);

        FileInputFormat.setInputPaths(conf, args[0]);
        FileOutputFormat.setOutputPath(conf, new Path(args[1]));
        JobClient.runJob(conf);
    }
}

• 编译上述两个文件

javac UFORecordValidationMapper.java UFOLocation.java

• 将编译好的文件打包成jar

jar cvf ufo.jar UFO*class

• 提交打包好的jar包到hadoop上运行

hadoop jar ufo.jar UFOLocation ufo.tsv output

• 从hadoop上获取结果到本地

hadoop dfs -get output/part-00000 ufo_result.txt

• 查看结果

more ufo_result.txt