AggregateWordHistogram源代码注释
package org.apache.hadoop.examples;import java.io.IOException;import java.util.ArrayList;import java.util.Map.Entry;import org.apache.hadoop.io.Text;import org.apache.hadoop.mapred.JobClient;import org.apache.hadoop.mapred.JobConf;import org.apache.hadoop.mapred.lib.aggregate.ValueAggregatorBaseDescriptor;import org.apache.hadoop.mapred.lib.aggregate.ValueAggregatorJob;/** hadoop的map/reduce例子，求的是单词在文本中的直方图，本人不解其意。执行的命令也是需要-libjars参数：hadoop jar hadoop-example.jar -libjars hadoop-example.jar shakepoems.text out_aggregate_his 3 textinputformat利用系统已经实现的算法来写map/reduce程序简化开发，算法类型如下：UniqValueCountLongValueSumDoubleValueSumValueHistogramLongValueMaxLongValueMinStringValueMaxStringValueMin运行的结果是：WORD_HISTOGRAM 11247 1 1 1710 4.4786165199608785 32.29810604730843这个和直方图有啥关系？希望高人指点迷津。 * This is an example Aggregated Hadoop Map/Reduce application. Computes the * histogram of the words in the input texts. * * To run: bin/hadoop jar hadoop-*-examples.jar aggregatewordhist <i>in-dir</i> * <i>out-dir</i> <i>numOfReducers</i> textinputformat * */public class AggregateWordHistogram { public static class AggregateWordHistogramPlugin extends ValueAggregatorBaseDescriptor { /**指定算法类型是VALUE_HISTOGRAM * Parse the given value, generate an aggregation-id/value pair per word. * The ID is of type VALUE_HISTOGRAM, with WORD_HISTOGRAM as the real id. * The value is WORD\t1. * * @return a list of the generated pairs. */ @Override public ArrayList<Entry<Text, Text>> generateKeyValPairs(Object key, Object val) { String words[] = val.toString().split(" |\t"); ArrayList<Entry<Text, Text>> retv = new ArrayList<Entry<Text, Text>>(); for (int i = 0; i < words.length; i++) { Text valCount = new Text(words[i] + "\t" + "1"); Entry<Text, Text> en = generateEntry(VALUE_HISTOGRAM, "WORD_HISTOGRAM", valCount); retv.add(en); } return retv; } } /** * The main driver for word count map/reduce program. Invoke this method to * submit the map/reduce job. * * @throws IOException * When there is communication problems with the job tracker. */ @SuppressWarnings("unchecked") public static void main(String[] args) throws IOException { JobConf conf = ValueAggregatorJob.createValueAggregatorJob(args , new Class[] {AggregateWordHistogramPlugin.class}); JobClient.runJob(conf); } }?