public void write(KEYOUT key, VALUEOUT value
          ) throws IOException, InterruptedException {
  output.write(key, value);
 }

@Override
  public void write(K key, V value) throws IOException, InterruptedException {
   collector.collect(key, value,
            partitioner.getPartition(key, value, partitions));
  }

@SuppressWarnings("unchecked")
  NewOutputCollector(org.apache.hadoop.mapreduce.JobContext jobContext,
            JobConf job,
            TaskUmbilicalProtocol umbilical,
            TaskReporter reporter
            ) throws IOException, ClassNotFoundException {
  // 創(chuàng)建實際用來收集key-value的緩存區(qū)對象
   collector = createSortingCollector(job, reporter);
  // 獲取總的分區(qū)個數(shù)
   partitions = jobContext.getNumReduceTasks();
   if (partitions > 1) {
    partitioner = (org.apache.hadoop.mapreduce.Partitioner<K,V>)
     ReflectionUtils.newInstance(jobContext.getPartitionerClass(), job);
   } else {
    // 默認情況，直接創(chuàng)建一個匿名內部類，所有的key-value都分配到0號分區(qū)
    partitioner = new org.apache.hadoop.mapreduce.Partitioner<K,V>() {
     @Override
     public int getPartition(K key, V value, int numPartitions) {
      return partitions - 1;
     }
    };
   }
  }

@SuppressWarnings("unchecked")
 private <KEY, VALUE> MapOutputCollector<KEY, VALUE>
     createSortingCollector(JobConf job, TaskReporter reporter)
  throws IOException, ClassNotFoundException {
  MapOutputCollector.Context context =
   new MapOutputCollector.Context(this, job, reporter);
  // 從當前Job的配置中，獲取mapreduce.job.map.output.collector.class，如果沒有設置，使用MapOutputBuffer.class
  Class<?>[] collectorClasses = job.getClasses(
   JobContext.MAP_OUTPUT_COLLECTOR_CLASS_ATTR, MapOutputBuffer.class);
  int remainingCollectors = collectorClasses.length;
  Exception lastException = null;
  for (Class clazz : collectorClasses) {
   try {
    if (!MapOutputCollector.class.isAssignableFrom(clazz)) {
     throw new IOException("Invalid output collector class: " + clazz.getName() +
      " (does not implement MapOutputCollector)");
    }
    Class<? extends MapOutputCollector> subclazz =
     clazz.asSubclass(MapOutputCollector.class);
    LOG.debug("Trying map output collector class: " + subclazz.getName());
   // 創(chuàng)建緩沖區(qū)對象
    MapOutputCollector<KEY, VALUE> collector =
     ReflectionUtils.newInstance(subclazz, job);
   // 創(chuàng)建完緩沖區(qū)對象后，執(zhí)行初始化
    collector.init(context);
    LOG.info("Map output collector class = " + collector.getClass().getName());
    return collector;
   } catch (Exception e) {
    String msg = "Unable to initialize MapOutputCollector " + clazz.getName();
    if (--remainingCollectors > 0) {
     msg += " (" + remainingCollectors + " more collector(s) to try)";
    }
    lastException = e;
    LOG.warn(msg, e);
   }
  }
  throw new IOException("Initialization of all the collectors failed. " +
   "Error in last collector was :" + lastException.getMessage(), lastException);
 }

