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，下面是一个最基本的WordCount单词统计的MapReduce程序，包括自定义的Mapper类和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 Text word = new Text();
    private LongWritable one = new LongWritable(1);

    @Override
    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String line = value.toString();
        String[] words = line.split("\\s+");
        for (String w : words) {
            word.set(w);
            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> {
    private LongWritable count = new LongWritable();

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

在Driver类中，需要进行如下配置：

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 {
        if (args.length != 2) {
            System.err.println("Usage: WordCount <input path> <output path>");
            System.exit(-1);
        }

        Job job = new Job();
        job.setJarByClass(WordCount.class);
        job.setJobName("Word Count");

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

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

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

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

在运行程序时，需要指定输入文件路径和输出文件路径，例如：

$ hadoop jar WordCount.jar input output

程序会读取输入路径下的文件，统计每个单词出现的次数，并将结果输出到输出路径下的文件中。