当job提交至yarn之后，就会开始调度运行map任务，这里开始讲解map输入的源码分析。
一个map任务的入口就是 MapTask.class 中的run() 方法
1、首先看看MapTask.run() 方法
MapTask.class
//---------------------------------MapTask.java
public void run(JobConf job, TaskUmbilicalProtocol umbilical) throws IOException, ClassNotFoundException, InterruptedException {
this.umbilical = umbilical;
if (this.isMapTask()) {
if (this.conf.getNumReduceTasks() == 0) {
this.mapPhase = this.getProgress().addPhase("map", 1.0F);
} else {
this.mapPhase = this.getProgress().addPhase("map", 0.667F);
this.sortPhase = this.getProgress().addPhase("sort", 0.333F);
}
}
TaskReporter reporter = this.startReporter(umbilical);
boolean useNewApi = job.getUseNewMapper();
//进行map任务的初始化
this.initialize(job, this.getJobID(), reporter, useNewApi);
if (this.jobCleanup) {
this.runJobCleanupTask(umbilical, reporter);
} else if (this.jobSetup) {
this.runJobSetupTask(umbilical, reporter);
} else if (this.taskCleanup) {
this.runTaskCleanupTask(umbilical, reporter);
} else {
//启动map任务，判断是使用新的还是旧的api
if (useNewApi) {
this.runNewMapper(job, this.splitMetaInfo, umbilical, reporter);
} else {
this.runOldMapper(job, this.splitMetaInfo, umbilical, reporter);
}
this.done(umbilical, reporter);
}
}上面重点有两个方法，一个是 this.initialize()以及 this.runNewMapper()。
2、下面看看this.initialize()
//---------------------------------Task.java
public void initialize(JobConf job, JobID id, Reporter reporter, boolean useNewApi) throws IOException, ClassNotFoundException, InterruptedException {
//创建task以及job上下文对象
this.jobContext = new JobContextImpl(job, id, reporter);
this.taskContext = new TaskAttemptContextImpl(job, this.taskId, reporter);
//将task任务的状态改为正在运行
if (this.getState() == org.apache.hadoop.mapred.TaskStatus.State.UNASSIGNED) {
this.setState(org.apache.hadoop.mapred.TaskStatus.State.RUNNING);
}
if (useNewApi) {
if (LOG.isDebugEnabled()) {
LOG.debug("using new api for output committer");
}
//获取job中配置的输出格式类，并通过反射获取该类的Class对象
this.outputFormat = (OutputFormat)ReflectionUtils.newInstance(this.taskContext.getOutputFormatClass(), job);
//通过outputformat类获取commiter
this.committer = this.outputFormat.getOutputCommitter(this.taskContext);
} else {
this.committer = this.conf.getOutputCommitter();
}
//从FileOutputFormat获取任务结果输出路径。
/*
可能有的人会奇怪，为啥mapper这里要获取outputformat 的输出路径。
首先我们要知道，一个MapReduce任务可以只有mapper，而没有reducer的，
那么这时候程序的输出是有mapper直接输出的，这时候自然就需要知道输出的路径，这里就派上用场了
*/
Path outputPath = FileOutputFormat.getOutputPath(this.conf);
if (outputPath != null) {
if (this.committer instanceof FileOutputCommitter) {
FileOutputFormat.setWorkOutputPath(this.conf, ((FileOutputCommitter)this.committer).getTaskAttemptPath(this.taskContext));
} else {
FileOutputFormat.setWorkOutputPath(this.conf, outputPath);
}
}
this.committer.setupTask(this.taskContext);
Class<? extends ResourceCalculatorProcessTree> clazz = this.conf.getClass("mapreduce.job.process-tree.class", (Class)null, ResourceCalculatorProcessTree.class);
this.pTree = ResourceCalculatorProcessTree.getResourceCalculatorProcessTree((String)System.getenv().get("JVM_PID"), clazz, this.conf);
LOG.info(" Using ResourceCalculatorProcessTree : " + this.pTree);
if (this.pTree != null) {
this.pTree.updateProcessTree();
this.initCpuCumulativeTime = this.pTree.getCumulativeCpuTime();
}
}这个方法主要做了一些初始化工作，比如创建上下文对象，获取输出outputFormat类，以及路径等。
3、下面接着看看this.runNewMapper()
//---------------------------------MapTask.java
private <INKEY, INVALUE, OUTKEY, OUTVALUE> void runNewMapper(JobConf job, TaskSplitIndex splitIndex, TaskUmbilicalProtocol umbilical, TaskReporter reporter) throws IOException, ClassNotFoundException, InterruptedException {
