Scala常见问题解决

这俩天总结了在写Spark Job的时候遇到的一些问题，写在这里，以后遇到了方便查看。

1.Error:(64, 64) Unable to find encoder for type stored in a Dataset. Primitive types (Int, String, etc) and Product types (case classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.

问题的源码：

...
val originRdd: RDD[(String, String, Int)] = dataFrame.map(row => (row.getString(0), row.getString(1), row.getInt(2))).rdd
...

这里是对DataFrame的三列值进行一个特定数据类型的获取动作，但是报错了，这是因为spark数据集在存储数据类型的时候需要编码器，对于常见的数据类型，spark已经有现成的编码器可用，但是需要从spark.implicits._中引入才可以运行。

解决方案：在执行DataFrame转换为RDD之前引入sparkSession.implicits._即可：

import sparkSession.implicits._
val originRdd: RDD[(String, String, Int)] = dataFrame.map(row => (row.getString(0), row.getString(1), row.getInt(2))).rdd

参考链接： https://*.com/questions/38664972/why-is-unable-to-find-encoder-for-type-stored-in-a-dataset-when-creating-a-dat

2.Error:(21, 19) missing parameter type lists.forEach(no => {})

有可能是在遍历java.util.List对象的时候出错，这个时候需要引入一个转换器：

import scala.collection.JavaConversions._

其他的问题可以参考：

https://*.com/questions/13115053/scala-anonymous-function-missing-parameter-type-error

3.Scala测试用例报错java.lang.Exception: Test class should have exactly one public constructor

java.lang.Exception: Test class should have exactly one public constructor

	at org.junit.runners.BlockJUnit4ClassRunner.validateOnlyOneConstructor(BlockJUnit4ClassRunner.java:158)
	at org.junit.runners.BlockJUnit4ClassRunner.validateConstructor(BlockJUnit4ClassRunner.java:147)
	at org.junit.runners.BlockJUnit4ClassRunner.collectInitializationErrors(BlockJUnit4ClassRunner.java:127)
	at org.junit.runners.ParentRunner.validate(ParentRunner.java:416)
	at org.junit.runners.ParentRunner.<init>(ParentRunner.java:84)
	at org.junit.runners.BlockJUnit4ClassRunner.<init>(BlockJUnit4ClassRunner.java:65)
	at org.junit.internal.builders.JUnit4Builder.runnerForClass(JUnit4Builder.java:10)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:26)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.requests.ClassRequest.getRunner(ClassRequest.java:33)
	at org.junit.internal.requests.FilterRequest.getRunner(FilterRequest.java:36)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:49)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.sJUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

解决办法：将object 改为class

4.org.apache.spark.SparkException: Task not serializable错误

Caused by: org.apache.spark.SparkException: Task not serializable
	at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:298)
	at org.apache.spark.util.ClosureCleaner$.org$apache$spark$util$ClosureCleaner$$clean(ClosureCleaner.scala:288)
	at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:108)
	at org.apache.spark.SparkContext.clean(SparkContext.scala:2287)
	at org.apache.spark.rdd.PairRDDFunctions$$anonfun$mapValues$1.apply(PairRDDFunctions.scala:758)
	at org.apache.spark.rdd.PairRDDFunctions$$anonfun$mapValues$1.apply(PairRDDFunctions.scala:757)
	at org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:151)
	at org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:112)
	at org.apache.spark.rdd.RDD.withScope(RDD.scala:362)
	at org.apache.spark.rdd.PairRDDFunctions.mapValues(PairRDDFunctions.scala:757)
	at com.test.Test2$.main(Test2.scala:70)

在进行RDD的处理时，报了这个错，这是因为Spark在进行分布式计算的时候，需要从Driver节点往Worker节点传输数据内容，比如一个公用的Object，在传输过程中就会使用到序列化(实现Serializable接口)，如果传输的对象不能序列化，就会导致Worker不能正常接收到该数据，所以就会报错。

所以出现了这个错误，首先看看rdd的action里面有没有使用到外部的对象，一定要使用可序列化的对象。

val groupedRdd: RDD[(String, Iterable[(String, String, Long)])] = originRdd.keyBy(row => row._1).groupByKey
groupedRdd.mapValues(values => { //这里的values是一个Iterable对象，非序列化对象
    //action内容
}

比如我这里是因为使用了一个Iterable对象，不能序列化导致了错误。

所以我改为了

groupedRdd.mapValues(iter => iter.toList) // 将Iterable转换为List，List是可序列化的，具体可看源码scala.collection.immutable.List
mapValues(values => { //这里的values是一个Iterable对象，非序列化对象
    //action内容
}