Hadoop_MapReduce工作原理
六个阶段:
- Input 文件输入
- Splitting 分片
- Mapping
- Shuffling
- Reducing
- Final result
mapper的输入数据为KV对形式,每一个KV对都会调用map()方法,输出数据也是KV对形式。
mapper从context中获得输入数据,将处理后的结果写入context中(context.write(text, iw);),输入(LongWritable, Text)和输出(Text, IntWritable)的数据格式由用户设置。
context通过RecordReader获取输入数据,通过RecordWriter保存mapper处理后的数据
InputFormat负责处理MR的输入
InputFormat是一个抽象类,有以下几个子类:
- ComposableInputFormat
- CompositeInputFormat
- DBInputFormat
- DelegatingInputFormat
- FileInputFormat
InputFormat有三个方法:
- InputFormat() :构造器
- createRecordReader() :提供RecordReader的实现类,把切片读到Mapper中进行处理。
- getSplits() :把输入文件进行切分
InputFormat的子类FileInputFormat还是一个抽象类,有以下几个子类:
- CombineFileInputFormat
- FixedLengthInputFormat
- KeyValueTextInputFormat
- NLineInputFormat
- SequenceFileInputFormat
- TextInputFormat
其中的 TextInputFormat 是MapReduce默认的InputFormat,它是按行读取每条记录。
Key(LongWritable):用来存储该行在整个文件中的起始字节偏移量
Value(Text):为该行的内容。
TextInputFormat对文件切分的逻辑是使用父类(FileInputFormat)的 getSplits() 方法。
切片方式为:对每个文件进行切分,默认的切片大小为128M.
NLineInputFormat
切片方式:以文件N行作为一个切片,默认一行一个切片。
KEY类型:LongWritable
VALUE类型:Text
示例:输入12行数据,以3行为一个切片,分成4个切片:
修改 Hadoop_WordCount单词统计 工程
- 修改 MyWordCount.java
package com.blu.mywordcount;
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.io.compress.BZip2Codec;
import org.apache.hadoop.io.compress.CompressionCodec;
import org.apache.hadoop.io.compress.GzipCodec;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.NLineInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class MyWordCount {
public static void main(String[] args) {
try {
Configuration conf = new Configuration();
conf.set("mapreduce.map.output.compress", "true");
conf.setClass("mapreduce.map.output.compress.codec", BZip2Codec.class, CompressionCodec.class);
Job job = Job.getInstance(conf);
job.setJarByClass(MyWordCount.class);
job.setMapperClass(MyWordCountMapper.class);
job.setReducerClass(MyWordCountReducer.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(IntWritable.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
//指定划分切片的行数
NLineInputFormat.setNumLinesPerSplit(job, 3);
//指定InputFormat的类型
job.setInputFormatClass(NLineInputFormat.class);
boolean flag = job.waitForCompletion(true);
System.exit(flag ?0 : 1);
} catch (Exception e) {
e.printStackTrace();
}
}
}
- 在D:\data下的testdata.txt文件中写入12行的数据:
good morning
good afternoon
good evening
zhangsan male
lisi female
wangwu male
good morning
good afternoon
good evening
zhangsan male
lisi female
wangwu male
- 设置以下参数运行MyWordCount的main方法
D:\data\testdata.txt D:\data\output
- 运行结果
afternoon 2
evening 2
female 2
good 6
lisi 2
male 4
morning 2
wangwu 2
zhangsan 2
- 控制台打印切片数量为4:
[INFO ] 2020-04-26 17:12:41,643 method:org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:204)
number of splits:4
- 修改的关键代码:
//指定划分切片的行数
NLineInputFormat.setNumLinesPerSplit(job, 3);
//指定InputFormat的类型
job.setInputFormatClass(NLineInputFormat.class);
KeyValueTextInputFormat
KEY类型:Text :以分隔符前的数据作为key
VALUE类型:Text :以分隔符后的数据作为value
示例,使用 KeyValueTextInputFormat 统计以下txt中人名出现的次数
D:\data\money.txt ( 注意该文件中每一行的人名与后面的数据的分割符为Tab )
zhangsan 500 450 jan
lisi 200 150 jan
lilei 150 160 jan
zhangsan 500 500 feb
lisi 200 150 feb
lilei 150 160 feb
- 创建 Kvmapper 类
package com.blu.kvdemo;
import java.io.IOException;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
/**
* 输出格式:
* zhangsan 1
* lisi 1
* zhangsan 1
*
* @author BLU
*
*/
public class Kvmapper extends Mapper<Text, Text, Text, IntWritable>{
/**
* 输入格式:
* zhangsan 500 450 jan
* key:zhangsan
* value:500 450 jan
*/
private IntWritable iw = new IntWritable(1);
@Override
protected void map(Text key, Text value, Mapper<Text, Text, Text, IntWritable>.Context context)
throws IOException, InterruptedException {
context.write(key, iw);
}
}
- KvReducer类
package com.blu.kvdemo;
import java.io.IOException;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
public class KvReducer extends Reducer<Text, IntWritable, Text, IntWritable>{
IntWritable iw = new IntWritable();
@Override
protected void reduce(Text key, Iterable<IntWritable> value,
Reducer<Text, IntWritable, Text, IntWritable>.Context context) throws IOException, InterruptedException {
int sum = 0;
for(IntWritable iw : value) {
sum += iw.get();
}
iw.set(sum);
context.write(key, iw);
}
}
- KeyValueDemo
package com.blu.kvdemo;
import java.io.IOException;
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.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.KeyValueLineRecordReader;
import org.apache.hadoop.mapreduce.lib.input.KeyValueTextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class KeyValueDemo {
public static void main(String[] args) throws Exception {
Job job = Job.getInstance();
job.setInputFormatClass(KeyValueTextInputFormat.class);
Configuration conf = new Configuration();
//设置以tab为分隔符
conf.set(KeyValueLineRecordReader.KEY_VALUE_SEPERATOR, "\t");
job.setJarByClass(KeyValueDemo.class);
job.setMapperClass(com.blu.kvdemo.Kvmapper.class);
job.setReducerClass(com.blu.kvdemo.KvReducer.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(IntWritable.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.setInputPaths(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
boolean flag = job.waitForCompletion(true);
System.exit(flag?0:1);
}
}
- 设置以下参数运行KeyValueDemo的main方法
D:\data\money.txt D:\data\output
- 运行结果
lilei 2
lisi 2
zhangsan 2