TaskAttemptContext taskContext = new TaskAttemptContextImpl(job, this.getTaskID(), reporter);
//通过反射获取job中配置的mapper实现类
Mapper<INKEY, INVALUE, OUTKEY, OUTVALUE> mapper = (Mapper)ReflectionUtils.newInstance(taskContext.getMapperClass(), job);
//通过反射获取job中配置的输入格式类,默认是TextInputFormat
InputFormat<INKEY, INVALUE> inputFormat = (InputFormat)ReflectionUtils.newInstance(taskContext.getInputFormatClass(), job);
org.apache.hadoop.mapreduce.InputSplit split = null;
//获取切片详细信息,传入输出路径以及偏移量作为参数.也就是当前mapper处理的某个切片
split = (org.apache.hadoop.mapreduce.InputSplit)this.getSplitDetails(new Path(splitIndex.getSplitLocation()), splitIndex.getStartOffset());
LOG.info("Processing split: " + split);
//获取输入的读取数据文件的 RecordReader 的对象，默认inputformat为TextInputFormat，对应默认的RecordReader为LineRecordReader
org.apache.hadoop.mapreduce.RecordReader<INKEY, INVALUE> input = new MapTask.NewTrackingRecordReader(split, inputFormat, reporter, taskContext);
job.setBoolean("mapreduce.job.skiprecords", this.isSkipping());
RecordWriter output = null;
//获取RecordWriter输出对象
if (job.getNumReduceTasks() == 0) {
output = new MapTask.NewDirectOutputCollector(taskContext, job, umbilical, reporter);
} else {
output = new MapTask.NewOutputCollector(taskContext, job, umbilical, reporter);
}
MapContext<INKEY, INVALUE, OUTKEY, OUTVALUE> mapContext = new MapContextImpl(job, this.getTaskID(), input, (RecordWriter)output, this.committer, reporter, split);
org.apache.hadoop.mapreduce.Mapper.Context mapperContext = (new WrappedMapper()).getMapContext(mapContext);
try {
//初始化RecordReader中的数据
input.initialize(split, mapperContext);
//运行mapper中的run方法，也就是Mapper类中的run方法，开始运行map任务
mapper.run(mapperContext);
this.mapPhase.complete();
this.setPhase(Phase.SORT);
this.statusUpdate(umbilical);
//map运行完，关闭输入、输出流
input.close();
input = null;
((RecordWriter)output).close(mapperContext);
output = null;
} finally {
this.closeQuietly((org.apache.hadoop.mapreduce.RecordReader)input);
this.closeQuietly((RecordWriter)output, mapperContext);
}
}可以看到，这里就是整个map任务的核心流程，做了以下工作：
（1）获取mapper类对象，下面要执行里面的map方法
（2）获取InputFormat对象，默认是默认inputformat为TextInputFormat
（3）通过InputFormat对象获取RecordReader对象，后面用于读取数据文件
（4）获取用于输出map的结果的RecordWriter对象
（5）获取切片信息，比如切片所在文件的路径，起始偏移量等
（6）初始化切片数据
（7）开始运行mapper中的run()方法
（8）运行完毕，关闭输入流，将结果通过RecordWriter刷写。
（9）刷写完毕后，关闭输入流以及输出流
下面看看其中的核心方法
4、this.getSplitDetails() 获取切片信息
//---------------------------------MapTask.java
private <T> T getSplitDetails(Path file, long offset) throws IOException {
//获取文件系统对象，并打开文件输出流
FileSystem fs = file.getFileSystem(this.conf);
FSDataInputStream inFile = fs.open(file);
//跳过指定的偏移量，也就是从指定偏移量的位置开始读取数据，其实就是切片开始的偏移量
inFile.seek(offset);
String className = StringInterner.weakIntern(Text.readString(inFile));
Class cls;
try {
cls = this.conf.getClassByName(className);
} catch (ClassNotFoundException var13) {
IOException wrap = new IOException("Split class " + className + " not found");
wrap.initCause(var13);
throw wrap;
}
SerializationFactory factory = new SerializationFactory(this.conf);
//反序列化方式打开输入流
Deserializer<T> deserializer = factory.getDeserializer(cls);
deserializer.open(inFile);
T split = deserializer.deserialize((Object)null);
long pos = inFile.getPos();
((Counter)this.getCounters().findCounter(TaskCounter.SPLIT_RAW_BYTES)).increment(pos - offset);
inFile.close();
//返回切片经过反序列化之后的可读取对象
return split;
}可以看到这里主要是返回切片的反序列化之后可以读取的信息对象
5、接着看看 input.initialize()
在看这个方法之前，首先我们看看input这个对象是由哪个类创建的。它是由NewTrackingRecordReader 这个类创建的。这是个静态内部类
//---------------------------------MapTask.java
