inputformat读取数据流程

public abstract class InputFormat<K, V> {public abstractList<InputSplit> getSplits(JobContext context) throws IOException, InterruptedException;public abstractRecordReader<K,V> createRecordReader(InputSplit split,TaskAttemptContext context) throws IOException,InterruptedException;}public abstract class FileInputFormat<K, V> extends InputFormat<K, V> {public class TextInputFormat extends FileInputFormat<LongWritable, Text> {getSplits方法，获得对输入文件的切分数量，每一个split对应一个map。
创建RecordReader，该RecordReader接收切分好的split，实现nextKeyValue、getCurrentKey、getCurrentValue。如下所示，每个map类都会继承Mapper类，在Mapper类中，run方法会调用InputFormat中的RecordReader来获得key、valuepublic class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {/*** 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);try {while (context.nextKeyValue()) {map(context.getCurrentKey(), context.getCurrentValue(), context);}} finally {cleanup(context);}}
}

reduce写出数据流程：

reduce读取map输出的中间结果
org.apache.hadoop.mapreduce.Reducer<KEYIN, VALUEIN, KEYOUT, VALUEOUT>
public void run(Context context) throws IOException, InterruptedException {setup(context);try {while (context.nextKey()) {//调用reduce方法，该方法一般由自定义的业务reducer重写reduce(context.getCurrentKey(), context.getValues(), context);Iterator<VALUEIN> iter = context.getValues().iterator();if(iter instanceof ReduceContext.ValueIterator) {((ReduceContext.ValueIterator<VALUEIN>)iter).resetBackupStore();        }}} finally {cleanup(context);}
}/*** 自定义reducer，重写父类reduce方法*/
public class IssueDataRecordDistinctReducer extends Reducer<Text, LongWritable, NullWritable,Text> {private NullWritable nullKey = NullWritable.get();protected void reduce(Text key, Iterable<LongWritable> values, Context context)  throws IOException, InterruptedException {context.write(nullKey, new Text("hello"));
}}context.write(nullKey, new Text("hello"));
reduce中，每一次context.write操作都会调用自定义OutputFormat中的RecordWrite类中的write方法