Hadoop會對原始輸入文件進行文件切割，然後把每個split傳入mapper程序中進行處理，FileInputFormat是所有以文件作 為數據源的InputFormat實現的基類，FileInputFormat保存作為job輸入的所有文件，並實現了對輸入文件計算splits的方 法。至於獲得記錄的方法是有不同的子類進行實現的。
那麼，FileInputFormat是怎樣將他們劃分成splits的呢？FileInputFormat只劃分比HDFS block大的文件，所以如果一個文件的大小比block小，將不會被劃分，這也是Hadoop處理大文件的效率要比處理很多小文件的效率高的原因。
hadoop默認的InputFormat是TextInputFormat，重寫了FileInputFormat中的createRecordReader和isSplitable方法。該類使用的reader是LineRecordReader，即以回車鍵(CR = 13)或換行符(LF = 10)為行分隔符。
但大多數情況下，回車鍵或換行符作為輸入文件的行分隔符並不能滿足我們的需求，通常用戶很有可能會輸入回車鍵、換行符，所以通常我們會定義不可見字符(即用戶無法輸入的字符)為行分隔符，這種情況下，就需要新寫一個InputFormat。
又或者，一條記錄的分隔符不是字符，而是字符串，這種情況相對麻煩；還有一種情況，輸入文件的主鍵key已經是排好序的了，需要hadoop做的只是把相 同的key作為一個數據塊進行邏輯處理，這種情況更麻煩，相當於免去了mapper的過程，直接進去reduce，那麼InputFormat的邏輯就相 對較為復雜了，但並不是不能實現。
1、改變一條記錄的分隔符，不用默認的回車或換行符作為記錄分隔符，甚至可以采用字符串作為記錄分隔符。
1）自定義一個InputFormat，繼承FileInputFormat，重寫createRecordReader方法，如果不需要分片或者需要改變分片的方式，則重寫isSplitable方法，具體代碼如下：
public class FileInputFormatB extends FileInputFormat<LongWritable, Text> {
@Override
public RecordReader<LongWritable, Text> createRecordReader( InputSplit split, TaskAttemptContext context) {
return new SearchRecordReader("\b");
}
@Override
protected boolean isSplitable(FileSystem fs, Path filename) {
// 輸入文件不分片
return false;
}
}
2）關鍵在於定義一個新的SearchRecordReader繼承RecordReader，支持自定義的行分隔符，即一條記錄的分隔符。標紅的地方為與hadoop默認的LineRecordReader不同的地方。
public class IsearchRecordReader extends RecordReader<LongWritable, Text> {
private static final Log LOG = LogFactory.getLog(IsearchRecordReader.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;
//行分隔符，即一條記錄的分隔符
private byte[] separator = {'\b'};
private int sepLength = 1;
public IsearchRecordReader(){
}
public IsearchRecordReader(String seps){
this.separator = seps.getBytes();
sepLength = separator.length;
}
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);
this.start = split.getStart();
this.end = (this.start + split.getLength());
Path file = split.getPath();
this.compressionCodecs = new CompressionCodecFactory(job);
CompressionCodec codec = this.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());
boolean skipFirstLine = false;
if (codec != null) {
this.in = new LineReader(codec.createInputStream(fileIn), job);
this.end = Long.MAX_VALUE;
} else {
if (this.start != 0L) {
skipFirstLine = true;
this.start -= sepLength;
fileIn.seek(this.start);
}
this.in = new LineReader(fileIn, job);
}
if (skipFirstLine) { // skip first line and re-establish "start".
int newSize = in.readLine(new Text(), 0, (int) Math.min( (long) Integer.MAX_VALUE, end - start));
if(newSize > 0){
start += newSize;
}
}
this.pos = this.start;
}
public boolean nextKeyValue() throws IOException {
if (this.key == null) {
this.key = new LongWritable();
}
this.key.set(this.pos);
if (this.value == null) {
this.value = new Text();
}
int newSize = 0;
while (this.pos < this.end) {
newSize = this.in.readLine(this.value, this.maxLineLength, Math.max(
(int) Math.min(Integer.MAX_VALUE, this.end - this.pos), this.maxLineLength));
if (newSize == 0) {
break;
}
this.pos += newSize;
if (newSize < this.maxLineLength) {
break;
}
LOG.info("Skipped line of size " + newSize + " at pos " + (this.pos - newSize));
}
if (newSize == 0) {
//讀下一個buffer
this.key = null;
this.value = null;
return false;
}
//讀同一個buffer的下一個記錄
return true;
}
public LongWritable getCurrentKey() {
return this.key;
}
public Text getCurrentValue() {
return this.value;
}
public float getProgress() {
if (this.start == this.end) {
return 0.0F;
}
return Math.min(1.0F, (float) (this.pos - this.start) / (float) (this.end - this.start));
}
public synchronized void close() throws IOException {
if (this.in != null)
this.in.close();
}
}
3）重寫SearchRecordReader需要的LineReader，可作為SearchRecordReader內部類。特別需要注意的地方就 是，讀取文件的方式是按指定大小的buffer來讀，必定就會遇到一條完整的記錄被切成兩半，甚至如果分隔符大於1個字符時分隔符也會被切成兩半的情況， 這種情況一定要加以拼接處理。
public class LineReader {
//回車鍵(hadoop默認)
//private static final byte CR = 13;
//換行符(hadoop默認)
//private static final byte LF = 10;
//按buffer進行文件讀取
private static final int DEFAULT_BUFFER_SIZE = 32 * 1024 * 1024;
