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<h2 title="Class ChainMapper" class="title">Class ChainMapper</h2>
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<li>org.apache.hadoop.mapred.lib.ChainMapper</li>
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<dt>All Implemented Interfaces:</dt>
<dd><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html?is-external=true" title="class or interface in java.lang">AutoCloseable</a>, <a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</a>, <a href="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a></dd>
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<pre>@InterfaceAudience.Public
@InterfaceStability.Stable
public class <span class="typeNameLabel">ChainMapper</span>
extends <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>
implements <a href="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a></pre>
<div class="block">The ChainMapper class allows to use multiple Mapper classes within a single
Map task.
<p>
The Mapper classes are invoked in a chained (or piped) fashion, the output of
the first becomes the input of the second, and so on until the last Mapper,
the output of the last Mapper will be written to the task's output.
<p>
The key functionality of this feature is that the Mappers in the chain do not
need to be aware that they are executed in a chain. This enables having
reusable specialized Mappers that can be combined to perform composite
operations within a single task.
<p>
Special care has to be taken when creating chains that the key/values output
by a Mapper are valid for the following Mapper in the chain. It is assumed
all Mappers and the Reduce in the chain use maching output and input key and
value classes as no conversion is done by the chaining code.
<p>
Using the ChainMapper and the ChainReducer classes is possible to compose
Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And
immediate benefit of this pattern is a dramatic reduction in disk IO.
<p>
IMPORTANT: There is no need to specify the output key/value classes for the
ChainMapper, this is done by the addMapper for the last mapper in the chain.
<p>
ChainMapper usage pattern:
<p>
<pre>
...
conf.setJobName("chain");
conf.setInputFormat(TextInputFormat.class);
conf.setOutputFormat(TextOutputFormat.class);
JobConf mapAConf = new JobConf(false);
...
ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class,
Text.class, Text.class, true, mapAConf);
JobConf mapBConf = new JobConf(false);
...
ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class,
LongWritable.class, Text.class, false, mapBConf);
JobConf reduceConf = new JobConf(false);
...
ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class,
Text.class, Text.class, true, reduceConf);
ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class,
LongWritable.class, Text.class, false, null);
ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class,
LongWritable.class, LongWritable.class, true, null);
FileInputFormat.setInputPaths(conf, inDir);
FileOutputFormat.setOutputPath(conf, outDir);
...
JobClient jc = new JobClient(conf);
RunningJob job = jc.submitJob(conf);
...
</pre></div>
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<td class="colOne"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainMapper.html#ChainMapper--">ChainMapper</a></span>()</code>
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<td class="colFirst"><code>static &lt;K1,V1,K2,V2&gt;<br>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainMapper.html#addMapper-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-">addMapper</a></span>(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;job,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends <a href="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends K1&gt;&nbsp;inputKeyClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends V1&gt;&nbsp;inputValueClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends K2&gt;&nbsp;outputKeyClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends V2&gt;&nbsp;outputValueClass,
boolean&nbsp;byValue,
<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;mapperConf)</code>
<div class="block">Adds a Mapper class to the chain job's JobConf.</div>
</td>
</tr>
<tr id="i1" class="rowColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainMapper.html#close--">close</a></span>()</code>
<div class="block">Closes the ChainMapper and all the Mappers in the chain.</div>
</td>
</tr>
<tr id="i2" class="altColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainMapper.html#configure-org.apache.hadoop.mapred.JobConf-">configure</a></span>(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;job)</code>
<div class="block">Configures the ChainMapper and all the Mappers in the chain.</div>
</td>
</tr>
<tr id="i3" class="rowColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainMapper.html#map-java.lang.Object-java.lang.Object-org.apache.hadoop.mapred.OutputCollector-org.apache.hadoop.mapred.Reporter-">map</a></span>(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>&nbsp;key,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>&nbsp;value,
<a href="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</a>&nbsp;output,
<a href="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</a>&nbsp;reporter)</code>
<div class="block">Chains the <code>map(...)</code> methods of the Mappers in the chain.</div>
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<code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#clone--" title="class or interface in java.lang">clone</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#equals-java.lang.Object-" title="class or interface in java.lang">equals</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#finalize--" title="class or interface in java.lang">finalize</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#getClass--" title="class or interface in java.lang">getClass</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#hashCode--" title="class or interface in java.lang">hashCode</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#notify--" title="class or interface in java.lang">notify</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#notifyAll--" title="class or interface in java.lang">notifyAll</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#toString--" title="class or interface in java.lang">toString</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#wait--" title="class or interface in java.lang">wait</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#wait-long-" title="class or interface in java.lang">wait</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true#wait-long-int-" title="class or interface in java.lang">wait</a></code></li>
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<h4>ChainMapper</h4>
<pre>public&nbsp;ChainMapper()</pre>
<div class="block">Constructor.</div>
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<a name="addMapper-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-">
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<h4>addMapper</h4>
<pre>public static&nbsp;&lt;K1,V1,K2,V2&gt;&nbsp;void&nbsp;addMapper(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;job,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends <a href="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends K1&gt;&nbsp;inputKeyClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends V1&gt;&nbsp;inputValueClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends K2&gt;&nbsp;outputKeyClass,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a>&lt;? extends V2&gt;&nbsp;outputValueClass,
boolean&nbsp;byValue,
<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;mapperConf)</pre>
<div class="block">Adds a Mapper class to the chain job's JobConf.
