【前言】hadoop2.X后就不能再用eclipse的hadoop插件了，所以在书写hadoop工程的时候尤其是mapreduce计算工程运行时与hadoop1.x稍有区别，在此简短做一记录

注意：这里以jar包的方式运行mapreduce工程

典型案例wordCount

【1】书写WCMapper

import java.io.IOException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class WCMapper extends Mapper<LongWritable, Text, Text, LongWritable>{

    @Override
    protected void map(LongWritable key, Text value, Context context)
            throws IOException, InterruptedException {
        //accept
        String line = value.toString();
        //split
        String[] words = line.split(" ");
        //loop
        for(String w : words){
            //send
            context.write(new Text(w), new LongWritable(1));
        }
    }
}

【2】书写WCReduce

import java.io.IOException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class WCReducer extends Reducer<Text, LongWritable, Text, LongWritable>{

    @Override
    protected void reduce(Text key, Iterable<LongWritable> values, Context context)
            throws IOException, InterruptedException {
        //define a counter
        long counter = 0;
        //loop
        for(LongWritable l : values){
            counter += l.get();
        }
        //write
        context.write(key, new LongWritable(counter));
    }
}

【3】书写WordCount

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
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.output.FileOutputFormat;

public class WordCount {
    public static void main(String[] args) throws Exception {
        Job job = Job.getInstance(new Configuration());
        
        //notice
        job.setJarByClass(WordCount.class);
        
        //set mapper`s property
        job.setMapperClass(WCMapper.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(LongWritable.class);
        FileInputFormat.setInputPaths(job, new Path("/mrDemo/input/words.txt"));
        
        //set reducer`s property
        job.setReducerClass(WCReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(LongWritable.class);
        FileOutputFormat.setOutputPath(job, new Path("/mrDemo/output/wcCount"));
        
        //submit
        job.waitForCompletion(true);
    }
}

【4】打jar包

步骤：
        右击 > export > java > JAR file > next > 填写jar名称 > next > next > 选择main方法入口

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【5】将jar包和所要计算的文件上传至linux
注意：这里所要分析的文件必须存入再次上传到HDFS，而且其路径要与代码中inputPath的路径一致
【6】hadoop运行jar

hadoop jar /jar包路径

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