private int bufferSize = DEFAULT_BUFFER_SIZE;
private InputStream in;
private byte[] buffer;
private int bufferLength = 0;
private int bufferPosn = 0;
LineReader(InputStream in, int bufferSize) {
this.bufferLength = 0;
this.bufferPosn = 0;
this.in = in;
this.bufferSize = bufferSize;
this.buffer = new byte[this.bufferSize];
}
public LineReader(InputStream in, Configuration conf) throws IOException {
this(in, conf.getInt("io.file.buffer.size", DEFAULT_BUFFER_SIZE));
}
public void close() throws IOException {
in.close();
}
public int readLine(Text str, int maxLineLength) throws IOException {
return readLine(str, maxLineLength, Integer.MAX_VALUE);
}
public int readLine(Text str) throws IOException {
return readLine(str, Integer.MAX_VALUE, Integer.MAX_VALUE);
}
//以下是需要改寫的部分_start，核心代碼
public int readLine(Text str, int maxLineLength, int maxBytesToConsume) throws IOException{
str.clear();
Text record = new Text();
int txtLength = 0;
long bytesConsumed = 0L;
boolean newline = false;
int sepPosn = 0;
do {
//已經讀到buffer的末尾了，讀下一個buffer
if (this.bufferPosn >= this.bufferLength) {
bufferPosn = 0;
bufferLength = in.read(buffer);
//讀到文件末尾了，則跳出，進行下一個文件的讀取
if (bufferLength <= 0) {
break;
}
}
int startPosn = this.bufferPosn;
for (; bufferPosn < bufferLength; bufferPosn ++) {
//處理上一個buffer的尾巴被切成了兩半的分隔符(如果分隔符中重復字符過多在這裡會有問題)
if(sepPosn > 0 && buffer[bufferPosn] != separator[sepPosn]){
sepPosn = 0;
}
//遇到行分隔符的第一個字符
if (buffer[bufferPosn] == separator[sepPosn]) {
bufferPosn ++;
int i = 0;
//判斷接下來的字符是否也是行分隔符中的字符
for(++ sepPosn; sepPosn < sepLength; i ++, sepPosn ++){
//buffer的最後剛好是分隔符，且分隔符被不幸地切成了兩半
if(bufferPosn + i >= bufferLength){
bufferPosn += i - 1;
break;
}
//一旦其中有一個字符不相同，就判定為不是分隔符
if(this.buffer[this.bufferPosn + i] != separator[sepPosn]){
sepPosn = 0;
break;
}
}
//的確遇到了行分隔符
if(sepPosn == sepLength){
bufferPosn += i;
newline = true;
sepPosn = 0;
break;
}
}
}
int readLength = this.bufferPosn - startPosn;
bytesConsumed += readLength;
//行分隔符不放入塊中
//int appendLength = readLength - newlineLength;
if (readLength > maxLineLength - txtLength) {
readLength = maxLineLength - txtLength;
}
if (readLength > 0) {
record.append(this.buffer, startPosn, readLength);
txtLength += readLength;
//去掉記錄的分隔符
if(newline){
str.set(record.getBytes(), 0, record.getLength() - sepLength);
}
}
} while (!newline && (bytesConsumed < maxBytesToConsume));
if (bytesConsumed > (long)Integer.MAX_VALUE) {
throw new IOException("Too many bytes before newline: " + bytesConsumed);
}
return (int) bytesConsumed;
}
//以下是需要改寫的部分_end
//以下是hadoop-core中LineReader的源碼_start
public int readLine(Text str, int maxLineLength, int maxBytesToConsume) throws IOException{
str.clear();
int txtLength = 0;
int newlineLength = 0;
boolean prevCharCR = false;
long bytesConsumed = 0L;
do {
int startPosn = this.bufferPosn;
if (this.bufferPosn >= this.bufferLength) {
startPosn = this.bufferPosn = 0;
if (prevCharCR) bytesConsumed ++;
this.bufferLength = this.in.read(this.buffer);
if (this.bufferLength <= 0) break;
}
for (; this.bufferPosn < this.bufferLength; this.bufferPosn ++) {
if (this.buffer[this.bufferPosn] == LF) {
newlineLength = (prevCharCR) ? 2 : 1;
this.bufferPosn ++;
break;
}
if (prevCharCR) {
newlineLength = 1;
break;
}
prevCharCR = this.buffer[this.bufferPosn] == CR;
}
int readLength = this.bufferPosn - startPosn;
if ((prevCharCR) && (newlineLength == 0))
--readLength;
bytesConsumed += readLength;
int appendLength = readLength - newlineLength;
if (appendLength > maxLineLength - txtLength) {
appendLength = maxLineLength - txtLength;
}
if (appendLength > 0) {
str.append(this.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;
}
//以下是hadoop-core中LineReader的源碼_end
}
2、已經按主鍵key排好序了，並保證相同主鍵key一定是在一起的，假設每條記錄的第一個字段為主鍵，那麼如 果沿用上面的LineReader，需要在核心方法readLine中對前後兩條記錄的id進行equals判斷，如果不同才進行split，如果相同繼 續下一條記錄的判斷。代碼就不再貼了，但需要注意的地方，依舊是前後兩個buffer進行交接的時候，非常有可能一條記錄被切成了兩半，一半在前一個buffer中，一半在後一個buffer中。
這種方式的好處在於少去了reduce操作，會大大地提高效率，其實mapper的過程相當的快，費時的通常是reduce。
更多Hadoop相關信息見Hadoop 專題頁面 http://www.linuxidc.com/topicnews.aspx?tid=13