Hadoop MapReduce 入门实例

一、准备工作

1. 从hadoop官网下载了最新的3.1.2版本的hadoop
2. 配置hadoop相关的环境变量

export HADOOP_HOME=/work/dev_tools/hadoop-3.1.2
export PATH=$HADOOP_HOME/bin:$PATH:.
export HADOOP_CLASSPATH=${JAVA_HOME}/lib/tools.jar

二、MapReduce代码示例

功能：给定文本，统计所有单词的词频

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
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 java.io.IOException;
import java.util.StringTokenizer;

/**
 * @author lvsheng
 * @date 2019-09-01
 **/
public class WordCount {
    
    
	
	public static class TokenizerMapper extends Mapper<Object, Text, Text, IntWritable> {
    
    
		
		private final static IntWritable one  = new IntWritable(1);
		private              Text        word = new Text();
		
		@Override
		public void map(Object key, Text value, Context context
		) throws IOException, InterruptedException {
    
    
			StringTokenizer itr = new StringTokenizer(value.toString());
			while (itr.hasMoreTokens()) {
    
    
				word.set(itr.nextToken());
				context.write(word, one);
			}
		}
	}
	
	public static class IntSumReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
    
    
		
		private IntWritable result = new IntWritable();
		
		@Override
		public void reduce(Text key, Iterable<IntWritable> values,
						   Context context
		) throws IOException, InterruptedException {
    
    
			int sum = 0;
			for (IntWritable val : values) {
    
    
				sum += val.get();
			}
			result.set(sum);
			context.write(key, result);
		}
	}
	
	public static void main(String[] args) throws Exception {
    
    
		long          start = System.currentTimeMillis();
		Configuration conf  = new Configuration();
		Job job = Job.getInstance(conf, "word count");
		job.setJarByClass(WordCount.class);
		job.setMapperClass(TokenizerMapper.class);
		job.setCombinerClass(IntSumReducer.class);
		job.setReducerClass(IntSumReducer.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		FileInputFormat.addInputPath(job, new Path(args[0]));
		FileOutputFormat.setOutputPath(job, new Path(args[1]));
		job.waitForCompletion(true);
		System.out.println("time cost ： " + (System.currentTimeMillis() - start) / 1000 + " s");
	}
}

注意：我的类放在默认的包下载，是没有包路径的。在有包路径的情况下，执行的时候报错，后面细说。

三、运行Job作业

1. 先编译这个类

hadoop com.sun.tools.javac.Main WordCount.java

2. 将编译好的字节码文件打jar包

jar cf WordCount.jar WordCount*.class

3. 运行程序

hadoop jar WordCount.jar WordCount /Users/lvsheng/Movies/aclImdb/train/pos /temp/output2

我给的输入文件比较大，程序单机跑了一个多小时才出结果。

遇到的一个小问题

当我的作业类是有包路径的时候，运行程序的时候一致报找不到了类，不管是加了类路径还是没有加类路径。

带路径的执行命令：

✗ hadoop jar WordCount.jar com.alibaba.ruzun.WordCount /Users/lvsheng/Movies/aclImdb/train/pos /temp/output2

错误堆栈：

Exception in thread "main" java.lang.ClassNotFoundException: com.alibaba.ruzun.WordCount
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:348)
        at org.apache.hadoop.util.RunJar.run(RunJar.java:311)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:232)

把命令里的包路径去掉更不行。既然是包路径引起的，干脆把作业类移到java文件下，这样就没有包路径了，问题解决。至于怎么造成的，后面深入学习的时候再排查吧。