MapReduce的WordCount

import org.apache.hadoop.conf.Configuration;
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.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;

public class WordCount {
    //map端继承hadoop的Mapper先把泛型填上
    // 前面两个是进入map端的key和value的数据类型，
    // 后面是从map端出去时候的数据类型
    public static class WordCountMap extends Mapper{
        //重写map方法
        @Override
        protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
            //输出key和value
            context.write(value,new LongWritable(1));
        }
    }
    //reduce端与map端道理相同，不同的是进入的key和value数据类型是与map端输出的一样，
    // 输出的数据类型根据具体要求
    public  static class WordCountReduce extends Reducer{
        @Override
        protected void reduce(Text key, Iterable values, Context context) throws IOException, InterruptedException {
            long sum=0;
            for (LongWritable value : values) {
                sum=sum+value.get();
            }
            context.write(key,new LongWritable(sum));
        }
    }

    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        //配置 通过job进行配置mapreduce
        Job job = Job.getInstance();
        //设置名字
        job.setJobName("第一个mapreduce程序");
        //map端所在类的位置
        job.setMapperClass(WordCountMap.class);
        //reduce端所在类的位置
        job.setReducerClass(WordCountReduce.class);
        //指定jar包
        job.setJarByClass(WordCount.class);
        //设置map端输出类型
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(LongWritable.class);
        //指定reduce端输出数据
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(LongWritable.class);
        //指定路径
        Path path = new Path("/data/test.txt");
        Path path1 = new Path("/output");
        //如果不存在就将路径添加进去，否则删除然后再输出
        FileSystem fileSystem = FileSystem.get(new Configuration());
        if(fileSystem.exists(path1)){
            fileSystem.delete(path1,true);
        }
        FileInputFormat.addInputPath(job, path);
        FileOutputFormat.setOutputPath(job, path1);
        job.waitForCompletion(true);
    }
}

hadoop jar 打包的jar包名 路径 (路径写从java开始的全路径）

注意：1： 要指定jar包所在类。2：数据要传输到指定的hdfs路径。3：要开启hdfs，在虚拟机上运行。

 会进行mapreduce