平时我们写MapReduce程序的时候，在设置输入格式的时候，总会调用形如job.setInputFormatClass(KeyValueTextInputFormat.class)来保证输入文件按照我们想要的格式被读取。所有的输入格式都继承于InputFormat，这是一个抽象类，其子类有专门用于读取普通文件的FileInputFormat，用来读取数据库的DBInputFormat等等。
不同的InputFormat都会按自己的实现来读取输入数据并产生输入分片，一个输入分片会被单独的MapTask作为数据源，下面我们先看看这些输入分片(InputSplit)是什么样的。
InPutSplit：
我们知道Mapper的输入是一个一个的输入分片，称为InputSplit。InputSplit是一个抽象类，它在逻辑上包含了提供给处理这个InputSplit的Mapper的所有K-V对。
public abstract class InputSplit {
public abstract long getLength() throws IOException, InterruptedException;
public abstract
String[] getLocations() throws IOException, InterruptedException;
}
getLength()用来获取InputSplit的大小，以支持对InputSplit进行排序，而getLocations()则用来获取存储分片的位置列表。
我们来看一个简单的InputSplit子类：FileSplit，源码如下：
public class FileSplit extends InputSplit implements Writable {
private Path file;
private long start;
private long length;
private String[] hosts;
FileSplit() {}
/** Constructs a split with host information
*
* @param file the file name
* @param start the position of the first byte in the file to process
* @param length the number of bytes in the file to process
* @param hosts the list of hosts containing the block, possibly null
*/
public FileSplit(Path file, long start, long length, String[] hosts) {
this.file = file;
this.start = start;
this.length = length;
this.hosts = hosts;
}
/** The file containing this split's data. */
public Path getPath() { return file; }
/** The position of the first byte in the file to process. */
public long getStart() { return start; }
/** The number of bytes in the file to process. */
@Override
public long getLength() { return length; }
@Override
public String toString() { return file + ":" + start + "+" + length; }
////////////////////////////////////////////
// 序列化和反序列化
////////////////////////////////////////////
@Override
public void write(DataOutput out) throws IOException {
Text.writeString(out, file.toString());
out.writeLong(start);
out.writeLong(length);
}
@Override
public void readFields(DataInput in) throws IOException {
file = new Path(Text.readString(in));
start = in.readLong();
length = in.readLong();
hosts = null;
}
@Override
public String[] getLocations() throws IOException {
if (this.hosts == null) {
return new String[]{};
} else {
return this.hosts;
}
}
}
从上面的源码我们可以看到，一个FileSplit是由文件路径，分片开始位置，分片大小和存储分片数据的hosts列表组成，由这些信息我们就可以从输入文件中切分出提供给单个Mapper的输入数据。这些属性会在Constructor设置，我们在后面会看到这会在InputFormat的getSplits()构造这些分片。
我们再来看看CombinerFileSplit的源码：
@InterfaceAudience.Public
@InterfaceStability.Stable
public class CombineFileSplit extends InputSplit implements Writable {
private Path[] paths;
private long[] startoffset;
private long[] lengths;
private String[] locations;
private long totLength;
/**
* default constructor
*/
public CombineFileSplit() {}
public CombineFileSplit(Path[] files, long[] start,
long[] lengths, String[] locations) {
initSplit(files, start, lengths, locations);
}
public CombineFileSplit(Path[] files, long[] lengths) {
long[] startoffset = new long[files.length];
for (int i = 0; i < startoffset.length; i++) {
startoffset[i] = 0;
}
String[] locations = new String[files.length];
for (int i = 0; i < locations.length; i++) {
locations[i] = "";
}
initSplit(files, startoffset, lengths, locations);
}
private void initSplit(Path[] files, long[] start,
long[] lengths, String[] locations) {
this.startoffset = start;
this.lengths = lengths;
this.paths = files;
this.totLength = 0;
this.locations = locations;
for(long length : lengths) {
totLength += length;
}
}
/**
* Copy constructor
*/
public CombineFileSplit(CombineFileSplit old) throws IOException {
this(old.getPaths(), old.getStartOffsets(),
old.getLengths(), old.getLocations());
}
public long getLength() {
return totLength;
}
/** Returns an array containing the start offsets of the files in the split*/
