Hadoop学习：MapReduce实现WordCount经典案例

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一、✌题目要求

> 统计文本中每个单词的数量

二、✌实现思想

> Map阶段默认输入为TextInputFormat，键值对对应为行的偏移量和每行的文本内容
> 在map函数中将每行文本进行切分，提取出每个单词
> 在Reduce阶段根据相同Key值进行累加求和
> 

三、✌代码实现

1.✌Map类

public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    
    

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
    
    

        //获取每一行的文本
        String line = value.toString();

        //将每行文本进行切分
        String[] words = line.split(" ");

        //循环写入，写出格式为：hello 1
        for (String word : words) {
    
    
            context.write(new Text(word), new IntWritable(1));
        }
    }

}

2.✌Reduce类

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

    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
    
    

        //创建计数变量
        int sum = 0;

        //统计相同key对应的value值进行累加
        for (IntWritable value : values) {
    
    
            sum += value.get();
        }

        //写出，写出格式为：hello 3
        context.write(key, new IntWritable(sum));
    }
}

3.✌Driver类

public class WordCountDriver {
    
    
    
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
    
    

        //设置文件输入输出路径
        args=new String[]{
    
    "D:/input/inputword","D:/output"};
        
        //打印日志信息
        BasicConfigurator.configure();
        
        //创建配置信息对象
        Configuration conf=new Configuration();

        //获取job对象
        Job job= Job.getInstance(conf);

        //关联Driver、Map、Reduce类
        job.setJarByClass(WordCountDriver.class);
        job.setMapperClass(WordCountMapper.class);
        job.setReducerClass(WordCountReducer.class);

        //设置Map阶段的输出格式
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);

        //设置最终的输出格式
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

        //配置文件路径
        FileInputFormat.setInputPaths(job,new Path(args[0]));
        FileOutputFormat.setOutputPath(job,new Path(args[1]));

        //提交任务
        boolean result=job.waitForCompletion(true);

        System.exit(result?0:1);
        
    }
    
}

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