尝试在 Spark Streaming 中使用持久表时出现空指针异常 [英] Null Pointer Exception When Trying to Use Persisted Table in Spark Streaming

问题描述

我在开始时创建了gpsLookUpTable"并保留它，这样我就不需要一遍又一遍地拉它来做映射.但是，当我尝试在 foreach 内部访问它时，我得到了空指针异常.感谢任何帮助.

I am creating "gpsLookUpTable" at the beginning and persisting it so that i do not need to pull it over and over again to do mapping. However, when i try to access it inside foreach i get null pointer exception. Any help is appreciated thanks.

以下是代码片段:

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

val conf = new SparkConf() ... 

val sc = new SparkContext(conf) 
val ssc = new StreamingContext(sc, Seconds(20)) 
val sqc = new SQLContext(sc) 

//////Trying to cache table here to use it below 
val gpsLookUpTable = MapInput.cacheMappingTables(sc, sqc).persist(StorageLevel.MEMORY_AND_DISK_SER_2) 
//sc.broadcast(gpsLookUpTable) 
ssc.textFileStream("hdfs://localhost:9000/inputDirectory/") 
.foreachRDD { rdd => 
if (!rdd.partitions.isEmpty) { 

val allRows = sc.textFile("hdfs://localhost:9000/supportFiles/GeoHashLookUpTable") 
sqc.read.json(allRows).registerTempTable("GeoHashLookUpTable") 
val header = rdd.first().split(",") 
val rowsWithoutHeader = Utils.dropHeader(rdd) 

rowsWithoutHeader.foreach { row => 

val singleRowArray = row.split(",") 
singleRowArray.foreach(println) 
(header, singleRowArray).zipped 
.foreach { (x, y) => 
///Trying to access persisted table but getting null pointer exception 
val selectedRow = gpsLookUpTable 
.filter("geoCode LIKE '" + GeoHash.subString(lattitude, longitude) + "%'") 
.withColumn("Distance", calculateDistance(col("Lat"), col("Lon"))) 
.orderBy("Distance") 
.select("TrackKM", "TrackName").take(1) 
if (selectedRow.length != 0) { 
// do something
} 
else { 
// do something
} 
} 
} }}

推荐答案

我假设您在集群中运行；您的 foreach 将在其他节点上作为闭包运行.引发 Nullpointer 是因为该闭包在没有初始化 gpsLookUpTable 的节点上运行.您显然确实尝试在

I assume you are running in a cluster; your foreach will run as a closure on other nodes. The Nullpointer is raised because that closure runs on a node which doesn't have a initialized gpsLookUpTable. You did obviously try to broadcast gpsLookUpTable in

//sc.broadcast(gpsLookUpTable) 

但是这个需要绑定一个变量，基本上是这样的:

But this need to be bound to a variable, basically like this:

val tableBC = sc.broadcast(gpsLookUpTable) 

在 foreach 中，您将替换为:

in foreach, you would replace this:

foreach { (x, y) => 
///Trying to access persisted table but getting null pointer exception 
val selectedRow = gpsLookUpTable 

这样:

foreach { (x, y) => 
///Trying to access persisted table but getting null pointer exception 
val selectedRow = tableBC.value 

有效地让您访问广播值.

which effectively give you access to the broadcast value.

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