package org.apache.hadoop.mapred;
import java.io.*;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.compress.*;
/** An {@link InputFormat} for plain text files. Files are broken into lines.
* Either linefeed or carriage-return are used to signal end of line. Keys are
* the position in the file, and values are the line of text..
*/
public class TextInputFormat extends FileInputFormat<LongWritable, Text>
implements JobConfigurable {
private CompressionCodecFactory compressionCodecs = null;
public void configure(JobConf conf) {
compressionCodecs = new CompressionCodecFactory(conf); //获取所拥有的所有压缩器——工厂
}
protected boolean isSplitable(FileSystem fs, Path file) {
return compressionCodecs.getCodec(file) == null; //根据后缀得到相应的压缩器 ,如果返回的是null，return false
}
public RecordReader<LongWritable, Text> getRecordReader(
InputSplit genericSplit, JobConf job,
Reporter reporter)
throws IOException {
reporter.setStatus(genericSplit.toString());
return new LineRecordReader(job, (FileSplit) genericSplit);//进入定位记录边界和解析key/value阶段
}
}
在TextInputFormat类中最后一句return new LineRecordReader(job, (FileSplit) genericSplit);进入LineRecordReader类里面，使用以下构造函数：
public LineRecordReader(Configuration job, FileSplit split) throws IOException {
this.maxLineLength = job.getInt("mapred.linerecordreader.maxlength",
Integer.MAX_VALUE);
start = split.getStart(); //得到当前分片第一个字节在文件中的起始位置
end = start + split.getLength(); //得到当前分片最后一个字节在文件中的位置
final Path file = split.getPath(); //得到当前分片所在文件路径
compressionCodecs = new CompressionCodecFactory(job);
final CompressionCodec codec = compressionCodecs.getCodec(file);//根据文件后缀得到相应的压缩器，注意有可能是对压缩文件的处理
// open the file and seek to the start of the split
FileSystem fs = file.getFileSystem(job);
FSDataInputStream fileIn = fs.open(split.getPath());
boolean skipFirstLine = false;
if (codec != null) { //如果是压缩文件，则存在相应的压缩器
//codec.createInputStream(fileIn)从输入流fileIn读取出来的数据进行解压缩，从而获得一个CompressionInputStream,从底层的流读取未压缩的数据。
in = new LineReader(codec.createInputStream(fileIn), job);//调用LineReader的构造函数，构造LineReader对象in
end = Long.MAX_VALUE;
} else {//如果不是压缩文件，正常处理
if (start != 0) {
skipFirstLine = true;
--start; //为什么--还没弄懂，跳过的第一行会是什么？
fileIn.seek(start);//找到当前start的偏移位置，下一个read（）函数从该偏移位置开始读取
}
in = new LineReader(fileIn, job);
}
if (skipFirstLine) { // skip first line and re-establish "start".如果不是这个文件的第一条记录，跳过，value值给new Text（），start重新设置
start += in.readLine(new Text(), 0,
(int)Math.min((long)Integer.MAX_VALUE, end - start));
}
this.pos = start;//更改当前类中的pos值，在next（）函数中将会用到
}LineRecordReader类里面另一个函数，将解析出的结果存入key/value： /** Read a line. */
public synchronized boolean next(LongWritable key, Text value)
throws IOException {
while (pos < end) {//当前分片偏移小于当前分片最后一个字节在文件中的下一个偏移时（对于当前分片split，体现split是MR的处理单位），循环读取写入key/value对
key.set(pos);
int newSize = in.readLine(value, maxLineLength,
Math.max((int)Math.min(Integer.MAX_VALUE, end-pos),
maxLineLength));
if (newSize == 0) {
return false;
}
pos += newSize;
if (newSize < maxLineLength) {
return true;
}
// line too long. try again
LOG.info("Skipped line of size " + newSize + " at pos " + (pos - newSize));
}
return false;
}
LineReader中的构造函数，主要用于读取输入流中的一行，主要定位记录边界，并返回当前记录的字节数：
/**
* Read one line from the InputStream into the given Text. A line
* can be terminated by one of the following: '
' (LF) , '
' (CR),
* or '
' (CR+LF). EOF also terminates an otherwise unterminated
* line.
