spark作业序列化问题解决方案

开发spark作业过程中经常会遇到序列化问题，并出现Task not serializable、java.io.NotSerializableException这样的报错。

org.apache.spark.SparkException: Task not serializable
	at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:416)
	at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:406)
	at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:162)
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	at org.apache.spark.sql.execution.streaming.ProgressReporter.reportTimetaken$(ProgressReporter.scala:350)
	at org.apache.spark.sql.execution.streaming.StreamExecution.reportTimetaken(StreamExecution.scala:69)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution.$anonfun$runActivatedStream$1(MicroBatchExecution.scala:191)
	at org.apache.spark.sql.execution.streaming.ProcessingTimeExecutor.execute(TriggerExecutor.scala:57)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution.runActivatedStream(MicroBatchExecution.scala:185)
	at org.apache.spark.sql.execution.streaming.StreamExecution.org$apache$spark$sql$execution$streaming$StreamExecution$$runStream(StreamExecution.scala:334)
	at org.apache.spark.sql.execution.streaming.StreamExecution$$anon$1.run(StreamExecution.scala:245)
Caused by: java.io.NotSerializableException: okhttp3.Request$Builder

首先来说一下产生这个问题的原因，spark作为分布式计算引擎，当你提交作业后：

1. driver节点会生成执行计划，并将任务(transformation)序列化
2. 将序列化后的任务分发到集群上适合的工作节点
3. 工作节点对任务进行反序列化，并执行任务
4. 将任务执行结果返回给driver节点

在step1，driver节点会尝试序列化要发送到worker节点的对象，如果对象无法序列化，作业就会失败并抛出上面的异常。

• 把对象序列化
  此方法仅适用于内部对象，将对象 implements Serializable接口即可
  对于一些无法序列化的对象，可以尝试使用以下方法

• 把对象实例化放到transformation的lambda表达式或udf中 

  JavaRDD rdd = sc.textFile("/tmp/myfile");
  rdd.map(s -> {
      NotSerializable notSerializable = new NotSerializable();
      return notSerializable.run(s);
  }).collect();

• 将无法序列化的对象置为静态变量

  private static NotSerializable notSerializable = new NotSerializable();
  
  public static void main( String[] args ) throws Exception {
      JavaRDD rdd = sc.textFile("/tmp/myfile");
  
      rdd.map(s -> notSerializable.run(s)).collect();
  
  }
  

• 调用rdd.forEachPartition, 将对象示例化放入其中

  JavaRDD rdd = sc.textFile("/tmp/myfile");
  rdd.forEachPartition(iter -> {
      NotSerializable notSerializable = new NotSerializable();
      
      // 业务处理代码
  
  });