public long[] getStartOffsets() {
return startoffset;
}
/** Returns an array containing the lengths of the files in the split*/
public long[] getLengths() {
return lengths;
}
/** Returns the start offset of the i<sup>th</sup> Path */
public long getOffset(int i) {
return startoffset[i];
}
/** Returns the length of the i<sup>th</sup> Path */
public long getLength(int i) {
return lengths[i];
}
/** Returns the number of Paths in the split */
public int getNumPaths() {
return paths.length;
}
/** Returns the i<sup>th</sup> Path */
public Path getPath(int i) {
return paths[i];
}
/** Returns all the Paths in the split */
public Path[] getPaths() {
return paths;
}
/** Returns all the Paths where this input-split resides */
public String[] getLocations() throws IOException {
return locations;
}
public void readFields(DataInput in) throws IOException {
totLength = in.readLong();
int arrLength = in.readInt();
lengths = new long[arrLength];
for(int i=0; i<arrLength;i++) {
lengths[i] = in.readLong();
}
int filesLength = in.readInt();
paths = new Path[filesLength];
for(int i=0; i<filesLength;i++) {
paths[i] = new Path(Text.readString(in));
}
arrLength = in.readInt();
startoffset = new long[arrLength];
for(int i=0; i<arrLength;i++) {
startoffset[i] = in.readLong();
}
}
public void write(DataOutput out) throws IOException {
out.writeLong(totLength);
out.writeInt(lengths.length);
for(long length : lengths) {
out.writeLong(length);
}
out.writeInt(paths.length);
for(Path p : paths) {
Text.writeString(out, p.toString());
}
out.writeInt(startoffset.length);
for(long length : startoffset) {
out.writeLong(length);
}
}
@Override
public String toString() {
StringBuffer sb = new StringBuffer();
for (int i = 0; i < paths.length; i++) {
if (i == 0 ) {
sb.append("Paths:");
}
sb.append(paths[i].toUri().getPath() + ":" + startoffset[i] +
"+" + lengths[i]);
if (i < paths.length -1) {
sb.append(",");
}
}
if (locations != null) {
String locs = "";
StringBuffer locsb = new StringBuffer();
for (int i = 0; i < locations.length; i++) {
locsb.append(locations[i] + ":");
}
locs = locsb.toString();
sb.append(" Locations:" + locs + "; ");
}
return sb.toString();
}
}
与FileSPlit类似，CombineFileSplit同样包含文件路径，分片起始位置，分片大小和存储分片数据的host列表，由于CombineFileSplit是针对小文件的，它把很多小文件包在一个InputSplit中，这样一个Mapper就可以处理很多小文件。要知道我们上面的FileSplit是对应一个输入文件的也就是说如果用FileSplit对应的FileInputFormat来作为输入格式。那么即使文件特别小，也是单独计算成一个分片来处理的。当我们的输入是由大量小文件组成的，就会导致同样大量的InputSplit，从而需要同样大量的Mapper来处理，这将很慢，想想一堆Map
Task要运行(运行一个新的MapTask可是要启动虚拟机的)，这是不符合Hadoop的设计理念的，所以使用CombineFileSplit可以优化Hadoop处理众多小文件的场景。
最后介绍TagInputSplit，这个类就是封装了一个InputSplit，然后加了一些tags在里面满足我们需要这些tags数据的情况，我们从下面就可以一目了然。
class TaggedInputSplit extends InputSplit implements Configurable, Writable {
private Class<? extends InputSplit> inputSplitClass;
private InputSplit inputSplit;
@SuppressWarnings("unchecked")
private Class<? extends InputFormat> inputFormatClass;
@SuppressWarnings("unchecked")
private Class<? extends Mapper> mapperClass;
private Configuration conf;
public TaggedInputSplit() {
// Default constructor.
}
/**
* Creates a new TaggedInputSplit.
*
* @param inputSplit The InputSplit to be tagged
* @param conf The configuration to use
* @param inputFormatClass The InputFormat class to use for this job
* @param mapperClass The Mapper class to use for this job
*/
@SuppressWarnings("unchecked")
public TaggedInputSplit(InputSplit inputSplit, Configuration conf,
Class<? extends InputFormat> inputFormatClass,
Class<? extends Mapper> mapperClass) {
this.inputSplitClass = inputSplit.getClass();
this.inputSplit = inputSplit;
this.conf = conf;
this.inputFormatClass = inputFormatClass;
this.mapperClass = mapperClass;
}
/**
* Retrieves the original InputSplit.
