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7Mapper (Hadoop 0.20.1 API)
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89<H2>
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91org.apache.hadoop.mapreduce</FONT>
92<BR>
93Class Mapper&lt;KEYIN,VALUEIN,KEYOUT,VALUEOUT&gt;</H2>
94<PRE>
95<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</A>
96  <IMG SRC="../../../../resources/inherit.gif" ALT="extended by "><B>org.apache.hadoop.mapreduce.Mapper&lt;KEYIN,VALUEIN,KEYOUT,VALUEOUT&gt;</B>
97</PRE>
98<DL>
99<DT><B>Direct Known Subclasses:</B> <DD><A HREF="../../../../org/apache/hadoop/mapreduce/lib/map/InverseMapper.html" title="class in org.apache.hadoop.mapreduce.lib.map">InverseMapper</A>, <A HREF="../../../../org/apache/hadoop/mapreduce/lib/map/MultithreadedMapper.html" title="class in org.apache.hadoop.mapreduce.lib.map">MultithreadedMapper</A>, <A HREF="../../../../org/apache/hadoop/examples/SecondarySort.MapClass.html" title="class in org.apache.hadoop.examples">SecondarySort.MapClass</A>, <A HREF="../../../../org/apache/hadoop/mapreduce/lib/map/TokenCounterMapper.html" title="class in org.apache.hadoop.mapreduce.lib.map">TokenCounterMapper</A>, <A HREF="../../../../org/apache/hadoop/examples/WordCount.TokenizerMapper.html" title="class in org.apache.hadoop.examples">WordCount.TokenizerMapper</A></DD>
100</DL>
101<HR>
102<DL>
103<DT><PRE>public class <B>Mapper&lt;KEYIN,VALUEIN,KEYOUT,VALUEOUT&gt;</B><DT>extends <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A></DL>
104</PRE>
105
106<P>
107Maps input key/value pairs to a set of intermediate key/value pairs. 
108 
109 <p>Maps are the individual tasks which transform input records into a
110 intermediate records. The transformed intermediate records need not be of
111 the same type as the input records. A given input pair may map to zero or
112 many output pairs.</p> 
113 
114 <p>The Hadoop Map-Reduce framework spawns one map task for each
115 <A HREF="../../../../org/apache/hadoop/mapreduce/InputSplit.html" title="class in org.apache.hadoop.mapreduce"><CODE>InputSplit</CODE></A> generated by the <A HREF="../../../../org/apache/hadoop/mapreduce/InputFormat.html" title="class in org.apache.hadoop.mapreduce"><CODE>InputFormat</CODE></A> for the job.
116 <code>Mapper</code> implementations can access the <A HREF="../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf"><CODE>Configuration</CODE></A> for
117 the job via the <A HREF="../../../../org/apache/hadoop/mapreduce/JobContext.html#getConfiguration()"><CODE>JobContext.getConfiguration()</CODE></A>.
118 
119 <p>The framework first calls
120 <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#setup(org.apache.hadoop.mapreduce.Mapper.Context)"><CODE>setup(org.apache.hadoop.mapreduce.Mapper.Context)</CODE></A>, followed by
121 <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#map(KEYIN, VALUEIN, org.apache.hadoop.mapreduce.Mapper.Context)"><CODE>map(Object, Object, Context)</CODE></A> 
122 for each key/value pair in the <code>InputSplit</code>. Finally
123 <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#cleanup(org.apache.hadoop.mapreduce.Mapper.Context)"><CODE>cleanup(Context)</CODE></A> is called.</p>
124 
125 <p>All intermediate values associated with a given output key are
126 subsequently grouped by the framework, and passed to a <A HREF="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><CODE>Reducer</CODE></A> to 
127 determine the final output. Users can control the sorting and grouping by
128 specifying two key <A HREF="../../../../org/apache/hadoop/io/RawComparator.html" title="interface in org.apache.hadoop.io"><CODE>RawComparator</CODE></A> classes.</p>
129
130 <p>The <code>Mapper</code> outputs are partitioned per
131 <code>Reducer</code>. Users can control which keys (and hence records) go to
132 which <code>Reducer</code> by implementing a custom <A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="class in org.apache.hadoop.mapreduce"><CODE>Partitioner</CODE></A>.
133 
134 <p>Users can optionally specify a <code>combiner</code>, via
135 <A HREF="../../../../org/apache/hadoop/mapreduce/Job.html#setCombinerClass(java.lang.Class)"><CODE>Job.setCombinerClass(Class)</CODE></A>, to perform local aggregation of the
136 intermediate outputs, which helps to cut down the amount of data transferred
137 from the <code>Mapper</code> to the <code>Reducer</code>.
