* Licensed to the Apache Software Foundation (ASF) under one
package org.apache.hadoop.mapreduce;
import java.io.IOException;
/**
* Maps input key/value pairs to a set of intermediate key/value pairs.
*
* <p>Maps are the individual tasks which transform input records into a
* intermediate records. The transformed intermediate records need not be of
* the same type as the input records. A given input pair may map to zero or
* many output pairs.</p>
*
* <p>The Hadoop Map-Reduce framework spawns one map task for each
* {@link InputSplit} generated by the {@link InputFormat} for the job.
* <code>Mapper</code> implementations can access the {@link Configuration} for
* the job via the {@link JobContext#getConfiguration()}.
*
* <p>The framework first calls
* {@link #setup(org.apache.hadoop.mapreduce.Mapper.Context)}, followed by
* {@link #map(Object, Object, Context)}
* for each key/value pair in the <code>InputSplit</code>. Finally
* {@link #cleanup(Context)} is called.</p>
*
* <p>All intermediate values associated with a given output key are
* subsequently grouped by the framework, and passed to a {@link Reducer} to
* determine the final output. Users can control the sorting and grouping by
* specifying two key {@link RawComparator} classes.</p>
*
* <p>The <code>Mapper</code> outputs are partitioned per
* <code>Reducer</code>. Users can control which keys (and hence records) go to
* which <code>Reducer</code> by implementing a custom {@link Partitioner}.
*
* <p>Users can optionally specify a <code>combiner</code>, via
* {@link Job#setCombinerClass(Class)}, to perform local aggregation of the
* intermediate outputs, which helps to cut down the amount of data transferred
* from the <code>Mapper</code> to the <code>Reducer</code>.
*
* <p>Applications can specify if and how the intermediate
* outputs are to be compressed and which {@link CompressionCodec}s are to be
* used via the <code>Configuration</code>.</p>
*
* <p>If the job has zero
* reduces then the output of the <code>Mapper</code> is directly written
* to the {@link OutputFormat} without sorting by keys.</p>
*
* <p>Example:</p>
* <p><blockquote><pre>
* public class TokenCounterMapper
* extends Mapper<Object, Text, Text, IntWritable>{
*
* private final static IntWritable one = new IntWritable(1);
* private Text word = new Text();
*
* public void map(Object key, Text value, Context context) throws IOException {
* StringTokenizer itr = new StringTokenizer(value.toString());
* while (itr.hasMoreTokens()) {
* word.set(itr.nextToken());
* context.collect(word, one);
* }
* }
* }
* </pre></blockquote></p>
*
* <p>Applications may override the {@link #run(Context)} method to exert
* greater control on map processing e.g. multi-threaded <code>Mapper</code>s
* etc.</p>
*
* @see InputFormat
* @see JobContext
* @see Partitioner
* @see Reducer
*/
public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
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 {
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);
}
}
