Hadoop MapReduce 学习笔记(三) MapReduce实现类似SQL的SELECT MAX(ID)

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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
　　下一篇: Hadoop MapReduce 学习笔记(四) MapReduce实现类似SQL的SELECT MAX(ID) 2 一些改进
　　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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