编写一个最基本的wordcount单词统计的mapreduce,要求写出自定义的maper类和reducer类.

好的，下面是一个基本的Word Count MapReduce程序，包括自定义的Mapper和Reducer类。

Mapper类：

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

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

  private final static LongWritable ONE = new LongWritable(1);
  private Text word = new Text();

  @Override
  protected void map(LongWritable key, Text value, Context context)
      throws IOException, InterruptedException {
    String[] words = value.toString().split("\\s+");
    for (String s : words) {
      word.set(s);
      context.write(word, ONE);
    }
  }
}

Reducer类：

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

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

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

主程序：

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 {
    Configuration conf = new Configuration();

    Job job = Job.getInstance(conf, "Word Count");
    job.setJarByClass(WordCount.class);

    job.setMapperClass(WordCountMapper.class);
    job.setReducerClass(WordCountReducer.class);

    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(LongWritable.class);

    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));

    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

在运行时，需要将输入文件和输出文件的路径作为参数传递给程序。例如：

hadoop jar WordCount.jar WordCount input output

其中，WordCount.jar是打包好的程序文件，input是输入文件路径，output是输出文件路径。