Hadoop MapReduce 学习笔记(四) MapReduce实现类似SQL的max 2 一些改进

       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!

     贴上代码:

       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.Reducer.Context;
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() throws Exception {
    super(); 
  }

  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();
    }
    

  }

}