用java写一个MapReduce程序将csv文件中的第六列20171128这种类型的数据拆分成2017-11-28

在Java中编写MapReduce程序来解析CSV文件并拆分日期字符串通常涉及到`FileInputFormat`, `Mapper`, 和 `Reducer` 的配合。以下是一个简单的示例，假设我们使用Hadoop MapReduce框架，CSV文件格式为"key, value, 20171128"，我们想把日期转换成"YYYY-MM-DD"格式：

// Mapper class
import java.io.IOException;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class DateSplitMapper extends Mapper<Object, Text, Text, IntWritable> {
    private final static IntWritable one = new IntWritable(1);
    private Text dateKey = new Text();

    @Override
    protected void map(Object key, Text value, Context context) throws IOException, InterruptedException {
        String[] fields = value.toString().split(",");
        if (fields.length >= 3) {
            String dateStr = fields[2];
            // Split the date string and format it as YYYY-MM-DD
            String[] dateParts = dateStr.split("\\d+");
            dateKey.set(dateParts[0] + "-" + dateParts[1] + "-" + dateParts[2]);
            context.write(dateKey, one);
        }
    }
}

// Configuration in Driver class
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
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 CSVDateParser {
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "CSV Date Parser");
        job.setJarByClass(CSVDateParser.class); // Set your driver class here

        FileInputFormat.addInputPath(job, new Path(args[0])); // Replace with your input file path
        FileOutputFormat.setOutputPath(job, new Path(args[1])); // Replace with your output directory

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

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