MapReduce 重要组件——Recordreader组件

（1）以怎样的方式从分片中读取一条记录，每读取一条记录都会调用RecordReader类；

（2）系统默认的RecordReader是LineRecordReader，如TextInputFormat；而SequenceFileInputFormat的RecordReader是SequenceFileRecordReader；

（3）LineRecordReader是用每行的偏移量作为map的key，每行的内容作为map的value；

（4）应用场景：自定义读取每一条记录的方式；自定义读入key的类型，如希望读取的key是文件的路径或名字而不是该行在文件中的偏移量。

 

自定义RecordReader：

（1）继承抽象类RecordReader，实现RecordReader的一个实例；

（2）实现自定义InputFormat类，重写InputFormat中createRecordReader（）方法，返回值是自定义的RecordReader实例；

（3）配置job.setInputFormatClass()设置自定义的InputFormat实例；

 

源码见org.apache.mapreduce.lib.input.TextInputFormat类；

 

RecordReader例子：

应用场景：

数据：

1

2

3

4

5

6

7

......

要求：分别计算奇数行与偶数行数据之和

奇数行综合：10+30+50+70=160

偶数行综合：20+40+60=120

 

新建项目TestRecordReader，包com.recordreader,

源代码MyMapper.java：

package com.recordreader;

 

import java.io.IOException;

 

import org.apache.hadoop.io.LongWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.Mapper;

 

public class MyMapper extends Mapper {

 

@Override

protected void map(LongWritable key, Text value,Context context)

throws IOException, InterruptedException {

// TODO Auto-generated method stub

context.write(key, value);

}

 

}

 

 

源代码MyPartitioner.java：

package com.recordreader;

 

import org.apache.hadoop.io.LongWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.Partitioner;

 

public class MyPartitioner extends Partitioner {

 

@Override

public int getPartition(LongWritable key, Text value, int numPartitions) {

// TODO Auto-generated method stub

if(key.get() % 2 == 0){

key.set(1);

return 1;

}

else {

key.set(0);

return 0;

}

}

 

}

 

源代码MyReducer.java：

package com.recordreader;

 

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.Reducer;

 

public class MyReducer extends Reducer {

 

@Override

protected void reduce(LongWritable key, Iterable value,Context context)

throws IOException, InterruptedException {

// TODO Auto-generated method stub

int sum = 0;

for(Text val: value){

sum += Integer.parseInt(val.toString());

}

Text write_key = new Text();

IntWritable write_value = new IntWritable();

if(key.get() == 0)

write_key.set("odd:");

else 

write_key.set("even:");

write_value.set(sum);

context.write(write_key, write_value);

}

 

}

 

源代码MyRecordReader.java：

package com.recordreader;

 

import java.io.IOException;

 

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.fs.FSDataInputStream;

import org.apache.hadoop.fs.FileSystem;

import org.apache.hadoop.fs.Path;

import org.apache.hadoop.io.LongWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.InputSplit;

import org.apache.hadoop.mapreduce.RecordReader;

import org.apache.hadoop.mapreduce.TaskAttemptContext;

import org.apache.hadoop.mapreduce.lib.input.FileSplit;

import org.apache.hadoop.util.LineReader;

 

public class MyRecordReader extends RecordReader {

private long start;

private long end;

private long pos;

private FSDataInputStream fin = null;

private LongWritable key = null;

private Text value = null;

private LineReader reader = null;

@Override

public void close() throws IOException {

// TODO Auto-generated method stub

fin.close();

}

 

@Override

public LongWritable getCurrentKey() throws IOException,

InterruptedException {

// TODO Auto-generated method stub

return key;

}

 

@Override

public Text getCurrentValue() throws IOException, InterruptedException {

// TODO Auto-generated method stub

return value;

}

 

@Override

public float getProgress() throws IOException, InterruptedException {

// TODO Auto-generated method stub

return 0;

}

 

@Override

public void initialize(InputSplit inputSplit, TaskAttemptContext context)

throws IOException, InterruptedException {

// TODO Auto-generated method stub

FileSplit fileSplit = (FileSplit)inputSplit;

start = fileSplit.getStart();

end = start + fileSplit.getLength();

Configuration conf = context.getConfiguration();

Path path = fileSplit.getPath();

FileSystem fs = path.getFileSystem(conf);

fin = fs.open(path);

fin.seek(start);

reader = new LineReader(fin);

pos = 1;

}

 

@Override

public boolean nextKeyValue() throws IOException, InterruptedException {

// TODO Auto-generated method stub

if(key == null)

key = new LongWritable();

key.set(pos);

if(value == null)

value = new Text();

if(reader.readLine(value) == 0)

return false;

pos++;

return true;

}

 

}

 

源代码MyFileInputFormat.java：

package com.recordreader;

 

import java.io.IOException;

 

import org.apache.hadoop.fs.Path;

import org.apache.hadoop.io.LongWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.InputSplit;

import org.apache.hadoop.mapreduce.JobContext;

import org.apache.hadoop.mapreduce.RecordReader;

import org.apache.hadoop.mapreduce.TaskAttemptContext;

import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;

 

public class MyFileInputFormat extends FileInputFormat {

 

@Override

public RecordReader createRecordReader(InputSplit arg0,

TaskAttemptContext arg1) throws IOException, InterruptedException {

// TODO Auto-generated method stub

return new MyRecordReader();

}

 

@Override

protected boolean isSplitable(JobContext context, Path filename) {

// TODO Auto-generated method stub

return false;

}

 

}

 

源代码TestRecordReader.java：

package com.recordreader;

 

import java.io.IOException;

 

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;

import org.apache.hadoop.util.GenericOptionsParser;

 

 

 

public class TestRecordReader {

public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException{

  Configuration conf = new Configuration();

   String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();

   if (otherArgs.length != 2) {

     System.err.println("Usage: wordcount ");

     System.exit(2);

   }

   Job job = new Job(conf, "word count");

   job.setJarByClass(TestRecordReader.class);

   job.setMapperClass(MyMapper.class);

   

   job.setReducerClass(MyReducer.class);

   job.setPartitionerClass(MyPartitioner.class);

   job.setNumReduceTasks(2);

   job.setInputFormatClass(MyFileInputFormat.class);

   

   

   

   FileInputFormat.addInputPath(job, new Path(otherArgs[0]));

   FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));

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

  }

}