<p>
It has to be specified how key and values are passed from one element of
the chain to the next, by value or by reference. If a Mapper leverages the
assumed semantics that the key and values are not modified by the collector
'by value' must be used. If the Mapper does not expect this semantics, as
an optimization to avoid serialization and deserialization 'by reference'
can be used.
<p>
For the added Mapper the configuration given for it,
<code>mapperConf</code>, have precedence over the job's JobConf. This
precedence is in effect when the task is running.
<p>
IMPORTANT: There is no need to specify the output key/value classes for the
ChainMapper, this is done by the addMapper for the last mapper in the chain
<p></div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>job</code> - job's JobConf to add the Mapper class.</dd>
<dd><code>klass</code> - the Mapper class to add.</dd>
<dd><code>inputKeyClass</code> - mapper input key class.</dd>
<dd><code>inputValueClass</code> - mapper input value class.</dd>
<dd><code>outputKeyClass</code> - mapper output key class.</dd>
<dd><code>outputValueClass</code> - mapper output value class.</dd>
<dd><code>byValue</code> - indicates if key/values should be passed by value
to the next Mapper in the chain, if any.</dd>
<dd><code>mapperConf</code> - a JobConf with the configuration for the Mapper
class. It is recommended to use a JobConf without default values using the
<code>JobConf(boolean loadDefaults)</code> constructor with FALSE.</dd>
</dl>
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<h4>configure</h4>
<pre>public&nbsp;void&nbsp;configure(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a>&nbsp;job)</pre>
<div class="block">Configures the ChainMapper and all the Mappers in the chain.
<p>
If this method is overriden <code>super.configure(...)</code> should be
invoked at the beginning of the overwriter method.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure-org.apache.hadoop.mapred.JobConf-">configure</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>job</code> - the configuration</dd>
</dl>
</li>
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<h4>map</h4>
<pre>public&nbsp;void&nbsp;map(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>&nbsp;key,
<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>&nbsp;value,
<a href="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</a>&nbsp;output,
<a href="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</a>&nbsp;reporter)
throws <a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></pre>
<div class="block">Chains the <code>map(...)</code> methods of the Mappers in the chain.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../org/apache/hadoop/mapred/Mapper.html#map-K1-V1-org.apache.hadoop.mapred.OutputCollector-org.apache.hadoop.mapred.Reporter-">map</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - the input key.</dd>
<dd><code>value</code> - the input value.</dd>
<dd><code>output</code> - collects mapped keys and values.</dd>
<dd><code>reporter</code> - facility to report progress.</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></code></dd>
</dl>
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<h4>close</h4>
<pre>public&nbsp;void&nbsp;close()
throws <a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></pre>
<div class="block">Closes the ChainMapper and all the Mappers in the chain.
<p>
If this method is overriden <code>super.close()</code> should be
invoked at the end of the overwriter method.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true#close--" title="class or interface in java.io">close</a></code>&nbsp;in interface&nbsp;<code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</a></code></dd>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
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