为什么Spark Streaming应用创建输入流，但该输入流无输出逻辑时，应用从checkpoint恢复启动失败

问题

Spark Streaming应用创建1个输入流，但该输入流无输出逻辑。应用从checkpoint恢复启动失败，报错如下：

17/04/24 10:13:57 ERROR Utils: Exception encountered
java.lang.NullPointerException
at org.apache.spark.streaming.dstream.DStreamCheckpointData$$anonfun$writeObject$1.apply$mcV$sp(DStreamCheckpointData.scala:125)
at org.apache.spark.streaming.dstream.DStreamCheckpointData$$anonfun$writeObject$1.apply(DStreamCheckpointData.scala:123)
at org.apache.spark.streaming.dstream.DStreamCheckpointData$$anonfun$writeObject$1.apply(DStreamCheckpointData.scala:123)
at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1195)
at org.apache.spark.streaming.dstream.DStreamCheckpointData.writeObject(DStreamCheckpointData.scala:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:441)
at org.apache.spark.streaming.dstream.DStream$$anonfun$writeObject$1.apply$mcV$sp(DStream.scala:515)
at org.apache.spark.streaming.dstream.DStream$$anonfun$writeObject$1.apply(DStream.scala:510)
at org.apache.spark.streaming.dstream.DStream$$anonfun$writeObject$1.apply(DStream.scala:510)
at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1195)
at org.apache.spark.streaming.dstream.DStream.writeObject(DStream.scala:510)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1378)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:441)
at org.apache.spark.streaming.DStreamGraph$$anonfun$writeObject$1.apply$mcV$sp(DStreamGraph.scala:191)
at org.apache.spark.streaming.DStreamGraph$$anonfun$writeObject$1.apply(DStreamGraph.scala:186)
at org.apache.spark.streaming.DStreamGraph$$anonfun$writeObject$1.apply(DStreamGraph.scala:186)
at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1195)
at org.apache.spark.streaming.DStreamGraph.writeObject(DStreamGraph.scala:186)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
at org.apache.spark.streaming.Checkpoint$$anonfun$serialize$1.apply$mcV$sp(Checkpoint.scala:142)
at org.apache.spark.streaming.Checkpoint$$anonfun$serialize$1.apply(Checkpoint.scala:142)
at org.apache.spark.streaming.Checkpoint$$anonfun$serialize$1.apply(Checkpoint.scala:142)
at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1230)
at org.apache.spark.streaming.Checkpoint$.serialize(Checkpoint.scala:143)
at org.apache.spark.streaming.StreamingContext.validate(StreamingContext.scala:566)
at org.apache.spark.streaming.StreamingContext.liftedTree1$1(StreamingContext.scala:612)
at org.apache.spark.streaming.StreamingContext.start(StreamingContext.scala:611)
at com.spark.test.kafka08LifoTwoInkfk$.main(kafka08LifoTwoInkfk.scala:21)
at com.spark.test.kafka08LifoTwoInkfk.main(kafka08LifoTwoInkfk.scala)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:772)
at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:183)
at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:208)
at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:123)
at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

回答

Streaming Context启动时，若应用设置了checkpoint，则需要对应用中的DStream checkpoint对象进行序列化，序列化时会用到dstream.context。

dstream.context是Streaming Context启动时从output Streams反向查找所依赖的DStream，逐个设置context。若Spark Streaming应用创建1个输入流，但该输入流无输出逻辑时，则不会给它设置context。所以在序列化时报“NullPointerException”。

解决办法：应用中如果有无输出逻辑的输入流，则在代码中删除该输入流，或添加该输入流的相关输出逻辑。