*
* @return The InputSplit that was tagged
*/
public InputSplit getInputSplit() {
return inputSplit;
}
/**
* Retrieves the InputFormat class to use for this split.
*
* @return The InputFormat class to use
*/
@SuppressWarnings("unchecked")
public Class<? extends InputFormat> getInputFormatClass() {
return inputFormatClass;
}
/**
* Retrieves the Mapper class to use for this split.
*
* @return The Mapper class to use
*/
@SuppressWarnings("unchecked")
public Class<? extends Mapper> getMapperClass() {
return mapperClass;
}
public long getLength() throws IOException, InterruptedException {
return inputSplit.getLength();
}
public String[] getLocations() throws IOException, InterruptedException {
return inputSplit.getLocations();
}
@SuppressWarnings("unchecked")
public void readFields(DataInput in) throws IOException {
inputSplitClass = (Class<? extends InputSplit>) readClass(in);
inputFormatClass = (Class<? extends InputFormat<?, ?>>) readClass(in);
mapperClass = (Class<? extends Mapper<?, ?, ?, ?>>) readClass(in);
inputSplit = (InputSplit) ReflectionUtils
.newInstance(inputSplitClass, conf);
SerializationFactory factory = new SerializationFactory(conf);
Deserializer deserializer = factory.getDeserializer(inputSplitClass);
deserializer.open((DataInputStream)in);
inputSplit = (InputSplit)deserializer.deserialize(inputSplit);
}
private Class<?> readClass(DataInput in) throws IOException {
String className = Text.readString(in);
try {
return conf.getClassByName(className);
} catch (ClassNotFoundException e) {
throw new RuntimeException("readObject can't find class", e);
}
}
@SuppressWarnings("unchecked")
public void write(DataOutput out) throws IOException {
Text.writeString(out, inputSplitClass.getName());
Text.writeString(out, inputFormatClass.getName());
Text.writeString(out, mapperClass.getName());
SerializationFactory factory = new SerializationFactory(conf);
Serializer serializer =
factory.getSerializer(inputSplitClass);
serializer.open((DataOutputStream)out);
serializer.serialize(inputSplit);
}
public Configuration getConf() {
return conf;
}
public void setConf(Configuration conf) {
this.conf = conf;
}
}
InputFormat：
通过使用InputFormat，MapReduce框架可以做到：
1.验证作业输入的正确性。
2.将输入文件切分成逻辑的InputSplits，一个InputSplit将被分配给一个单独的MapTask。
3.提供RecordReader的实现，这个RecordReader会从InputSplit中正确读出一条一条的K-V对供Mapper使用。
public abstract class InputFormat<K, V> {
public abstract
List<InputSplit> getSplits(JobContext context
) throws IOException, InterruptedException;
public abstract
RecordReader<K,V> createRecordReader(InputSplit split,
TaskAttemptContext context
) throws IOException,
InterruptedException;
}
上面是InputFormat的源码，getSplits()是用来获取由输入文件计算出来的InputSplits，我们在后面会看到计算InputSplit的时候会考虑到输入文件是否可分割、文件存储时分块的大小和文件大小等因素；而createRecordReader()提供了前面第三点所说的RecordReader的实现，以将K-V对从InputSplit中正确读取出来，比如LineRecordReader就以偏移值为key，一行数据为value的形式读取分片的。
FileInputFormat：
PathFilter被用来进行文件刷选，这样我们就可以控制哪些文件要被作为输入，哪些不作为输入，PathFIlter有一个accept(Path)方法，当接收的Path要被包含进来，就返回true，否则返回false。可以通过设置mapred.input.pathFIlter.class来设置用户自定义的PathFilter。
public interface PathFilter {
/**
* Tests whether or not the specified abstract pathname should be
* included in a pathname list.
