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EXCELSIOR
매퍼(Mapper) 와 리듀서(Reducer) 클래스 본문
1. Mapper.java
public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> { //<입력키 유형, 입력값 유형, 출력키 유형, 출력값 유형>을 의미 public class Context extends MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> { public Context(Configuration conf, TaskAttemptID taskid, RecordReader<KEYIN,VALUEIN> reader, RecordWriter<KEYOUT,VALUEOUT> writer, OutputCommitter committer, StatusReporter reporter, InputSplit split) throws IOException, InterruptedException { super(conf, taskid, reader, writer, committer, reporter, split); } } /** * Called once at the beginning of the task. */ protected void setup(Context context ) throws IOException, InterruptedException { // NOTHING } /** * Called once for each key/value pair in the input split. Most applications * should override this, but the default is the identity function. */ @SuppressWarnings("unchecked") protected void map(KEYIN key, VALUEIN value, Context context) throws IOException, InterruptedException { //맵 메서드, 맵리듀스 프로그램을 개발할 때 이 메서드를 재정의한다. context.write((KEYOUT) key, (VALUEOUT) value); } /** * Called once at the end of the task. */ protected void cleanup(Context context ) throws IOException, InterruptedException { // NOTHING } /** * 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); while (context.nextKeyValue()) { map(context.getCurrentKey(), context.getCurrentValue(), context); } cleanup(context); } }
2. Reducer.java
public class Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT> { public class Context extends ReduceContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> { public Context(Configuration conf, TaskAttemptID taskid, RawKeyValueIterator input, Counter inputKeyCounter, Counter inputValueCounter, RecordWriter<KEYOUT,VALUEOUT> output, OutputCommitter committer, StatusReporter reporter, RawComparator<KEYIN> comparator, Class<KEYIN> keyClass, Class<VALUEIN> valueClass ) throws IOException, InterruptedException { super(conf, taskid, input, inputKeyCounter, inputValueCounter, output, committer, reporter, comparator, keyClass, valueClass); } } /** * Called once at the start of the task. */ protected void setup(Context context ) throws IOException, InterruptedException { // NOTHING } /** * This method is called once for each key. Most applications will define * their reduce class by overriding this method. The default implementation * is an identity function. */ @SuppressWarnings("unchecked") protected void reduce(KEYIN key, Iterable<VALUEIN> values, Context context //이 메서드를 재정의한다. ) throws IOException, InterruptedException { for(VALUEIN value: values) { context.write((KEYOUT) key, (VALUEOUT) value); } } /** * Called once at the end of the task. */ protected void cleanup(Context context ) throws IOException, InterruptedException { // NOTHING } /** * Advanced application writers can use the * {@link #run(org.apache.hadoop.mapreduce.Reducer.Context)} method to * control how the reduce task works. */ public void run(Context context) throws IOException, InterruptedException { setup(context); while (context.nextKey()) { context.progress(); reduce(context.getCurrentKey(), context.getValues(), context); } cleanup(context); } }
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