138 
139 <p>Applications can specify if and how the intermediate
140 outputs are to be compressed and which <A HREF="../../../../org/apache/hadoop/io/compress/CompressionCodec.html" title="interface in org.apache.hadoop.io.compress"><CODE>CompressionCodec</CODE></A>s are to be
141 used via the <code>Configuration</code>.</p>
142 
143 <p>If the job has zero
144 reduces then the output of the <code>Mapper</code> is directly written
145 to the <A HREF="../../../../org/apache/hadoop/mapreduce/OutputFormat.html" title="class in org.apache.hadoop.mapreduce"><CODE>OutputFormat</CODE></A> without sorting by keys.</p>
146 
147 <p>Example:</p>
148 <p><blockquote><pre>
149 public class TokenCounterMapper
150     extends Mapper<Object, Text, Text, IntWritable>{
151   
152   private final static IntWritable one = new IntWritable(1);
153   private Text word = new Text();
154   
155   public void map(Object key, Text value, Context context) throws IOException {
156     StringTokenizer itr = new StringTokenizer(value.toString());
157     while (itr.hasMoreTokens()) {
158       word.set(itr.nextToken());
159       context.collect(word, one);
160     }
161   }
162 }
163 </pre></blockquote></p>
164
165 <p>Applications may override the <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#run(org.apache.hadoop.mapreduce.Mapper.Context)"><CODE>run(Context)</CODE></A> method to exert
166 greater control on map processing e.g. multi-threaded <code>Mapper</code>s
167 etc.</p>
168<P>
169
170<P>
171<DL>
172<DT><B>See Also:</B><DD><A HREF="../../../../org/apache/hadoop/mapreduce/InputFormat.html" title="class in org.apache.hadoop.mapreduce"><CODE>InputFormat</CODE></A>,
173<A HREF="../../../../org/apache/hadoop/mapreduce/JobContext.html" title="class in org.apache.hadoop.mapreduce"><CODE>JobContext</CODE></A>,
174<A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="class in org.apache.hadoop.mapreduce"><CODE>Partitioner</CODE></A>,
175<A HREF="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><CODE>Reducer</CODE></A></DL>
176<HR>
177
178<P>
179<!-- ======== NESTED CLASS SUMMARY ======== -->
180
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184<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
185<B>Nested Class Summary</B></FONT></TH>
186</TR>
187<TR BGCOLOR="white" CLASS="TableRowColor">
188<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
189<CODE>&nbsp;class</CODE></FONT></TD>
190<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A></B></CODE>
191
192<BR>
193&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TD>
194</TR>
195</TABLE>
196&nbsp;
197<!-- ======== CONSTRUCTOR SUMMARY ======== -->
198
199<A NAME="constructor_summary"><!-- --></A>
200<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
201<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
202<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
203<B>Constructor Summary</B></FONT></TH>
204</TR>
205<TR BGCOLOR="white" CLASS="TableRowColor">
206<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#Mapper()">Mapper</A></B>()</CODE>
207
208<BR>
209&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TD>
210</TR>
211</TABLE>
212&nbsp;
213<!-- ========== METHOD SUMMARY =========== -->
214
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217<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
218<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
219<B>Method Summary</B></FONT></TH>
220</TR>
221<TR BGCOLOR="white" CLASS="TableRowColor">
222<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
223<CODE>protected &nbsp;void</CODE></FONT></TD>
224<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#cleanup(org.apache.hadoop.mapreduce.Mapper.Context)">cleanup</A></B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)</CODE>
225
226<BR>
227&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Called once at the end of the task.</TD>
228</TR>
229<TR BGCOLOR="white" CLASS="TableRowColor">
230<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
231<CODE>protected &nbsp;void</CODE></FONT></TD>
232<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#map(KEYIN, VALUEIN, org.apache.hadoop.mapreduce.Mapper.Context)">map</A></B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="type parameter in Mapper">KEYIN</A>&nbsp;key,
233    <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="type parameter in Mapper">VALUEIN</A>&nbsp;value,
234    <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)</CODE>
235
236<BR>
237&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Called once for each key/value pair in the input split.</TD>
238</TR>
239<TR BGCOLOR="white" CLASS="TableRowColor">
240<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
241<CODE>&nbsp;void</CODE></FONT></TD>
242<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#run(org.apache.hadoop.mapreduce.Mapper.Context)">run</A></B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)</CODE>
243
244<BR>
245&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Expert users can override this method for more complete control over the
246 execution of the Mapper.</TD>
247</TR>
248<TR BGCOLOR="white" CLASS="TableRowColor">
249<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
250<CODE>protected &nbsp;void</CODE></FONT></TD>
251<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html#setup(org.apache.hadoop.mapreduce.Mapper.Context)">setup</A></B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)</CODE>
252
253<BR>
254&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Called once at the beginning of the task.</TD>
255</TR>
256</TABLE>
257&nbsp;<A NAME="methods_inherited_from_class_java.lang.Object"><!-- --></A>
