我在 Amazon emr-5.2.1 上以客户端模式使用 Spark 2.0.2。我使用 Kryo 序列化并在我们自己的 KryoRegistrator 中注册我们的类：
val sparkConf = new SparkConf()
.set("spark.serializer", "org.apache.spark.serializer.KryoSerializer")
.set("spark.kryo.registrator", classOf[de.gaf.ric.workflow.RicKryoRegistrator].getName)
.set("spark.kryo.registrationRequired", "true")
.set("spark.kryoserializer.buffer.max", "512m")
implicit val sc = new SparkContext(sparkConf)
该过程开始正常，但几分钟后，我在执行器上收到以下异常：
17/02/02 16:22:34 ERROR RetryingBlockFetcher: Failed to fetch block rdd_3641_12, and will not retry (0 retries)
java.lang.RuntimeException: org.apache.spark.SparkException: Failed to register classes with Kryo
at org.apache.spark.serializer.KryoSerializer.newKryo(KryoSerializer.scala:129)
at org.apache.spark.serializer.KryoSerializerInstance.borrowKryo(KryoSerializer.scala:274)
at org.apache.spark.serializer.KryoSerializerInstance.<init>(KryoSerializer.scala:259)
at org.apache.spark.serializer.KryoSerializer.newInstance(KryoSerializer.scala:175)
at org.apache.spark.serializer.SerializerManager.dataSerializeWithExplicitClassTag(SerializerManager.scala:141)
at org.apache.spark.storage.BlockManager.org$apache$spark$storage$BlockManager$$doGetLocalBytes(BlockManager.scala:499)
at org.apache.spark.storage.BlockManager$$anonfun$getLocalBytes$2.apply(BlockManager.scala:474)
at org.apache.spark.storage.BlockManager$$anonfun$getLocalBytes$2.apply(BlockManager.scala:474)
at scala.Option.map(Option.scala:146)
at org.apache.spark.storage.BlockManager.getLocalBytes(BlockManager.scala:474)
at org.apache.spark.storage.BlockManager.getBlockData(BlockManager.scala:280)
at org.apache.spark.network.netty.NettyBlockRpcServer$$anonfun$2.apply(NettyBlockRpcServer.scala:60)
at org.apache.spark.network.netty.NettyBlockRpcServer$$anonfun$2.apply(NettyBlockRpcServer.scala:60)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:186)
at org.apache.spark.network.netty.NettyBlockRpcServer.receive(NettyBlockRpcServer.scala:60)
at org.apache.spark.network.server.TransportRequestHandler.processRpcRequest(TransportRequestHandler.java:159)
at org.apache.spark.network.server.TransportRequestHandler.handle(TransportRequestHandler.java:107)
at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:119)
at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:51)
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at org.apache.spark.network.util.TransportFrameDecoder.channelRead(TransportFrameDecoder.java:85)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: de.gaf.ric.workflow.RicKryoRegistrator
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at org.apache.spark.serializer.KryoSerializer$$anonfun$newKryo$5.apply(KryoSerializer.scala:124)
at org.apache.spark.serializer.KryoSerializer$$anonfun$newKryo$5.apply(KryoSerializer.scala:124)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:186)
at org.apache.spark.serializer.KryoSerializer.newKryo(KryoSerializer.scala:124)
... 43 more
at org.apache.spark.network.client.TransportResponseHandler.handle(TransportResponseHandler.java:189)
at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:121)
at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:51)
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at org.apache.spark.network.util.TransportFrameDecoder.channelRead(TransportFrameDecoder.java:85)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
at java.lang.Thread.run(Thread.java:745)
班上RicKryoRegistrator确实包含在我的 uber JAR 中，我仔细检查了这一点。它还被转移到执行者：
17/02/02 16:19:02 INFO Executor: Fetching spark://172.31.20.106:41032/jars/app-imageprocessing-0.13.0-20170202.112920-1.jar with timestamp 1486048690879
17/02/02 16:19:02 INFO TransportClientFactory: Successfully created connection to /172.31.20.106:41032 after 23 ms (0 ms spent in bootstraps)
17/02/02 16:19:02 INFO Utils: Fetching spark://172.31.20.106:41032/jars/app-imageprocessing-0.13.0-20170202.112920-1.jar to /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/spark-8172edd9-d1c7-40c9-ad9b-74b2bd9dbad9/fetchFileTemp6474512860106916303.tmp
17/02/02 16:19:03 INFO Utils: Copying /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/spark-8172edd9-d1c7-40c9-ad9b-74b2bd9dbad9/-68603321486048690879_cache to /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/container_1486039395474_0012_01_000011/./app-imageprocessing-0.13.0-20170202.112920-1.jar
17/02/02 16:19:03 INFO Executor: Adding file:/mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/container_1486039395474_0012_01_000011/./app-imageprocessing-0.13.0-20170202.112920-1.jar to class loader
阅读源码org.apache.spark.serializer.KryoSerializer，我看到它使用以下类加载器：
val classLoader = defaultClassLoader.getOrElse(Thread.currentThread.getContextClassLoader)
难道是defaultClassLoader未设置，并且我的 uber JAR 未包含在Thread.currentThread.getContextClassLoader？还有什么原因呢？