*
* @param path The abstract pathname to be tested
* @return <code>true</code> if and only if <code>pathname</code>
* should be included
*/
boolean accept(Path path);
}
FileInputFormat是InputFormat的子类，它包含了一个MultiPathFilter，这个MultiPathFilter由一个过滤隐藏文件(名字前缀'-'或'.')的PathFilter和一些可能存在的用户自定义的PathFilter组成，MultiPathFilter会在listStatus()方法中使用，而listStatus()方法又被getSplits()方法用来获取输入文件，也就是说实现了在获取输入分片前进行文件过滤。
private static class MultiPathFilter implements PathFilter {
private List<PathFilter> filters;
public MultiPathFilter() {
this.filters = new ArrayList<PathFilter>();
}
public MultiPathFilter(List<PathFilter> filters) {
this.filters = filters;
}
public void add(PathFilter one) {
filters.add(one);
}
public boolean accept(Path path) {
for (PathFilter filter : filters) {
if (filter.accept(path)) {
return true;
}
}
return false;
}
public String toString() {
StringBuffer buf = new StringBuffer();
buf.append("[");
for (PathFilter f: filters) {
buf.append(f);
buf.append(",");
}
buf.append("]");
return buf.toString();
}
}
这些PathFilter会在listStatus()方法中用到，listStatus()是用来获取输入数据列表的。
下面是FileInputFormat的getSplits()方法，它首先得到分片的最小值minSize和最大值maxSize，它们会被用来计算分片的大小。可以通过设置mapred.min.split.size和mapred.max.split.size来设置。splits集合可以用来存储计算得到的输入分片，files则存储作为由listStatus()获取的输入文件列表。然后对于每个输入文件，判断是否可以分割，通过computeSplits()计算出分片大小splitSize，计算方法是：Math.max(minSize,Math.min(maxSize,blockSize));也就是保证在minSize和maxSize之间，且如果minSize<=blockSize<=maxSize，则设blockSize。然后根据这个splitSize计算出每个文件的InputSplit集合，然后加入到列表splits集合中。注意到我们生成InputSplit的时候按上面说的使用文件路径，分片起始位置，分片大小和存放这个文件爱你的hosts列表来创建。最后我们还设置了输入文件数量：mapreduce.input.num.files。
public List<InputSplit> getSplits(JobContext job
) throws IOException {
long minSize = Math.max(getFormatMinSplitSize(), getMinSplitSize(job));
long maxSize = getMaxSplitSize(job);
// generate splits
List<InputSplit> splits = new ArrayList<InputSplit>();
List<FileStatus>files = listStatus(job);
for (FileStatus file: files) {
Path path = file.getPath();
FileSystem fs = path.getFileSystem(job.getConfiguration());
long length = file.getLen();
BlockLocation[] blkLocations = fs.getFileBlockLocations(file, 0, length);
if ((length != 0) && isSplitable(job, path)) {
long blockSize = file.getBlockSize();
long splitSize = computeSplitSize(blockSize, minSize, maxSize);
long bytesRemaining = length;
while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);
splits.add(new FileSplit(path, length-bytesRemaining, splitSize,
blkLocations[blkIndex].getHosts()));
bytesRemaining -= splitSize;
}
if (bytesRemaining != 0) {
splits.add(new FileSplit(path, length-bytesRemaining, bytesRemaining,
blkLocations[blkLocations.length-1].getHosts()));
}
} else if (length != 0) {
splits.add(new FileSplit(path, 0, length, blkLocations[0].getHosts()));
} else {
//Create empty hosts array for zero length files
splits.add(new FileSplit(path, 0, length, new String[0]));
}
}
// Save the number of input files in the job-conf
job.getConfiguration().setLong(NUM_INPUT_FILES, files.size());
LOG.debug("Total # of splits: " + splits.size());
return splits;
}
就这样，我们利用FileInputFormat的getSplits()方法，我们就计算出了我们作业的所有输入分片了。
那这些计算出来的分片是怎么被map读出来的呢？就是InputFormat中的另一个方法createRecordReader()，FileInputFormat并没有对这个方法做具体的要求，而是交给子类自行去实现它。
RecordReader：
RecordReader是用来从一个输入分片中读取一个一个的K-V对的抽象类，我们可以将其看做是在InputSplit上的迭代器。我们从类图中可以看到它的一些方法，最主要的方法就是nextKeyValue()方法，由它获取分片上的下一个K-V对。
我们呢再来看看RecordReader的一个子类：LineRecordReader，这也是我们用的最多的。
LineRecordReader由一个FileSplit构造出来，start是这个FileSplit的起始位置，pos是当前读取分片的位置，end是分片结束位置，in是打开一个读取这个分片的输入流，它是使用这个FIleSplit对应的文件名来打开的。key和value则分别是每次读取的K-V对。然后我们还看到可以利用getProgress()来跟踪读取分片的进度，这个函数就是根据已经读取的K-V对占总K-V对的比例显示进度的。
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);
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()) {
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;
}
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) {
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);
}
}
}
}
其它的一些RecordReader如SequenceFileRecordReader，CombineFileRecordReader则对应不同的InputFormat。
下面继续看看这些RecordReader是如何被MapReduce框架使用的。
首先看看Mapper类的源码：
public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
public class Context
extends MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {
public Context(Configuration conf, TaskAttemptID taskid,
RecordReader<KEYIN,VALUEIN> reader,
RecordWriter<KEYOUT,VALUEOUT> writer,
OutputCommitter committer,
StatusReporter reporter,
InputSplit split) throws IOException, InterruptedException {
super(conf, taskid, reader, writer, committer, reporter, split);
}
}
/**
* Called once at the beginning of the task.
