Hadoop 1.2.1 MapReduce 例子

自学hadoop真的很困难，主要是hadoop版本太混乱了，各个版本之间兼容性并不算太好。更主要的是网上的很多MapReduce的Java例子不写import！！！只写类名！！！偏偏Hadoop中有很多重名的类，不写Import根本不知道是哪个类！！！而且也不写上hadoop的版本号！！！让人根本看不明白！！！

所以这里我写下所有要注意的情况，特别要注意import的是哪一个类！！！

环境： hadoop1.2.1+jdk1.7+eclipse4.5+maven

maven的pom文件是：（如果不知道maven，那得稍微看看maven是什么）

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.howso</groupId>
  <artifactId>hadoopmaven</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>hadoopmaven</name>
  <properties>
      <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
      <hadoop.version>1.2.1</hadoop.version>
  </properties>
  <dependencies>
      <dependency>
          <groupId>org.apache.hadoop</groupId>
          <artifactId>hadoop-client</artifactId>
          <version>${hadoop.version}</version>
      </dependency>
      <dependency>
          <groupId>org.apache.hadoop</groupId>
          <artifactId>hadoop-core</artifactId>
          <version>${hadoop.version}</version>
      </dependency>
      <dependency>
          <groupId>org.hamcrest</groupId>
          <artifactId>hamcrest-all</artifactId>
          <version>1.1</version>
          <scope>test</scope>
      </dependency>
      <dependency>
          <groupId>junit</groupId>
          <artifactId>junit</artifactId>
          <version>4.11</version>
          <scope>test</scope>
      </dependency>
      <dependency>
          <groupId>org.apache.mrunit</groupId>
          <artifactId>mrunit</artifactId>
          <version>1.1.0</version>
          <classifier>hadoop2</classifier>
          <scope>test</scope>
      </dependency>
      
      <dependency>
          <groupId>org.apache.hadoop</groupId>
          <artifactId>hadoop-minicluster</artifactId>
          <version>${hadoop.version}</version>
          <scope>test</scope>
      </dependency>
      
      <dependency>
          <groupId>org.apache.hadoop</groupId>
          <artifactId>hadoop-test</artifactId>
          <version>${hadoop.version}</version>
      </dependency>
      <dependency>
          <groupId>com.sun.jersey</groupId>
          <artifactId>jersey-core</artifactId>
          <version>1.8</version>
          <scope>test</scope>
      </dependency>
      
  </dependencies>
  <build>
      <finalName>hadoopx</finalName>
      <plugins>
          <plugin>
              <groupId>org.apache.maven.plugins</groupId>
              <artifactId>maven-compilter-plugin</artifactId>
              <version>3.1</version>
              <configuration>
                  <source>1.6</source>
                  <target>1.6</target>
              </configuration>
          </plugin>
          <plugin>
              <groupId>org.apache.maven.plugins</groupId>
              <artifactId>maven-jar-plugin</artifactId>
              <version>2.5</version>
              <configuration>
                  <outputDirectory>basedir</outputDirectory>
                  <archive>
                      <manifest>
                          <mainClass>hadoopmaven.Driver</mainClass>
                      </manifest>
                  </archive>
              </configuration>
          </plugin>
      </plugins>
  </build>
</project>

这里面有一些组件是用来写hadoop的test的：mrunit，hadoop-test。

总共有3个类：Driver， MaxMapper, MaxReducer。 这三个类合力来获得每年最大的温度。这三个类都在hadoopmaven包下面。

一定要注意import的是哪个类，hadoop中相同的名字的类不少，特别是Mapper，Reducer这两个，竟然都有相同名称的，一定要注意。

Driver类：

package hadoopmaven;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
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.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

public class Driver extends Configured implements Tool{
    
    //这个能够运行成功
    public static void main(String[] args) throws Exception {
        int k =ToolRunner.run(new Driver(), args);
        System.out.println("ks is : "+k);
        System.exit(k);
    }
    
    
    public int run(String[] arg0) throws Exception {
        
        Job job = new Job(getConf(), "word count");
        job.setJarByClass(getClass());
        job.setJarByClass(Driver.class);
        job.setMapperClass(MaxMapper.class);
        job.setCombinerClass(MaxReducer.class);
        job.setReducerClass(MaxReducer.class);
        
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
        
        FileInputFormat.addInputPath(job, new Path("/input/temp.txt"));
        
        FileOutputFormat.setOutputPath(job, new Path("/output4"));
        
        return job.waitForCompletion(true)?0:1;
    }

}

MaxMapper类：

package hadoopmaven;

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public  class MaxMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    //输入的格式是
    // 1991,90
    // 1991,91
    // 1993,98
    @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Text, IntWritable>.Context context)
            throws IOException, InterruptedException {
        String[] line=value.toString().split(",");
        context.write(new Text(line[0]), new IntWritable(Integer.parseInt(line[1])));
    }
    
}

MaxReducer类：

package hadoopmaven;

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public  class MaxReducer extends Reducer<Text, IntWritable, Text, IntWritable>{

    @Override
    protected void reduce(Text arg0, Iterable<IntWritable> arg1,
            Reducer<Text, IntWritable, Text, IntWritable>.Context context) throws IOException, InterruptedException {
        int max=Integer.MIN_VALUE;
        for(IntWritable v:arg1){
            max=Math.max(max, v.get());
        }
        context.write(arg0, new IntWritable(max));
    }
    
    
}

这个MapReduce任务的作用是从hdfs的 /input/temp.txt文件中读取信息（/input/temp.txt的文件格式如下），获得每个年份对应的最大的数值，放到/output4文件夹中去。

1991,33
1991,45
1992,94
1992,85
1992,5
1993,78
1993,75

最后用maven的clean package打个包，maven会自动在打好的jar包中写上main class（因为在pom文件中配置了main class的名称了），打好的jar包在项目根目录下的basedir目录中，名字叫做hadoopx.jar(这些都是在pom中配置的。)

把temp.txt文件放入hdfs中去，把hadoopx.jar放入hadoop根目录，进入hadoop根目录，使用命令 bin/hadoop jar hadoopx.jar 运行