AggregateWordCount源代码注释

package org.apache.hadoop.examples;

import java.io.IOException;
import java.util.ArrayList;
import java.util.StringTokenizer;
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;

/**
  这个是hadoop的map/reduce的例子，是对例子WordCount利用系统已经实现的map/reduce类进行简化。系统已经实现的ValueAggregatorBaseDescriptor 和ValueAggregatorJob已经实现各种数据类型的求和最大值，最小值的算法。类型如下：
UniqValueCount
LongValueSum
DoubleValueSum
ValueHistogram
LongValueMax
LongValueMin
StringValueMax
StringValueMin
具体请看相关的源代码。
这个job的执行必须用-jarlibs执行，不然会报configured错误。
执行命令如下：
hadoop jar hadoop-example.jar -libjars hadoop-example.jar shakepoems.text out_aggregate_his 3 textinputformat

 * This is an example Aggregated Hadoop Map/Reduce application. It reads the
 * text input files, breaks each line into words and counts them. The output is
 * a locally sorted list of words and the count of how often they occurred.
 * 
 * To run: bin/hadoop jar hadoop-*-examples.jar aggregatewordcount <i>in-dir</i>
 * <i>out-dir</i> <i>numOfReducers</i> textinputformat
 * 
 */
public class AggregateWordCount {

/*继承类ValueAggregatorBaseDescriptor */
  public static class WordCountPlugInClass extends
      ValueAggregatorBaseDescriptor {
    @Override
    public ArrayList<Entry<Text, Text>> generateKeyValPairs(Object key,
                                                            Object val) {
      String countType = LONG_VALUE_SUM;//指定算法类型是long类型的求和
      ArrayList<Entry<Text, Text>> retv = new ArrayList<Entry<Text, Text>>();
      String line = val.toString();
      StringTokenizer itr = new StringTokenizer(line);
      while (itr.hasMoreTokens()) {
        Entry<Text, Text> e = generateEntry(countType, itr.nextToken(), ONE);
        if (e != null) {
          retv.add(e);
        }
      }
      return retv;
    }
  }

  /**用静态类ValueAggregatorJob执行job
   * 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[] {WordCountPlugInClass.class});
   
    JobClient.runJob(conf);
  }

}