一:简介
MapReduce主要是先读取文件数据,然后进行Map处理,接着Reduce处理,最后把处理结果写到文件中。
Hadoop读取数据:通过InputFormat决定读取的数据的类型,然后拆分成一个个InputSplit,每个InputSplit(输入分片)对应一个Map处理,RecordReader读取InputSplit的内容给Map
- InputFormat:输入格式,决定读取数据的格式,可以是文件或数据库等
- InputSplit: 输入分片,代表一个个逻辑分片,并没有真正存储数据,只是提供了一个如何将数据分片的方法,通常一个split就是一个block。
- RecordReader:将InputSplit拆分成一个个<key, value>对给Map处理
- Mapper:主要是读取InputSplit的每一个Key,Value对并进行处理
- Shuffle:对Map的结果进行合并、排序等操作并传输到Reduce进行处理
- Combiner:
- Reduce:对map进行统计
- select:直接分析输入数据,取出需要的字段数据即可
- where: 也是对输入数据处理的过程中进行处理,判断是否需要该数据
- aggregation: 聚合操作 min, max, sum
- group by: 通过Reducer实现
- sort:排序
- join: map join, reduce join
- 输出格式: 输出格式会转换最终的键值对并写入文件。默认情况下键和值以tab分割,各记录以换行符分割。输出格式也可以自定义。
二:准备数据
echo "Hadoop Common\nHadoop Distributed File System\nHadoop YARN\nHadoop MapReduce " > /tmp/foobar.txt
hadoop fs -put /tmp/foobar.txt /wordcount/input
hadoop fs -cat /wordcount/input/foobar.txt
三:Word Count
统计文件中每个单词出现的次数。
- 引入依赖
<dependency>
<groupId>org.apache.hadoop</groupId>
<artifactId>hadoop-common</artifactId>
<version>3.2.1</version>
</dependency>
<dependency>
<groupId>org.apache.hadoop</groupId>
<artifactId>hadoop-mapreduce-client-common</artifactId>
<version>3.2.1</version>
</dependency>
<dependency>
<groupId>org.apache.hadoop</groupId>
<artifactId>hadoop-mapreduce-client-core</artifactId>
<version>3.2.1</version>
</dependency>
- Java
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.Iterator;
import java.util.StringTokenizer;
public class WordCount {
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
// core-site.xml中配置的fs.defaultFS
conf.set("fs.defaultFS", "hdfs://localhost:8020");
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount.class);
job.setMapperClass(WordCount.TokenizerMapper.class);
job.setReducerClass(WordCount.IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path("/wordcount/input/foobar.txt"));
FileOutputFormat.setOutputPath(job, new Path("/wordcount/output"));
// 等待job完成后退出
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
public static class TokenizerMapper extends Mapper<Object, Text, Text, IntWritable> {
private static final IntWritable one = new IntWritable(1);
private Text word = new Text();
/**
* map方法会调用多次,每行文本都会调用一次
* @param key
* @param value 每一行对应的文本
* @param context
*/
@Override
public void map(Object key, Text value, Mapper<Object, Text, Text, IntWritable>.Context context) throws IOException, InterruptedException {
StringTokenizer itr = new StringTokenizer(value.toString());
while(itr.hasMoreTokens()) {
// 每个单词
String item = itr.nextToken();
this.word.set(item);
context.write(this.word, one);
}
}
}
public static class IntSumReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
private IntWritable result = new IntWritable();
/**
* @param key 相同单词归为一组
* @param values 根据key分组的每一项
*/
@Override
public void reduce(Text key, Iterable<IntWritable> values, Reducer<Text, IntWritable, Text, IntWritable>.Context context) throws IOException, InterruptedException {
System.out.println(key.toString());
int sum = 0;
Iterator iter = values.iterator();
while (iter.hasNext()) {
int value = ((IntWritable) iter.next()).get();
sum += value;
}
this.result.set(sum);
context.write(key, this.result);
}
}
}
四:执行.jar
在执行jar文件时需要指定mainClass, 否则会报错 RunJar jarFile [mainClass] args...
方式一:在命令行参数中指定mainClass
指定mainClass类的完全限定名hadoop jar xxx.jar <mainClass类的完全限定名>
mvn clean package
hadoop jar target/hadoop-mapreduce-wordcount-1.0-SNAPSHOT.jar org.example.WordCount
方式二:使用maven插件指定mainClass
配置maven-jar-plugin插件, 在插件中指定mainClass,在插件中配置了mainClass在命令行中就不需要再指定mainClass了。
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<configuration>
<archive>
<manifest>
<addClasspath>true</addClasspath>
<classpathPrefix></classpathPrefix>
<mainClass>org.example.WordCount</mainClass>
</manifest>
</archive>
</configuration>
</plugin>
</plugins>
</build>
mvn clean package
hadoop jar target/hadoop-mapreduce-wordcount-1.0-SNAPSHOT.jar