static class NewTrackingRecordReader<K, V> extends org.apache.hadoop.mapreduce.RecordReader<K, V> {
private final org.apache.hadoop.mapreduce.RecordReader<K, V> real;
private final org.apache.hadoop.mapreduce.Counter inputRecordCounter;
private final org.apache.hadoop.mapreduce.Counter fileInputByteCounter;
private final TaskReporter reporter;
private final List<Statistics> fsStats;
NewTrackingRecordReader(org.apache.hadoop.mapreduce.InputSplit split, InputFormat<K, V> inputFormat, TaskReporter reporter, TaskAttemptContext taskContext) throws InterruptedException, IOException {
this.reporter = reporter;
this.inputRecordCounter = reporter.getCounter(TaskCounter.MAP_INPUT_RECORDS);
this.fileInputByteCounter = reporter.getCounter(FileInputFormatCounter.BYTES_READ);
List<Statistics> matchedStats = null;
if (split instanceof org.apache.hadoop.mapreduce.lib.input.FileSplit) {
matchedStats = Task.getFsStatistics(((org.apache.hadoop.mapreduce.lib.input.FileSplit)split).getPath(), taskContext.getConfiguration());
}
this.fsStats = matchedStats;
long bytesInPrev = this.getInputBytes(this.fsStats);
//调用job任务中定义的inputformat类中的createRecordReader方法，获取RecordReader对象。返回的是 LineRecordReader
this.real = inputFormat.createRecordReader(split, taskContext);
long bytesInCurr = this.getInputBytes(this.fsStats);
this.fileInputByteCounter.increment(bytesInCurr - bytesInPrev);
}
...........
}我们可以看到构造方法中，是调用 inputFormat对象的createRecordReader() 方法来创建RecordReader对象的，上面也说了默认inputFormat为 TextInputFormat。
//---------------------------TextInputFormat.java
public class TextInputFormat extends FileInputFormat<LongWritable, Text> {
public TextInputFormat() {
}
public RecordReader<LongWritable, Text> createRecordReader(InputSplit split, TaskAttemptContext context) {
String delimiter = context.getConfiguration().get("textinputformat.record.delimiter");
byte[] recordDelimiterBytes = null;
if (null != delimiter) {
recordDelimiterBytes = delimiter.getBytes(Charsets.UTF_8);
}
return new LineRecordReader(recordDelimiterBytes);
}可以清楚看到，返回的就是 LineRecordReader 这个reader类。
接着我们继续看 input.initialize()
static class NewTrackingRecordReader<K, V> extends org.apache.hadoop.mapreduce.RecordReader<K, V> {
public void initialize(org.apache.hadoop.mapreduce.InputSplit split, TaskAttemptContext context) throws IOException, InterruptedException {
long bytesInPrev = this.getInputBytes(this.fsStats);
//调用 RecordReader对象的 initialize方法，初始化输入。上面说到默认的是LineRecordReader
//this.real已经在上面初始化了，就是LineRecordReader
this.real.initialize(split, context);
long bytesInCurr = this.getInputBytes(this.fsStats);
this.fileInputByteCounter.increment(bytesInCurr - bytesInPrev);
}
}可以看到，调用 RecordReader中的 initialize 方法，也就是调用LineRecordReader 中的 initialize() 方法，下面看看
//---------------------------------------LineRecordReader.java
public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException {
FileSplit split = (FileSplit)genericSplit;
Configuration job = context.getConfiguration();
this.maxLineLength = job.getInt("mapreduce.input.linerecordreader.line.maxlength", 2147483647);
//获取切片的数据开始位置以及终止位置
this.start = split.getStart();
this.end = this.start + split.getLength();
//获取切片对应的文件的输入流
Path file = split.getPath();
FileSystem fs = file.getFileSystem(job);
this.fileIn = fs.open(file);
//如果文件有压缩，则用压缩类解压
CompressionCodec codec = (new CompressionCodecFactory(job)).getCodec(file);
//以压缩方式读取切片
if (null != codec) {
this.isCompressedInput = true;
this.decompressor = CodecPool.getDecompressor(codec);
if (codec instanceof SplittableCompressionCodec) {
