Mapreduce分布式计算案例WordCount
时间: 2023-11-27 17:52:29 浏览: 37
WordCount是MapReduce的经典案例之一,它可以统计文本中每个单词出现的次数。下面是WordCount的MapReduce程序实现步骤:
1. Map阶段:将文本文件划分为若干个小块,每个小块由Map任务处理。Map任务读取文本文件中的每个单词,将单词作为Key,将出现次数作为Value,然后将Key-Value对发送给Reduce任务。
2. Reduce阶段:Reduce任务接收Map任务发送的Key-Value对,并将相同的单词的Value进行累加,得到最终的结果。Reduce任务将每个单词以及对应的出现次数输出到磁盘上。
下面是WordCount的Java代码实现:
Map阶段:
```java
public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
private Text word = new Text();
private final static IntWritable one = new IntWritable(1);
public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
String line = value.toString();
StringTokenizer tokenizer = new StringTokenizer(line);
while (tokenizer.hasMoreTokens()) {
word.set(tokenizer.nextToken());
context.write(word, one);
}
}
}
```
Reduce阶段:
```java
public class WordCountReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
private IntWritable result = new IntWritable();
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);
}
}
```
在MapReduce程序的主函数中,需要指定输入和输出路径,以及Map和Reduce任务的类:
```java
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount.class);
job.setMapperClass(WordCountMapper.class);
job.setCombinerClass(WordCountReducer.class);
job.setReducerClass(WordCountReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
```
在执行MapReduce程序之前,需要将输入文件上传到HDFS上。然后,通过Hadoop命令行工具运行程序:
```
hadoop jar <jar文件路径> <main函数所在类名> <输入路径> <输出路径>
```
其中,`<jar文件路径>`为编译后的jar包路径,`<main函数所在类名>`为包含主函数的类名,`<输入路径>`为输入文件所在的HDFS路径,`<输出路径>`为输出结果保存的HDFS路径。
执行完毕后,可以在输出路径中查看统计结果。