*/
protected void setup(Context context
) throws IOException, InterruptedException {
// NOTHING
}
/**
* Called once for each key/value pair in the input split. Most applications
* should override this, but the default is the identity function.
*/
@SuppressWarnings("unchecked")
protected void map(KEYIN key, VALUEIN value,
Context context) throws IOException, InterruptedException {
context.write((KEYOUT) key, (VALUEOUT) value);
}
/**
* Called once at the end of the task.
*/
protected void cleanup(Context context
) throws IOException, InterruptedException {
// NOTHING
}
/**
* Expert users can override this method for more complete control over the
* execution of the Mapper.
* @param context
* @throws IOException
*/
public void run(Context context) throws IOException, InterruptedException {
setup(context);
while (context.nextKeyValue()) {
map(context.getCurrentKey(), context.getCurrentValue(), context);
}
cleanup(context);
}
}
我们写MapReduce程序的时候，我们写的Mapper类都要继承这个Mapper类，通常我们会重写map()方法，map()每次接受一个K-V对，然后对这个K-V对进行处理，再分发出处理后的数据。我们也可能重写setUp()方法以对这个MapTask进行一些预处理，比如创建一个List之类集合，我们也可能重写cleanUp()方法做一些处理后的工作，当然我们也可能在cleanUp()中写出K-V对。举个例子就是：InputSplit的数据是一些整数，然后我们要在Mapper中计算他们的和。我们可以先设置个sum属性，然后map()函数处理一个K-V对就是将其加到sum上，最后在cleanUp()函数中调用context.write(key,value)。
最后我们看看Mapper.class中的run()方法，它相当于MapTask的驱动，我们可以看到run()方法首先调用setUp()方法进行初始化操作，然后遍历每个通过context.nextKeyValue()获取的K-V对，调用map()函数进行处理，最后调用cleanUp()方法做相关处理。
我们看看Mapper.class中的Context类，它继承自MapContext，使用了一个RecordReader进行构造。下面我们看看MapContext这个类的源码：
public class MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> extends TaskInputOutputContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
private RecordReader<KEYIN, VALUEIN> reader;
private InputSplit split;
public MapContext(Configuration conf, TaskAttemptID taskid, RecordReader<KEYIN, VALUEIN> reader, RecordWriter<KEYOUT, VALUEOUT> writer,
OutputCommitter committer, StatusReporter reporter, InputSplit split) {
super(conf, taskid, writer, committer, reporter);
this.reader = reader;
this.split = split;
}
/**
* Get the input split for this map.
