java和scala分別實現WordCount

        WordCount作爲大數據領域的經典範例，如同HelloWorld在程序設計中的地位一樣，是一個入門程序。在此使用並行化處理介紹WordCount程序過程。

1. 實例描述

輸入（txt）：

Hello World Bye World

Hello Hadoop Bye Hadoop

Bye Hadoop Hello Hadoop

輸出：

<Hello, 3>

<World, 2>

<Bye, 3>

<Hadoop, 4>

2. 設計思路

在map階段會將數據映射爲：

<Hello, 1>

<World, 1>

<Bye, 1>

<World, 1>

<Hello, 1>

<Hadoop, 1>

<Bye, 1>

<Hadoop, 1>

<Bye, 1>

<Hadoop, 1>

<Hello, 1>

<Hadoop, 1>

在reduce階段會將相同key的數據合併，並將合併結果相加：

<Hello, 1, 1, 1>

<World, 1, 1>

<Bye, 1, 1, 1>

<Hadoop, 1, 1, 1, 1>

3. 代碼示例

WordCount用java實現代碼示例如下：

import org.apache.spark.SparkConf;
import org.apache.spark.api.java.JavaPairRDD;
import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.JavaSparkContext;
import org.apache.spark.api.java.function.FlatMapFunction;
import org.apache.spark.api.java.function.Function2;
import org.apache.spark.api.java.function.PairFunction;
import scala.Tuple2;

import java.util.Arrays;
import java.util.regex.Pattern;

/**
 * Created by yhao on 2015/3/16.
 */
public class WordCount {
    private static final Pattern SPACE = Pattern.compile(" ");

    public static void main(String[] args) {
        String srcPath = null;
        String desPath = null;
        SparkConf sparkConf = new SparkConf().setAppName("wordcount");
  
        if (args.length != 2) {
            System.out.println("Parameter error!");
            System.exit(-1);
        }else {
            srcPath = args[0];
            desPath = args[1];
        }

        JavaSparkContext jsc = new JavaSparkContext();
        final JavaRDD<String> lines = jsc.textFile(srcPath);
        JavaRDD<String> words = lines.flatMap(new FlatMapFunction<String, String>() {
            @Override
            public Iterable<String> call(String s) throws Exception {
                return Arrays.asList(SPACE.split(s));
            }
        });

        JavaPairRDD<String, Integer> ones = words.mapToPair(new PairFunction<String, String, Integer>() {
            @Override
            public Tuple2<String, Integer> call(String s) throws Exception {
                return new Tuple2<String, Integer>(s, 1);
            }
        });

        JavaPairRDD<String, Integer> counts = ones.reduceByKey(new Function2<Integer, Integer, Integer>() {
            @Override
            public Integer call(Integer integer, Integer integer2) throws Exception {
                return integer + integer2;
            }
        });

        counts.saveAsTextFile(desPath);
        jsc.stop();
    }
}

WordCount用scala實現代碼如下：

import org.apache.spark.{SparkContext, SparkConf}
import org.apache.spark.SparkContext._

/**
 * Created by yhao on 2015/3/17.
 */
class WordCount {

}

object WordCount {
  def main(args: Array[String]) {
    if(args.length != 2) {
      System.out.println("Usage: <src> <des>")
      System.exit(2)
    }

    val sparkConf = new SparkConf().setAppName("WordCount")
    val sparkContext = new SparkContext(sparkConf)
    val lines = sparkContext.textFile(args(0))
    val words = lines.flatMap(_.split(" ")).map(a => (a, 1))
    val counts = words.reduceByKey((a, b) => a+b)

    counts.saveAsTextFile(args(1))
    sparkContext.stop()
  }
}

4. 應用場景

        WordCount模型可應用於很多場景中，如統計博客各日誌瀏覽量，文本搜索詞頻等等。