Hadoop LineRecordReader实现分析

直接上代码：

package org.apache.hadoop.mapreduce.lib.input;

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.fs.Seekable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.compress.CodecPool;
import org.apache.hadoop.io.compress.CompressionCodec;
import org.apache.hadoop.io.compress.CompressionCodecFactory;
import org.apache.hadoop.io.compress.Decompressor;
import org.apache.hadoop.io.compress.SplitCompressionInputStream;
import org.apache.hadoop.io.compress.SplittableCompressionCodec;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.util.LineReader;
import org.apache.commons.logging.LogFactory;
import org.apache.commons.logging.Log;

/**
 * Treats keys as offset in file and value as line. 
 */
public class LineRecordReader extends RecordReader<LongWritable, Text> {
  private static final Log LOG = LogFactory.getLog(LineRecordReader.class);

  private CompressionCodecFactory compressionCodecs = null;
  private long start;
  private long pos;
  private long end;
  private LineReader in;
  private int maxLineLength;
  private LongWritable key = null;
  private Text value = null;
  private Seekable filePosition;
  private CompressionCodec codec; //
  private Decompressor decompressor;

  public void initialize(InputSplit genericSplit,
                         TaskAttemptContext context) throws IOException {
    FileSplit split = (FileSplit) genericSplit;
    Configuration job = context.getConfiguration();
    this.maxLineLength = job.getInt("mapred.linerecordreader.maxlength",
                                    Integer.MAX_VALUE);
    start = split.getStart();
    end = start + split.getLength();
    final Path file = split.getPath();
    compressionCodecs = new CompressionCodecFactory(job);
	//根据job的配置信息，和split的信息，获取到读取实体文件的信息，这里包括文件的压缩信息。
	//这里压缩的code有：DEFAULT,GZIP,BZIP2,LZO,LZ4,SNAPPY
    codec = compressionCodecs.getCodec(file); 

    // open the file and seek to the start of the split
    FileSystem fs = file.getFileSystem(job);
    FSDataInputStream fileIn = fs.open(split.getPath());

    if (isCompressedInput()) {
	//通过CodecPool的getCompressor方法获得Compressor对象，该方法需要传入一个codec，
	//然后Compressor对象在createOutputStream中使用，使用完毕后再通过returnCompressor放回去
      decompressor = CodecPool.getDecompressor(codec);
      if (codec instanceof SplittableCompressionCodec) {
        final SplitCompressionInputStream cIn =
          ((SplittableCompressionCodec)codec).createInputStream(
            fileIn, decompressor, start, end,
            SplittableCompressionCodec.READ_MODE.BYBLOCK);
        in = new LineReader(cIn, job);
        start = cIn.getAdjustedStart();
        end = cIn.getAdjustedEnd();
        filePosition = cIn;
      } else {
        in = new LineReader(codec.createInputStream(fileIn, decompressor),
            job);
        filePosition = fileIn;
      }
    } else {
      fileIn.seek(start);
      in = new LineReader(fileIn, job);
      filePosition = fileIn;
    }
    // If this is not the first split, we always throw away first record
    // because we always (except the last split) read one extra line in
    // next() method.
    if (start != 0) {
      start += in.readLine(new Text(), 0, maxBytesToConsume(start));
    }
    this.pos = start;
  }
  
  private boolean isCompressedInput() {
    return (codec != null);
  }

  private int maxBytesToConsume(long pos) {
    return isCompressedInput()
      ? Integer.MAX_VALUE
      : (int) Math.min(Integer.MAX_VALUE, end - pos);
  }

  private long getFilePosition() throws IOException {
    long retVal;
    if (isCompressedInput() && null != filePosition) {
      retVal = filePosition.getPos();
    } else {
      retVal = pos;
    }
    return retVal;
  }

//读取每一行数据的时候，都会执行nextKeyValue()方法。
//返回为true的时候，就会再调用getCurrentKey和getCurrentValue方法获取，key，value值
  public boolean nextKeyValue() throws IOException {
    if (key == null) {
      key = new LongWritable();
    }
    key.set(pos);
    if (value == null) {
      value = new Text();
    }
    int newSize = 0;
    // We always read one extra line, which lies outside the upper
    // split limit i.e. (end - 1)
    while (getFilePosition() <= end) {
	//在这里进行数据读取，LineReader以\n作为分隔符，读取一行数据，放到Text value里面
     //读取一行，可以参考LineReader的源码实现
      newSize = in.readLine(value, maxLineLength,
          Math.max(maxBytesToConsume(pos), maxLineLength));
      if (newSize == 0) {
        break;
      }
      pos += newSize;
      if (newSize < maxLineLength) {
        break;
      }

      // line too long. try again
      LOG.info("Skipped line of size " + newSize + " at pos " + 
               (pos - newSize));
    }
    if (newSize == 0) {
      key = null;
      value = null;
      return false;
    } else {
      return true;
    }
  }

  @Override
  public LongWritable getCurrentKey() {
    return key;
  }

  @Override
  public Text getCurrentValue() {
    return value;
  }

  /**
   * Get the progress within the split
   */
  public float getProgress() throws IOException {
    if (start == end) {
      return 0.0f;
    } else {
      return Math.min(1.0f,
        (getFilePosition() - start) / (float)(end - start));
    }
  }

  public synchronized void close() throws IOException {
    try {
      if (in != null) {
        in.close();
      }
    } finally {
      if (decompressor != null) {
        CodecPool.returnDecompressor(decompressor);
      }
    }
  }
}

 

ref:http://blog.csdn.net/lastsweetop/article/details/9173061

http://www.myexception.cn/program/1345730.html