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## org.apache.hadoop.mapreduce
Class Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

    java.lang.Object
      org.apache.hadoop.mapreduce.Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

Direct Known Subclasses:
  InverseMapper, MultithreadedMapper, SecondarySort.MapClass, TokenCounterMapper, WordCount.TokenizerMapper

    public class Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

extends Object

Maps input key/value pairs to a set of intermediate key/value pairs.

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.

The Hadoop Map-Reduce framework spawns one map task for each `InputSplit` generated by the `InputFormat` for the job. `Mapper` implementations can access the `Configuration` for the job via the `JobContext.getConfiguration()`.

The framework first calls `setup(org.apache.hadoop.mapreduce.Mapper.Context)`, followed by `map(Object, Object, Context)` for each key/value pair in the `InputSplit`. Finally `cleanup(Context)` is called.

All intermediate values associated with a given output key are subsequently grouped by the framework, and passed to a `Reducer` to determine the final output. Users can control the sorting and grouping by specifying two key `RawComparator` classes.

The `Mapper` outputs are partitioned per `Reducer`. Users can control which keys (and hence records) go to which `Reducer` by implementing a custom `Partitioner`.

Users can optionally specify a `combiner`, via `Job.setCombinerClass(Class)`, to perform local aggregation of the intermediate outputs, which helps to cut down the amount of data transferred from the `Mapper` to the `Reducer`.

Applications can specify if and how the intermediate outputs are to be compressed and which `CompressionCodec`s are to be used via the `Configuration`.

If the job has zero reduces then the output of the `Mapper` is directly written to the `OutputFormat` without sorting by keys.

Example:

>      public class TokenCounterMapper
>          extends Mapper{

>        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);
>          }
>        }
>      }

Applications may override the `run(Context)` method to exert greater control on map processing e.g. multi-threaded `Mapper`s etc.

See Also:
  `InputFormat`, `JobContext`, `Partitioner`, `Reducer`

Nested Class Summary
`class` | `Mapper.Context`

Constructor Summary
`Mapper()`

Method Summary
`protected void` | `cleanup(Mapper.Context context)` Called once at the end of the task.
`protected void` | `map(KEYIN key, VALUEIN value, Mapper.Context context)` Called once for each key/value pair in the input split.
`void` | `run(Mapper.Context context)` Expert users can override this method for more complete control over the execution of the Mapper.
`protected void` | `setup(Mapper.Context context)` Called once at the beginning of the task.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### Mapper

    public Mapper()

Method Detail

### setup

    protected void setup(Mapper.Context context)
                  throws IOException,
                         InterruptedException

  Called once at the beginning of the task.

  Throws:
    `IOException`
    `InterruptedException`

### map

    protected void map(KEYIN key,
                       VALUEIN value,
                       Mapper.Context context)
                throws IOException,
                       InterruptedException

  Called once for each key/value pair in the input split. Most applications should override this, but the default is the identity function.

  Throws:
    `IOException`
    `InterruptedException`

### cleanup

    protected void cleanup(Mapper.Context context)
                    throws IOException,
                           InterruptedException

  Called once at the end of the task.

  Throws:
    `IOException`
    `InterruptedException`

### run

    public void run(Mapper.Context context)
             throws IOException,
                    InterruptedException

  Expert users can override this method for more complete control over the execution of the Mapper.

  Parameters:
    `context` -
  Throws:
    `IOException`
    `InterruptedException`

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Copyright © 2009 The Apache Software Foundation