What is the output output process of MapReduce

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1. First, take a look at the execution entry / /-- ReduceTask.javapublic void run (JobConf job, TaskUmbilicalProtocol umbilical) throws IOException, InterruptedException, ClassNotFoundException {job.setBoolean ("mapreduce.job.skiprecords", this.isSkipping ()); if (this.isMapOrReduce ()) {this.copyPhase = this.getProgress (). AddPhase ("copy") This.sortPhase = this.getProgress (). AddPhase ("sort"); this.reducePhase = this.getProgress (). AddPhase ("reduce");} TaskReporter reporter = this.startReporter (umbilical); boolean useNewApi = job.getUseNewReducer (); / / reducetask initialization work 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 {this.codec = this.initCodec (); RawKeyValueIterator rIter = null; ShuffleConsumerPlugin shuffleConsumerPlugin = null; Class combinerClass = this.conf.getCombinerClass (); CombineOutputCollector combineCollector = null! = combinerClass? New CombineOutputCollector (this.reduceCombineOutputCounter, reporter, this.conf): null; Class