How to write MapReduce programs in Java?

1 year ago
William Carter
2 minutes

The basic steps for writing a MapReduce program are as follows:

  1. Create a class that implements the Mapper interface and overrides the map method. The map method takes a key-value pair as input, processes the input data, and outputs it as an intermediate key-value pair.
public class MyMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

    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);
        }
    }
}
  1. Create a class that implements the Reducer interface and override the reduce method. This method takes in intermediate key-value pairs as input, aggregates the input data based on the key, and outputs the final result as key-value pairs.
public class MyReducer 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);
    }
}
  1. Create a configuration object to set the parameters of a MapReduce job.
Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount.class);
  1. Specify the path to the input data and the path to the output results.
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
  1. Specify the classes for Mapper and Reducer.
job.setMapperClass(MyMapper.class);
job.setCombinerClass(MyReducer.class);
job.setReducerClass(MyReducer.class);
  1. Define the key-value pair type of the final outcome.
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
  1. Submit the MapReduce job.
System.exit(job.waitForCompletion(true) ? 0 : 1);

The above are the basic steps for writing a MapReduce program. Depending on specific requirements, the logic of the Mapper and Reducer can be expanded and modified.

bannerAds