Hadoop MapReduce 学习笔记(三) MapReduce实现类似SQL的SELECT MAX(ID)-优快云博客

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从一大堆数中找出最大的数,类似SQL的SELECT MAX(NUMBER) FROM TABLE .这里写了个简单的MapReduce,实现了该功能.我这里会生成测试数据,同时在生成的时候会计算出最大值.待MapReduce跑玩后,你可以去输出路径查看并进行对比.具体请查看代码:

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.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 @GetMaxValueMapReduceImproveTest 这里速度会更慢
 * 
 */
public class GetMaxValueMapReduceTest extends MyMapReduceSIngleColumnTest{
  public static final Logger log=LoggerFactory.getLogger(GetMaxValueMapReduceTest.class);
  

  public GetMaxValueMapReduceTest(int dataLength) throws Exception {
    super(dataLength);
    // TODO Auto-generated constructor stub
  }

  public GetMaxValueMapReduceTest(long dataLength, String inputPath,
      String outputPath) throws Exception {
    super(dataLength, inputPath, outputPath);
    // TODO Auto-generated constructor stub
  }

  public GetMaxValueMapReduceTest(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;
        }
        
      }
    }
  }
  
  /**
   * 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_getMax";
    
    
    try {
      mapReduceTest=new GetMaxValueMapReduceTest(10000000);
      
      inputPath=new Path(mapReduceTest.getInputPath());
      outputPath=new Path(mapReduceTest.getOutputPath());
      
      conf=new Configuration();
      job=new Job(conf,"getMaxValue");
      
      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(GetMaxValueMapReduceTest.class);
      job.setMapOutputKeyClass(Text.class);
      job.setMapOutputValueClass(LongWritable.class);
      job.setOutputKeyClass(Text.class);
      job.setOutputValueClass(LongWritable.class);
      job.setMapperClass(MyMapper.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:18908
      
    } catch (Exception e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }

}

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