SplitCompressionInputStream cIn = ((SplittableCompressionCodec)codec).createInputStream(this.fileIn, this.decompressor, this.start, this.end, READ_MODE.BYBLOCK);
this.in = new CompressedSplitLineReader(cIn, job, this.recordDelimiterBytes);
this.start = cIn.getAdjustedStart();
this.end = cIn.getAdjustedEnd();
this.filePosition = cIn;
} else {
if (this.start != 0L) {
throw new IOException("Cannot seek in " + codec.getClass().getSimpleName() + " compressed stream");
}
this.in = new SplitLineReader(codec.createInputStream(this.fileIn, this.decompressor), job, this.recordDelimiterBytes);
this.filePosition = this.fileIn;
}
} else {
//无压缩方式读取切片
this.fileIn.seek(this.start);
//这里很重要，是真正用于读取数据的类
this.in = new UncompressedSplitLineReader(this.fileIn, job, this.recordDelimiterBytes, split.getLength());
this.filePosition = this.fileIn;
}
//对起始偏移量进行修正，并赋值给pos这个偏移量
if (this.start != 0L) {
this.start += (long)this.in.readLine(new Text(), 0, this.maxBytesToConsume(this.start));
}
this.pos = this.start;
}这里的工作主要是给 RecordReader对象读取文件做初始化工作。主要就是获取切片的输入流对象。
this.in 这里就用于后面读取数据的对象，这里就是完成了这个输入流对象的初始化。
6、接着我们回到3中，看mapper.run() 方法
这个其实就是写的mapper 的run方法：
//------------------------Mapper.java mapper.run(mapperContext);
public void run(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException {
this.setup(context);
try {
//这里循环读取key和value，给map方法处理
//关键在于 context这个对象,从上面runNewApi中可以看到，是MapContextImpl类型的
while(context.nextKeyValue()) {
this.map(context.getCurrentKey(), context.getCurrentValue(), context);
}
} finally {
this.cleanup(context);
}
}可以看到，这里是个while循环，通过context上下文对象获取KV，然后传入map方法中处理。
7、下面看看 context.nextKeyValue()
从3中可以看到，这个context是 MapContextImpl类型的，看看这个类
//-----------------------MapContextImpl.java..
public class MapContextImpl<KEYIN, VALUEIN, KEYOUT, VALUEOUT> extends TaskInputOutputContextImpl<KEYIN, VALUEIN, KEYOUT, VALUEOUT> implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
private RecordReader<KEYIN, VALUEIN> reader;
private InputSplit split;
//构造方法中包括获取 RecordReader对象，以及split
public MapContextImpl(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;
}
public InputSplit getInputSplit() {
return this.split;
}
//下面都是调用 RecordReader 中的get方法获取key value
public KEYIN getCurrentKey() throws IOException, InterruptedException {
return this.reader.getCurrentKey();
}
public VALUEIN getCurrentValue() throws IOException, InterruptedException {
return this.reader.getCurrentValue();
}
public boolean nextKeyValue() throws IOException, InterruptedException {
//这里就是调用reader 的方法
return this.reader.nextKeyValue();
}
}在它的构造方法中，主要从3中传入了 split切片，以及 RecordReader对象。下面就是三个获取KV的方法，也就是在 mapper.run() 中调用的方法。
下面看看 this.reader.nextKeyValue()
//----------------------------------LineRecordReader.java
public boolean nextKeyValue() throws IOException {
if (this.key == null) {
this.key = new LongWritable();
}
//设置key为偏移量
this.key.set(this.pos);
if (this.value == null) {
this.value = new Text();
}
int newSize = 0;
while(this.getFilePosition() <= this.end || this.in.needAdditionalRecordAfterSplit()) {
if (this.pos == 0L) {
newSize = this.skipUtfByteOrderMark();
} else {
/*读取数据到value中。this.in是UncompressedSplitLineReader类型的，在LineRecordReader的initialize方法中初始化了。该类父类为LineReader。*/
//调用 LineRreader 的readline 方法。读一行数据
newSize = this.in.readLine(this.value, this.maxLineLength, this.maxBytesToConsume(this.pos));
this.pos += (long)newSize;
}
if (newSize == 0 || newSize < this.maxLineLength) {
break;
}
LOG.info("Skipped line of size " + newSize + " at pos " + (this.pos - (long)newSize));
}
if (newSize == 0) {
this.key = null;
this.value = null;
return false;
} else {
return true;