*
* @param str the object to store the given line (without newline)
* @param maxLineLength the maximum number of bytes to store into str;
* the rest of the line is silently discarded.
* @param maxBytesToConsume the maximum number of bytes to consume
* in this call. This is only a hint, because if the line cross
* this threshold, we allow it to happen. It can overshoot
* potentially by as much as one buffer length.
*
* @return the number of bytes read including the (longest) newline
* found.
*
* @throws IOException if the underlying stream throws
*/
public int readLine(Text str, int maxLineLength,
int maxBytesToConsume) throws IOException {
/* We're reading data from in, but the head of the stream may be
* already buffered in buffer, so we have several cases:
* 1. No newline characters are in the buffer, so we need to copy
* everything and read another buffer from the stream.
* 2. An unambiguously terminated line is in buffer, so we just
* copy to str.
* 3. Ambiguously terminated line is in buffer, i.e. buffer ends
* in CR. In this case we copy everything up to CR to str, but
* we also need to see what follows CR: if it's LF, then we
* need consume LF as well, so next call to readLine will read
* from after that.
* We use a flag prevCharCR to signal if previous character was CR
* and, if it happens to be at the end of the buffer, delay
* consuming it until we have a chance to look at the char that
* follows.
*/
str.clear();
int txtLength = 0; //tracks str.getLength(), as an optimization
int newlineLength = 0; //length of terminating newline
boolean prevCharCR = false; //true of prev char was CR
long bytesConsumed = 0;
do {
int startPosn = bufferPosn; //starting from where we left off the last time
if (bufferPosn >= bufferLength) {
startPosn = bufferPosn = 0;
if (prevCharCR)
++bytesConsumed; //account for CR from previous read
bufferLength = in.read(buffer);
if (bufferLength <= 0)
break; // EOF
}
for (; bufferPosn < bufferLength; ++bufferPosn) { //search for newline
if (buffer[bufferPosn] == LF) {
newlineLength = (prevCharCR) ? 2 : 1;
++bufferPosn; // at next invocation proceed from following byte
break;
}
if (prevCharCR) { //CR + notLF, we are at notLF
newlineLength = 1;
break;
}
prevCharCR = (buffer[bufferPosn] == CR);
}
int readLength = bufferPosn - startPosn;
if (prevCharCR && newlineLength == 0)
--readLength; //CR at the end of the buffer
bytesConsumed += readLength;
int appendLength = readLength - newlineLength;
if (appendLength > maxLineLength - txtLength) {
appendLength = maxLineLength - txtLength;
}
if (appendLength > 0) {
str.append(buffer, startPosn, appendLength);
txtLength += appendLength;
}
} while (newlineLength == 0 && bytesConsumed < maxBytesToConsume);
if (bytesConsumed > (long)Integer.MAX_VALUE)
throw new IOException("Too many bytes before newline: " + bytesConsumed);
return (int)bytesConsumed; //返回当前已读字节数
}由上，还有两点说明：
1.CompressionCodeFactory提供了getCodec()方法，从而将文件扩展名映射到相应的CompressionCodec，从getCodec方法接受一个Path对象，要想对从输入流读取而来的数据进行解压缩，则调用createInStream(InputStream in)方法，从而获得一个compressionInputStream，从而获得一个CompressionInputStream，从而从底层的流读取未压缩的数据;另一方面，CompressionCodec还提供压缩功能，如果想对一个正在被写入的输出流的数据进行压缩，我们可以使用createOutStream(OutputStream out)方法创建一个CompressionOutputStream，将其压缩格式写入底层的流。
2.synchronized：把synchronized当作函数修饰符时，锁定的是调用这个同步方法对象。也就是说，当一个对象P1在不同的线程中执行这个同步方法时，他们之间会形成互斥，达到同步的效果。