Mapper 类Reducer 类 Driver 类怎么写

Mapper 类、Reducer 类和 Driver 类是 Hadoop MapReduce 中的三个重要组件。下面是它们的基本代码实现：

Mapper 类：

public class MyMapper extends Mapper<LongWritable, Text, Text, IntWritable> {

    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String line = value.toString();
        String[] words = line.split(" ");
        for (String w : words) {
            word.set(w);
            context.write(word, one);
        }
    }
}

Reducer 类：

public class MyReducer extends Reducer<Text, IntWritable, Text, IntWritable> {

    public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        int sum = 0;
        for (IntWritable val : values) {
            sum += val.get();
        }
        context.write(key, new IntWritable(sum));
    }
}

Driver 类：

public class MyDriver {

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "word count");
        job.setJarByClass(MyDriver.class);
        job.setMapperClass(MyMapper.class);
        job.setCombinerClass(MyReducer.class);
        job.setReducerClass(MyReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}

以上代码实现了一个简单的 WordCount 示例，其中 MyMapper 类和 MyReducer 类分别实现了 Mapper 和 Reducer 接口，并重写了 map 和 reduce 方法。MyDriver 类则是程序的入口，它配置了 Job 的各种参数，并提交运行任务。