*/
public InputSplit getInputSplit() {
return split;
}
@Override
public KEYIN getCurrentKey() throws IOException, InterruptedException {
return reader.getCurrentKey();
}
@Override
public VALUEIN getCurrentValue() throws IOException, InterruptedException {
return reader.getCurrentValue();
}
@Override
public boolean nextKeyValue() throws IOException, InterruptedException {
return reader.nextKeyValue();
}
}
我们可以看到MapContext直接是使用传入的RecordReader来进行K-V对的读取。
到现在，我们已经知道输入文件是如何被读取、过滤、分片、读出K-V对，然后交给我们的Mapper类来处理的了。
最后，我们来看看FIleInputFormat的几个子类。
TextInputFormat：
TextInputFormat是FileInputFormat的子类，其createRecordReader()方法返回的就是LineRecordReader。
public class TextInputFormat extends FileInputFormat<LongWritable, Text> {
@Override
public RecordReader<LongWritable, Text>
createRecordReader(InputSplit split,
TaskAttemptContext context) {
return new LineRecordReader();
}
@Override
protected boolean isSplitable(JobContext context, Path file) {
CompressionCodec codec =
new CompressionCodecFactory(context.getConfiguration()).getCodec(file);
if (null == codec) {
return true;
}
return codec instanceof SplittableCompressionCodec;
}
}
我们还看到isSplitable()方法，当文件使用压缩的形式，这个文件就不可分割，否则就读取不正确的数据了。这从某种程度上将影响分片的计算。有时我们希望一个文件只被一个Mapper处理的时候，我们就可以重写isSplitable()方法，告诉MapReduce框架，我哪些文件可以分割，哪些文件不能分割而只能作为一个分片。
NLineInputFormat：
NLineInputFormat也是FileInputFormat的子类，与名字一致，它是根据行数来划分InputSplits而不是像TextInputFormat那样依赖分片大小和行的长度的。也就是说，TextInputFormat当一行很长或分片比较小时，获取的分片可能只包含很少的K-V对，这样一个MapTask处理的K-V对就很少，这可能很不理想。因此我们可以使用NLineInputFormat来控制一个MapTask处理的K-V对，这是通过分割InputSplit时，按行数来分割的方法来实现的，这我们在代码中可以看出来。我们设置mapreduce.input.lineinputformat.linespermap来设置这个行数，源码如下：
@InterfaceAudience.Public
@InterfaceStability.Stable
public class NLineInputFormat extends FileInputFormat<LongWritable, Text> {
public static final String LINES_PER_MAP =
"mapreduce.input.lineinputformat.linespermap";
public RecordReader<LongWritable, Text> createRecordReader(
InputSplit genericSplit, TaskAttemptContext context)
throws IOException {
context.setStatus(genericSplit.toString());
return new LineRecordReader();
}
/**
* Logically splits the set of input files for the job, splits N lines
* of the input as one split.
*
* @see FileInputFormat#getSplits(JobContext)
*/
public List<InputSplit> getSplits(JobContext job)
throws IOException {
List<InputSplit> splits = new ArrayList<InputSplit>();
int numLinesPerSplit = getNumLinesPerSplit(job);
for (FileStatus status : listStatus(job)) {
splits.addAll(getSplitsForFile(status,
job.getConfiguration(), numLinesPerSplit));
}
return splits;
}
public static List<FileSplit> getSplitsForFile(FileStatus status,
Configuration conf, int numLinesPerSplit) throws IOException {
List<FileSplit> splits = new ArrayList<FileSplit> ();
Path fileName = status.getPath();
if (status.isDir()) {
throw new IOException("Not a file: " + fileName);
}
FileSystem fs = fileName.getFileSystem(conf);
LineReader lr = null;
try {
FSDataInputStream in = fs.open(fileName);
lr = new LineReader(in, conf);
Text line = new Text();
int numLines = 0;
long begin = 0;
long length = 0;
int num = -1;
while ((num = lr.readLine(line)) > 0) {
numLines++;
length += num;
if (numLines == numLinesPerSplit) {
splits.add(createFileSplit(fileName, begin, length));
begin += length;
length = 0;
numLines = 0;
}
}
if (numLines != 0) {
splits.add(createFileSplit(fileName, begin, length));
}
} finally {
if (lr != null) {
lr.close();
}
}
return splits;
}
/**
* NLineInputFormat uses LineRecordReader, which always reads
* (and consumes) at least one character out of its upper split
* boundary. So to make sure that each mapper gets N lines, we
* move back the upper split limits of each split
* by one character here.
* @param fileName Path of file
* @param begin the position of the first byte in the file to process
* @param length number of bytes in InputSplit
* @return FileSplit
*/
protected static FileSplit createFileSplit(Path fileName, long begin, long length) {
return (begin == 0)
? new FileSplit(fileName, begin, length - 1, new String[] {})
: new FileSplit(fileName, begin - 1, length, new String[] {});
}
/**
* Set the number of lines per split
* @param job the job to modify
* @param numLines the number of lines per split
*/
public static void setNumLinesPerSplit(Job job, int numLines) {
job.getConfiguration().setInt(LINES_PER_MAP, numLines);
}
/**
* Get the number of lines per split
* @param job the job
* @return the number of lines per split
*/
public static int getNumLinesPerSplit(JobContext job) {
return job.getConfiguration().getInt(LINES_PER_MAP, 1);
}
}