Spark自定义累加器实现WordCount

自定义累加器,继承AccumulatorV2

继承后需要实现几个方法

class AccWordCount extends AccumulatorV2[String, mutable.HashMap[String, Int]]
{
	override def isZero: Boolean = ???

    override def copy(): AccumulatorV2[String, mutable.HashMap[String, Int]] = ???

    override def reset(): Unit = ???

    override def add(v: String): Unit = ???

    override def merge(other: AccumulatorV2[String, mutable.HashMap[String, Int]]): Unit = ???

    override def value: mutable.HashMap[String, Int] = ???
}

方法的具体实现代码

private val hashAcc = new mutable.HashMap[String, Int]()

override def isZero: Boolean = hashAcc.isEmpty

override def copy(): AccumulatorV2[String, mutable.HashMap[String, Int]] = {
    val newAcc = new AccWordCount
    //可能存在多个task同时写入，出现线程安全问题
    hashAcc.synchronized
    {
        newAcc.hashAcc ++= this.hashAcc
    }
    newAcc
}

override def reset(): Unit = hashAcc.clear()

//局部累加
override def add(v: String): Unit = {
    hashAcc.synchronized
    {
        hashAcc.get(v) match
        {
            case None => hashAcc += ((v, 1))
            case Some(x) => hashAcc += ((v, x + 1))
        }
    }
}

//全局累加
override def merge(other: AccumulatorV2[String, mutable.HashMap[String, Int]]): Unit = {
    other match
    {
        case o: AccumulatorV2[String, mutable.HashMap[String, Int]] =>
        {
            for ((k, v) <- o.value)
            {
                hashAcc.get(k) match
                {
                    case None => hashAcc += ((k, v))
                    case Some(x) => hashAcc += ((k, v + 1))
                }
            }
        }
    }
}

override def value: mutable.HashMap[String, Int] = hashAcc

使用

//模板代码
private val conf: SparkConf = new SparkConf().setAppName(this.getClass.getSimpleName).setMaster("local[*]")
private val sc = new SparkContext(conf)

//数据
private val rdd: RDD[String] = sc.makeRDD(List("hello", "world", "python", "spark", "spark", "hello", "scala", "spark","python"))

//创建累加器对象
private val acc = new AccWordCount
sc.register(acc, "acc")

//分布式累加
rdd.foreach(x => acc.add(x))
println(acc.value)