Hadoop MapReduce 学习笔记(四) MapReduce实现类似SQL的SELECT MAX(ID) 2 一些改进
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????? 请先阅读:??????????
?????????? 1.Hadoop MapReduce 学习笔记(一) 序言和准备
??????????????2.Hadoop MapReduce 学习笔记(二) 序言和准备 2
??????????????3.Hadoop MapReduce 学习笔记(三) MapReduce实现类似SQL的SELECT MAX(ID)
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??? 下一篇:Hadoop MapReduce 学习笔记(五) MapReduce实现类似SQL的max和min
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????? ?? Hadoop MapReduce 学习笔记(三) MapReduce实现类似SQL的SELECT MAX(ID)只是大概的实现了找出最大值,这里我们要引入Hadoop Job的另一个参数,Combiner.据我现在所掌握的,map后会将结果写入本地文件,然后经过combine,merge等交给reduce.而reduce比较耗性能,需要大量通信(别人说的,待探索).所以尽可能的在map后输出尽可能少的数据交给Reduce.所以Combiner就是做这个用途.比如我这里要找出最大值,每一次map后都有一些数,我们可以对这些数找出最大值,再交给reduce,所以用到了Combiner.这里只是我的理解,还待考证!期待探索源码的这一天!
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? ? ?贴上代码:
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package com.guoyun.hadoop.mapreduce.study;import java.io.IOException;import java.util.StringTokenizer;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.output.FileOutputFormat;import org.slf4j.Logger;import org.slf4j.LoggerFactory;/** * 获得最大的数，类似SQL：SELECT MAX(NUMBER) FROM TABLE * 注意这里只有一列 * 相比 @GetMaxValueMapReduceTest,这里进行了一点小改进，引入了Combiner * 如果想对多列中的某一列或者某几列取最大和最小值请查看 @GetMaxAndMinValueMultiMapReduceTest */public class GetMaxValueMapReduceImproveTest extends MyMapReduceSIngleColumnTest{ public static final Logger log=LoggerFactory.getLogger(GetMaxValueMapReduceImproveTest.class); public GetMaxValueMapReduceImproveTest(String outputPath) { super(outputPath); // TODO Auto-generated constructor stub } /** * Map,to get the source datas */ public static class MyMapper extends Mapper<LongWritable,Text,Text,LongWritable>{ private final Text writeKey=new Text("K"); private LongWritable writeValue=new LongWritable(0); @Override protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { log.debug("begin to map"); StringTokenizer tokenizer=null; String lineValue=null; tokenizer=new StringTokenizer(value.toString().trim()); while(tokenizer.hasMoreTokens()){ lineValue=tokenizer.nextToken().trim(); if(lineValue.equals("")){ continue; } try { writeValue.set(Long.parseLong(lineValue)); context.write(writeKey, writeValue); } catch (NumberFormatException e) { continue; } } } } public static class MyCombiner extends Reducer<Text,LongWritable,Text,LongWritable>{ private final Text maxValueKey=new Text("maxValue"); @Override public void reduce(Text key, Iterable<LongWritable> values,Context context) throws IOException, InterruptedException { log.debug("begin to combine"); long maxValue=Long.MIN_VALUE; for(LongWritable value:values){ if(value.get()>maxValue){ maxValue=value.get(); } } context.write(maxValueKey, new LongWritable(maxValue)); } } /** * Reduce,to get the max value */ public static class MyReducer extends Reducer<Text,LongWritable,Text,LongWritable>{ private final Text maxValueKey=new Text("maxValue"); @Override public void reduce(Text key, Iterable<LongWritable> values,Context context) throws IOException, InterruptedException { log.debug("begin to reduce"); long maxValue=Long.MIN_VALUE; for(LongWritable value:values){ if(value.get()>maxValue){ maxValue=value.get(); } } context.write(maxValueKey, new LongWritable(maxValue)); } } /** * @param args */ public static void main(String[] args) { MyMapReduceTest mapReduceTest=null; Configuration conf=null; Job job=null; FileSystem fs=null; Path inputPath=null; Path outputPath=null; long begin=0; String output="testDatas/mapreduce/MROutput_SingleColumn_getMaxImprove"; try { mapReduceTest=new GetMaxValueMapReduceImproveTest(output); inputPath=new Path(mapReduceTest.getInputPath()); outputPath=new Path(mapReduceTest.getOutputPath()); conf=new Configuration(); job=new Job(conf,"getMaxValueImprove"); fs=FileSystem.getLocal(conf); if(fs.exists(outputPath)){ if(!fs.delete(outputPath,true)){ System.err.println("Delete output file:"+mapReduceTest.getOutputPath()+" failed!"); return; } } job.setJarByClass(GetMaxValueMapReduceImproveTest.class); job.setMapOutputKeyClass(Text.class); job.setMapOutputValueClass(LongWritable.class); job.setOutputKeyClass(Text.class); job.setOutputValueClass(LongWritable.class); job.setMapperClass(MyMapper.class); job.setCombinerClass(MyCombiner.class); job.setReducerClass(MyReducer.class); job.setNumReduceTasks(2); FileInputFormat.addInputPath(job, inputPath); FileOutputFormat.setOutputPath(job, outputPath); begin=System.currentTimeMillis(); job.waitForCompletion(true); System.out.println("==================================================="); if(mapReduceTest.isGenerateDatas()){ System.out.println("The maxValue is:"+mapReduceTest.getMaxValue()); System.out.println("The minValue is:"+mapReduceTest.getMinValue()); } System.out.println("Spend time:"+(System.currentTimeMillis()-begin)); // Spend time:11330 } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } }}