Java怎么编写Mapreduce程序

编写MapReduce程序的基本步骤如下：

创建一个实现了Mapper接口的类，重写map方法。map方法接收一个键值对作为输入，将输入数据处理并输出为中间键值对。

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();
        StringTokenizer tokenizer = new StringTokenizer(line);
        while (tokenizer.hasMoreTokens()) {
            word.set(tokenizer.nextToken());
            context.write(word, one);
        }
    }
}

创建一个实现了Reducer接口的类，重写reduce方法。reduce方法接收中间键值对作为输入，将输入数据根据键汇总并输出为最终结果键值对。

public class MyReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
    private IntWritable result = new IntWritable();

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

创建一个配置对象，设置MapReduce作业的相关参数。

Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount.class);

指定输入数据的路径和输出结果的路径。

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

设置Mapper和Reducer的类。

job.setMapperClass(MyMapper.class);
job.setCombinerClass(MyReducer.class);
job.setReducerClass(MyReducer.class);

设置最终结果的键值对类型。

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

提交MapReduce作业。

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

以上就是编写MapReduce程序的基本步骤。根据具体需求，可以对Mapper和Reducer的逻辑进行扩展和修改。