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

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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 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  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 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 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 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 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 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 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 extends org.apache.hadoop.mapreduce.RecordReader {
    private final org.apache.hadoop.mapreduce.RecordReader real;
    private final org.apache.hadoop.mapreduce.Counter inputRecordCounter;
    private final org.apache.hadoop.mapreduce.Counter fileInputByteCounter;
    private final TaskReporter reporter;
    private final List fsStats;

    NewTrackingRecordReader(org.apache.hadoop.mapreduce.InputSplit split, InputFormat 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 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 {
    public TextInputFormat() {
    }

    public RecordReader 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 extends org.apache.hadoop.mapreduce.RecordReader {
    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.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 extends TaskInputOutputContextImpl implements MapContext {
    private RecordReader 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 reader, RecordWriter 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 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 {
    public InputFormat() {
    }

    public abstract List getSplits(JobContext var1) throws IOException, InterruptedException;

    public abstract RecordReader 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。