先看一个例子：

/*
  首先我们定义了一个Search对象，带有一个String类型的参数
  该类拥有三个成员方法：
  1）isMatch：判断参数字符串s是否包含子串query
  2）getMatchRdd1：使用isMatch方法获取匹配结果后的RDD
  3）getMatchRdd1：在filter中实现方法获取匹配结果后的RDD
 */

class Search(query: String) {
  def isMatch(s: String): Boolean = {
    s.contains(query)
  }

  def getMatchRdd1(rdd: RDD[String]): RDD[String] = {
    rdd.filter(isMatch)
  }

  def getMatchRdd2(rdd: RDD[String]): RDD[String] = {
    rdd.filter(_.contains(query))
  }

}

object SerializableDemo {
  def main(args: Array[String]): Unit = {

    val conf = new SparkConf().setMaster("local[4]").setAppName("SerializableDemo")
    val sc = new SparkContext(conf)

    val rdd = sc.parallelize(Array("hello", "world", "hello", "spark"))

    val search = new Search("h")

    val matchRdd = search.getMatchRdd2(rdd)
    matchRdd.collect().foreach(println)

    sc.stop()

  }
}

运行后结果：

Exception in thread "main" org.apache.spark.SparkException: Task not serializable
at org.apache.spark.util.ClosureCleaner.orgsparkClosureCleaneranonfun1.apply(RDD.scala:388)
at org.apache.spark.rdd.RDD$$anonfun1.apply(RDD.scala:387)
at org.apache.spark.rdd.RDDOperationScope.withScope(RDDOperationScope.scala:112)
at org.apache.spark.rdd.RDD.withScope(RDD.scala:363)
at org.apache.spark.rdd.RDD.filter(RDD.scala:387)
at adamlee.spark.Search.getMatchRdd2(SerializableDemo.scala:34)
at adamlee.spark.SerializableDemo$.main(SerializableDemo.scala:16)
at adamlee.spark.SerializableDemo.main(SerializableDemo.scala)
Caused by: java.io.NotSerializableException: adamlee.spark.Search
Serialization stack:

• object not serializable (class: adamlee.spark.Search, value: adamlee.spark.Search@4cafa9aa)
• field (class: adamlee.spark.Search$$anonfun1, name: $outer, type: class adamlee.spark.Search)
• object (class adamlee.spark.Search$$anonfun1, <function1>)
  at org.apache.spark.serializer.SerializationDebugger.ensureSerializable(ClosureCleaner.scala:342)
  ... 12 more

报错提示Task未能序列化，再看Caused By提示：object not serializable，告诉我们Search这个类的对象未能序列化。

原因就是search对象初始化是在Driver端进行的，当我们执行collect是，触发计算，Driver需要将任务下发至Executor，这时候就产生了进程间通信，Driver和Executor间通信是通过网络传输，网络上传输的是二进制的比特流，由于Search类并未继承Serializable类，所以这个类的对象就不能被序列化。

现在我们新建一个类Search1，继承了Serializable：

class Search1(query: String) extends Serializable {
  def isMatch(s: String): Boolean = {
    s.contains(query)
  }

  def getMatchRdd1(rdd: RDD[String]): RDD[String] = {
    rdd.filter(isMatch)
  }

  def getMatchRdd2(rdd: RDD[String]): RDD[String] = {
    rdd.filter(_.contains(query))
  }

}

object SerializableDemo {
  def main(args: Array[String]): Unit = {

    val conf = new SparkConf().setMaster("local[4]").setAppName("SerializableDemo")
    val sc = new SparkContext(conf)

    val rdd = sc.parallelize(Array("hello", "world", "hello", "spark"))

    val search1 = new Search1("h")

    val matchRdd = search1.getMatchRdd2(rdd)
    matchRdd.collect().foreach(println)

    sc.stop()

  }
}

运行后结果：

hello
hello