Hadoop读书笔记系列文章：http://blog.csdn.net/caicongyang/article/category/2166855
1.combiner
问：什么是combiner：
答：Combiner发生在Mapper端，对数据进行归约处理，使传到reducer端的数据变小了，传输时间变端，作业时间变短，Combiner不能夸Mapper执行，（只有reduce可以接受多个Mapper的任务）。 并不是所有的算法都适合归约处理，例如求平均数
2.代码实现
WordCount.java
package combine;
import java.io.IOException;
import java.net.URI;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.mapreduce.lib.partition.HashPartitioner;
/**
*
* <p>
* Title: WordCount.java
* Package counter
* </p>
* <p>
* Description:
* 问：什么是combiner：
* 答：Combiner发生在Mapper端，对数据进行归约处理，使传到reducer端的数据变小了，传输时间变端，作业时间变短，Combiner不能夸Mapper执行，
* （只有reduce可以接受多个Mapper的任务）并不是多少的算法都适合归约处理，例如求平均数
*
* <p>
* @author Tom.Cai
* @created 2014-11-26 下午10:47:32
* @version V1.0
*
*/
public class WordCount {
private static final String INPUT_PATH = "hdfs://192.168.80.100:9000/hello";
private static final String OUT_PATH = "hdfs://192.168.80.100:9000/out";
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
FileSystem fileSystem = FileSystem.get(new URI(INPUT_PATH), conf);
Path outPath = new Path(OUT_PATH);
if (fileSystem.exists(outPath)) {
fileSystem.delete(outPath, true);
}
Job job = new Job(conf, WordCount.class.getSimpleName());
//1.1设定输入文件
FileInputFormat.setInputPaths(job, INPUT_PATH);
//1.2设定输入格式
job.setInputFormatClass(TextInputFormat.class);
//指定自定义Mapper类
job.setMapperClass(MyMapper.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(LongWritable.class);
//1.3设定分区
job.setPartitionerClass(HashPartitioner.class);
job.setNumReduceTasks(1);
//1.4排序分组
//1.5归约
job.setCombinerClass(MyReducer.class);
//2.2设定Reduce类
job.setReducerClass(MyReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(LongWritable.class);
//2.3指定输出地址
FileOutputFormat.setOutputPath(job, new Path(OUT_PATH));
job.setOutputFormatClass(TextOutputFormat.class);
job.waitForCompletion(true);
}
static class MyMapper extends Mapper<LongWritable, Text, Text, LongWritable> {
@Override
protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
String[] splited = value.toString().split("\t");
for (String word : splited) {
context.write(new Text(word), new LongWritable(1));
}
}
}
static class MyReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
@Override
protected void reduce(Text key, Iterable<LongWritable> value, Context context) throws IOException, InterruptedException {
long count = 0L;
for (LongWritable times : value) {
count += times.get();
}
context.write(key, new LongWritable(count));
}
}
}
</pre><p></p><pre>
3.加入Combiner后的计数器：
14/12/01 21:26:41 INFO mapred.JobClient: Counters: 19
14/12/01 21:26:41 INFO mapred.JobClient: File Output Format Counters
14/12/01 21:26:41 INFO mapred.JobClient: Bytes Written=20
14/12/01 21:26:41 INFO mapred.JobClient: FileSystemCounters
14/12/01 21:26:41 INFO mapred.JobClient: FILE_BYTES_READ=346
14/12/01 21:26:41 INFO mapred.JobClient: HDFS_BYTES_READ=40
14/12/01 21:26:41 INFO mapred.JobClient: FILE_BYTES_WRITTEN=128546
14/12/01 21:26:41 INFO mapred.JobClient: HDFS_BYTES_WRITTEN=20
14/12/01 21:26:41 INFO mapred.JobClient: File Input Format Counters
14/12/01 21:26:41 INFO mapred.JobClient: Bytes Read=20
14/12/01 21:26:41 INFO mapred.JobClient: Map-Reduce Framework
14/12/01 21:26:41 INFO mapred.JobClient: Map output materialized bytes=50
14/12/01 21:26:41 INFO mapred.JobClient: Map input records=2
14/12/01 21:26:41 INFO mapred.JobClient: Reduce shuffle bytes=0
14/12/01 21:26:41 INFO mapred.JobClient: Spilled Records=6
14/12/01 21:26:41 INFO mapred.JobClient: Map output bytes=52
14/12/01 21:26:41 INFO mapred.JobClient: Total committed heap usage (bytes)=532807680
14/12/01 21:26:41 INFO mapred.JobClient: SPLIT_RAW_BYTES=97
14/12/01 21:26:41 INFO mapred.JobClient: Combine input records=4
14/12/01 21:26:41 INFO mapred.JobClient: Reduce input records=3
14/12/01 21:26:41 INFO mapred.JobClient: Reduce input groups=3
14/12/01 21:26:41 INFO mapred.JobClient: Combine output records=3
14/12/01 21:26:41 INFO mapred.JobClient: Reduce output records=3
14/12/01 21:26:41 INFO mapred.JobClient: Map output records=4
4.未加入归约之前的计数器
14/12/01 21:35:27 INFO mapred.JobClient: Counters: 19
14/12/01 21:35:27 INFO mapred.JobClient: File Output Format Counters
14/12/01 21:35:27 INFO mapred.JobClient: Bytes Written=20
14/12/01 21:35:27 INFO mapred.JobClient: FileSystemCounters
14/12/01 21:35:27 INFO mapred.JobClient: FILE_BYTES_READ=362
14/12/01 21:35:27 INFO mapred.JobClient: HDFS_BYTES_READ=40
14/12/01 21:35:27 INFO mapred.JobClient: FILE_BYTES_WRITTEN=128090
14/12/01 21:35:27 INFO mapred.JobClient: HDFS_BYTES_WRITTEN=20
14/12/01 21:35:27 INFO mapred.JobClient: File Input Format Counters
14/12/01 21:35:27 INFO mapred.JobClient: Bytes Read=20
14/12/01 21:35:27 INFO mapred.JobClient: Map-Reduce Framework
14/12/01 21:35:27 INFO mapred.JobClient: Map output materialized bytes=66
14/12/01 21:35:27 INFO mapred.JobClient: Map input records=2
14/12/01 21:35:27 INFO mapred.JobClient: Reduce shuffle bytes=0
14/12/01 21:35:27 INFO mapred.JobClient: Spilled Records=8
14/12/01 21:35:27 INFO mapred.JobClient: Map output bytes=52
14/12/01 21:35:27 INFO mapred.JobClient: Total committed heap usage (bytes)=366034944
14/12/01 21:35:27 INFO mapred.JobClient: SPLIT_RAW_BYTES=97
14/12/01 21:35:27 INFO mapred.JobClient: Combine input records=0
14/12/01 21:35:27 INFO mapred.JobClient: Reduce input records=4
14/12/01 21:35:27 INFO mapred.JobClient: Reduce input groups=3
14/12/01 21:35:27 INFO mapred.JobClient: Combine output records=0
14/12/01 21:35:27 INFO mapred.JobClient: Reduce output records=3
14/12/01 21:35:27 INFO mapred.JobClient: Map output records=4
5.总结
从前后两个计数器输出可以看到：加了归约以后 Reduce input records从4变成了3，从Mapper端到Reduce端的作业变少了，传输时间变少了，从而提升了整体的作业时间。
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