@SuppressWarnings("unchecked")
  public void init(MapOutputCollector.Context context
          ) throws IOException, ClassNotFoundException {
   job = context.getJobConf();
   reporter = context.getReporter();
   mapTask = context.getMapTask();
   mapOutputFile = mapTask.getMapOutputFile();
   sortPhase = mapTask.getSortPhase();
   spilledRecordsCounter = reporter.getCounter(TaskCounter.SPILLED_RECORDS);
   // 獲取分區(qū)總個數(shù)，取決于ReduceTask的數(shù)量
   partitions = job.getNumReduceTasks();
   rfs = ((LocalFileSystem)FileSystem.getLocal(job)).getRaw();
   //sanity checks
   // 從當前配置中，獲取mapreduce.map.sort.spill.percent，如果沒有設置，就是0.8
   final float spillper =
    job.getFloat(JobContext.MAP_SORT_SPILL_PERCENT, (float)0.8);
   // 獲取mapreduce.task.io.sort.mb，如果沒設置，就是100MB
   final int sortmb = job.getInt(JobContext.IO_SORT_MB, 100);
   indexCacheMemoryLimit = job.getInt(JobContext.INDEX_CACHE_MEMORY_LIMIT,
                     INDEX_CACHE_MEMORY_LIMIT_DEFAULT);
   if (spillper > (float)1.0 || spillper <= (float)0.0) {
    throw new IOException("Invalid \"" + JobContext.MAP_SORT_SPILL_PERCENT +
      "\": " + spillper);
   }
   if ((sortmb & 0x7FF) != sortmb) {
    throw new IOException(
      "Invalid \"" + JobContext.IO_SORT_MB + "\": " + sortmb);
   }
// 在溢寫前，對key-value排序，采用的排序器，使用快速排序，只排索引
   sorter = ReflectionUtils.newInstance(job.getClass("map.sort.class",
      QuickSort.class, IndexedSorter.class), job);
   // buffers and accounting
   int maxMemUsage = sortmb << 20;
   maxMemUsage -= maxMemUsage % METASIZE;
   // 存放key-value
   kvbuffer = new byte[maxMemUsage];
   bufvoid = kvbuffer.length;
  // 存儲key-value的屬性信息，分區(qū)號，索引等
   kvmeta = ByteBuffer.wrap(kvbuffer)
     .order(ByteOrder.nativeOrder())
     .asIntBuffer();
   setEquator(0);
   bufstart = bufend = bufindex = equator;
   kvstart = kvend = kvindex;
   maxRec = kvmeta.capacity() / NMETA;
   softLimit = (int)(kvbuffer.length * spillper);
   bufferRemaining = softLimit;
   LOG.info(JobContext.IO_SORT_MB + ": " + sortmb);
   LOG.info("soft limit at " + softLimit);
   LOG.info("bufstart = " + bufstart + "; bufvoid = " + bufvoid);
   LOG.info("kvstart = " + kvstart + "; length = " + maxRec);
   // k/v serialization
    // 獲取快速排序的Key的比較器，排序只按照key進行排序！
   comparator = job.getOutputKeyComparator();
  // 獲取key-value的序列化器
   keyClass = (Class<K>)job.getMapOutputKeyClass();
   valClass = (Class<V>)job.getMapOutputValueClass();
   serializationFactory = new SerializationFactory(job);
   keySerializer = serializationFactory.getSerializer(keyClass);
   keySerializer.open(bb);
   valSerializer = serializationFactory.getSerializer(valClass);
   valSerializer.open(bb);
   // output counters
   mapOutputByteCounter = reporter.getCounter(TaskCounter.MAP_OUTPUT_BYTES);
   mapOutputRecordCounter =
    reporter.getCounter(TaskCounter.MAP_OUTPUT_RECORDS);
   fileOutputByteCounter = reporter
     .getCounter(TaskCounter.MAP_OUTPUT_MATERIALIZED_BYTES);
   // 溢寫到磁盤，可以使用一個壓縮格式！ 獲取指定的壓縮編解碼器
   // compression
   if (job.getCompressMapOutput()) {
    Class<? extends CompressionCodec> codecClass =
     job.getMapOutputCompressorClass(DefaultCodec.class);
    codec = ReflectionUtils.newInstance(codecClass, job);
   } else {
    codec = null;
   }
   // 獲取Combiner組件
   // combiner
   final Counters.Counter combineInputCounter =
    reporter.getCounter(TaskCounter.COMBINE_INPUT_RECORDS);
   combinerRunner = CombinerRunner.create(job, getTaskID(),
                       combineInputCounter,
                       reporter, null);
   if (combinerRunner != null) {
    final Counters.Counter combineOutputCounter =
     reporter.getCounter(TaskCounter.COMBINE_OUTPUT_RECORDS);
    combineCollector= new CombineOutputCollector<K,V>(combineOutputCounter, reporter, job);
   } else {
    combineCollector = null;
   }
   spillInProgress = false;
   minSpillsForCombine = job.getInt(JobContext.MAP_COMBINE_MIN_SPILLS, 3);
   // 設置溢寫線程在后臺運行，溢寫是在后臺運行另外一個溢寫線程！和收集是兩個線程！
   spillThread.setDaemon(true);
   spillThread.setName("SpillThread");
   spillLock.lock();
   try {
   // 啟動線程
    spillThread.start();
    while (!spillThreadRunning) {
     spillDone.await();
    }
   } catch (InterruptedException e) {
    throw new IOException("Spill thread failed to initialize", e);
   } finally {
    spillLock.unlock();
   }
   if (sortSpillException != null) {
    throw new IOException("Spill thread failed to initialize",
      sortSpillException);
   }
  }

@SuppressWarnings("unchecked")
 public Class<? extends Partitioner<?,?>> getPartitionerClass()
   throws ClassNotFoundException {
  return (Class<? extends Partitioner<?,?>>)
   conf.getClass(PARTITIONER_CLASS_ATTR, HashPartitioner.class);
 }

public class HashPartitioner<K, V> extends Partitioner<K, V> {
 /** Use {@link Object#hashCode()} to partition. **/
 public int getPartition(K key, V value,
             int numReduceTasks) {
  return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;
 }
}

if (partition < 0 || partition >= partitions) {
    throw new IOException("Illegal partition for " + key + " (" +
      partition + ")");
   }

Class<? extends Reducer<K,V,K,V>> cls =
    (Class<? extends Reducer<K,V,K,V>>) job.getCombinerClass();