}
}可以看到，这里已经看到key和value的踪影了。key就是数据偏移量，value就是通过readLine读取的数据。如果有数据返回true，mapper.run() 通过getKey和getValue对应的KV。下面看看 this.in.readLine，也就是 LineReader.readLine()。
8、LineReader.readLine() 按行读取的reader
//---------------------------LineReader.java
public int readLine(Text str, int maxLineLength, int maxBytesToConsume) throws IOException {
return this.recordDelimiterBytes != null ? this.readCustomLine(str, maxLineLength, maxBytesToConsume) : this.readDefaultLine(str, maxLineLength, maxBytesToConsume);
}
private int readCustomLine(Text str, int maxLineLength, int maxBytesToConsume) throws IOException {
str.clear();
int txtLength = 0;
long bytesConsumed = 0L;
int delPosn = 0;
int ambiguousByteCount = 0;
do {
int startPosn = this.bufferPosn;
if (this.bufferPosn >= this.bufferLength) {
startPosn = this.bufferPosn = 0;
this.bufferLength = this.fillBuffer(this.in, this.buffer, ambiguousByteCount > 0);
if (this.bufferLength <= 0) {
if (ambiguousByteCount > 0) {
str.append(this.recordDelimiterBytes, 0, ambiguousByteCount);
bytesConsumed += (long)ambiguousByteCount;
}
break;
}
}
for(; this.bufferPosn < this.bufferLength; ++this.bufferPosn) {
if (this.buffer[this.bufferPosn] == this.recordDelimiterBytes[delPosn]) {
++delPosn;
if (delPosn >= this.recordDelimiterBytes.length) {
++this.bufferPosn;
break;
}
} else if (delPosn != 0) {
this.bufferPosn -= delPosn;
if (this.bufferPosn < -1) {
this.bufferPosn = -1;
}
delPosn = 0;
}
}
int readLength = this.bufferPosn - startPosn;
bytesConsumed += (long)readLength;
int appendLength = readLength - delPosn;
if (appendLength > maxLineLength - txtLength) {
appendLength = maxLineLength - txtLength;
}
bytesConsumed += (long)ambiguousByteCount;
if (appendLength >= 0 && ambiguousByteCount > 0) {
//看到这里就很明显了，将数据追加到 value中
str.append(this.recordDelimiterBytes, 0, ambiguousByteCount);
ambiguousByteCount = 0;
this.unsetNeedAdditionalRecordAfterSplit();
}
if (appendLength > 0) {
str.append(this.buffer, startPosn, appendLength);
txtLength += appendLength;
}
if (this.bufferPosn >= this.bufferLength && delPosn > 0 && delPosn < this.recordDelimiterBytes.length) {
ambiguousByteCount = delPosn;
bytesConsumed -= (long)delPosn;
}
} while(delPosn < this.recordDelimiterBytes.length && bytesConsumed < (long)maxBytesToConsume);
if (bytesConsumed > 2147483647L) {
throw new IOException("Too many bytes before delimiter: " + bytesConsumed);
} else {
return (int)bytesConsumed;
}
}上面重要就是读取数据的过程了，过程过于长，抓住关键的看，其实就是将读取的一行数据追加到 this.value中。
9、总结
至此，map的整个输入流程涉及到两个重要的类
InputFormat -- 处理原始数据并切片；创建RecordReader 对象
RecordReader -- 读取切片中的数据，处理成KV，传递KV给map方法处理
这两个都是抽象类：
public abstract class RecordReader<KEYIN, VALUEIN> implements Closeable {
public RecordReader() {
}
public abstract void initialize(InputSplit var1, TaskAttemptContext var2) throws IOException, InterruptedException;
public abstract boolean nextKeyValue() throws IOException, InterruptedException;
public abstract KEYIN getCurrentKey() throws IOException, InterruptedException;
public abstract VALUEIN getCurrentValue() throws IOException, InterruptedException;
public abstract float getProgress() throws IOException, InterruptedException;
public abstract void close() throws IOException;
}public abstract class InputFormat<K, V> {
public InputFormat() {
}
public abstract List<InputSplit> getSplits(JobContext var1) throws IOException, InterruptedException;
public abstract RecordReader<K, V> createRecordReader(InputSplit var1, TaskAttemptContext var2) throws IOException, InterruptedException;
}当我们想自定义inputformat类和recordreader类时，就需要继承这两个类，并实现其中的方法。