Mapreduce Mapper解释

Question

Hadoop明确指南中有一个NCDC天气数据集示例 Mapper类代码如下

Example 2-3. Mapper for maximum temperature example
import java.io.IOException;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class MaxTemperatureMapper
 extends Mapper<LongWritable, Text, Text, IntWritable> {
 private static final int MISSING = 9999;

 @Override
 public void map(LongWritable key, Text value, Context context)
 throws IOException, InterruptedException {

 String line = value.toString();
 String year = line.substring(15, 19);
 int airTemperature;
 if (line.charAt(87) == '+') { // parseInt doesn't like leading plus signs
 airTemperature = Integer.parseInt(line.substring(88, 92));
 } else {
 airTemperature = Integer.parseInt(line.substring(87, 92));
 }
 String quality = line.substring(92, 93);
 if (airTemperature != MISSING && quality.matches("[01459]")) {
 context.write(new Text(year), new IntWritable(airTemperature));
 }
 }
}

驱动程序代码为：

Example 2-5. Application to find the maximum temperature in the weather dataset
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 MaxTemperature {
 public static void main(String[] args) throws Exception {
 if (args.length != 2) {
 System.err.println("Usage: MaxTemperature <input path> <output path>");
 System.exit(-1);
 }

 Job job = new Job();
 job.setJarByClass(MaxTemperature.class);
 job.setJobName("Max temperature");
 FileInputFormat.addInputPath(job, new Path(args[0]));
 FileOutputFormat.setOutputPath(job, new Path(args[1]));

 job.setMapperClass(MaxTemperatureMapper.class);
 job.setReducerClass(MaxTemperatureReducer.class);
 job.setOutputKeyClass(Text.class);
 job.setOutputValueClass(IntWritable.class);

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

我无法理解，因为我们传递包含多行的文件，为什么行上没有迭代。代码好像是在单行上处理。

Answer 1

这本书解释了Mapper<LongWritable, Text,的含义......关键是文件的偏移量。该值是一条线。

它还提到TextInputFormat是默认的mapreduce输入类型，它是一种FileInputFormat

public class TextInputFormat
extends FileInputFormat<LongWritable,Text>

因此，默认输入类型必须为Long，Text pairs

正如JavaDoc所说

  

文件被分成几行。换行或回车用于发出行尾信号。键是文件中的位置，值是文本行..

本书还有关于定义自定义RecordReaders的部分

您需要致电job.setInputFormatClass将其更改为读取除单行以外的任何内容