258<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
259<TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor">
260<TH ALIGN="left"><B>Methods inherited from class java.lang.<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A></B></TH>
261</TR>
262<TR BGCOLOR="white" CLASS="TableRowColor">
263<TD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#clone()" title="class or interface in java.lang">clone</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#equals(java.lang.Object)" title="class or interface in java.lang">equals</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#finalize()" title="class or interface in java.lang">finalize</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#getClass()" title="class or interface in java.lang">getClass</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#hashCode()" title="class or interface in java.lang">hashCode</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#notify()" title="class or interface in java.lang">notify</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#notifyAll()" title="class or interface in java.lang">notifyAll</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#toString()" title="class or interface in java.lang">toString</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait()" title="class or interface in java.lang">wait</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait(long)" title="class or interface in java.lang">wait</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait(long, int)" title="class or interface in java.lang">wait</A></CODE></TD>
264</TR>
265</TABLE>
266&nbsp;
267<P>
268
269<!-- ========= CONSTRUCTOR DETAIL ======== -->
270
271<A NAME="constructor_detail"><!-- --></A>
272<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
273<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
274<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
275<B>Constructor Detail</B></FONT></TH>
276</TR>
277</TABLE>
278
279<A NAME="Mapper()"><!-- --></A><H3>
280Mapper</H3>
281<PRE>
282public <B>Mapper</B>()</PRE>
283<DL>
284</DL>
285
286<!-- ============ METHOD DETAIL ========== -->
287
288<A NAME="method_detail"><!-- --></A>
289<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
290<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
291<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
292<B>Method Detail</B></FONT></TH>
293</TR>
294</TABLE>
295
296<A NAME="setup(org.apache.hadoop.mapreduce.Mapper.Context)"><!-- --></A><H3>
297setup</H3>
298<PRE>
299protected void <B>setup</B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)
300              throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A>,
301                     <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></PRE>
302<DL>
303<DD>Called once at the beginning of the task.
304<P>
305<DD><DL>
306
307<DT><B>Throws:</B>
308<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE>
309<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></CODE></DL>
310</DD>
311</DL>
312<HR>
313
314<A NAME="map(java.lang.Object,java.lang.Object,org.apache.hadoop.mapreduce.Mapper.Context)"><!-- --></A><A NAME="map(KEYIN, VALUEIN, org.apache.hadoop.mapreduce.Mapper.Context)"><!-- --></A><H3>
315map</H3>
316<PRE>
317protected void <B>map</B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="type parameter in Mapper">KEYIN</A>&nbsp;key,
318                   <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="type parameter in Mapper">VALUEIN</A>&nbsp;value,
319                   <A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)
320            throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A>,
321                   <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></PRE>
322<DL>
323<DD>Called once for each key/value pair in the input split. Most applications
324 should override this, but the default is the identity function.
325<P>
326<DD><DL>
327
328<DT><B>Throws:</B>
329<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE>
330<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></CODE></DL>
331</DD>
332</DL>
333<HR>
334
335<A NAME="cleanup(org.apache.hadoop.mapreduce.Mapper.Context)"><!-- --></A><H3>
336cleanup</H3>
337<PRE>
338protected void <B>cleanup</B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)
339                throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A>,
340                       <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></PRE>
341<DL>
342<DD>Called once at the end of the task.
343<P>
344<DD><DL>
345
346<DT><B>Throws:</B>
347<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE>
348<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></CODE></DL>
349</DD>
350</DL>
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352
353<A NAME="run(org.apache.hadoop.mapreduce.Mapper.Context)"><!-- --></A><H3>
354run</H3>
355<PRE>
356public void <B>run</B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Mapper.Context.html" title="class in org.apache.hadoop.mapreduce">Mapper.Context</A>&nbsp;context)
357         throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A>,
358                <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></PRE>
359<DL>
360<DD>Expert users can override this method for more complete control over the
361 execution of the Mapper.
362<P>
363<DD><DL>
364<DT><B>Parameters:</B><DD><CODE>context</CODE> -
365<DT><B>Throws:</B>
366<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE>
367<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</A></CODE></DL>
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