使用Spark实现Function的序列化问题

Question

我在理解Java中的Spark函数实现时遇到了麻烦.该文件提供了三种方式使用功能map和reduce:

1. 通过lambda
2. 通过内联类实现Function和Function2
3. 通过内部类实现Function和Function2

麻烦的是我无法创造2.和3.工作.例如,这段代码:

public int countInline(String path) {

    String master = "local";
    SparkConf conf = new SparkConf().setAppName("charCounterInLine")
            .setMaster(master);
    JavaSparkContext sc = new JavaSparkContext(conf);
    JavaRDD<String> lines = sc.textFile(path);

    JavaRDD<Integer> lineLengths = lines
            .map(new Function<String, Integer>() {
                public Integer call(String s) {
                    return s.length();
                }
            });
    return lineLengths.reduce(new Function2<Integer, Integer, Integer>() {
        public Integer call(Integer a, Integer b) {
            return a + b;
        }
    }); // the line causing the error 
}

给了我这个错误:

14/07/09 11:23:20 INFO DAGScheduler: Failed to run reduce at CharCounter.java:42
[WARNING]
java.lang.reflect.InvocationTargetException
        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:483)
        at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
        at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.spark.SparkException: Job aborted due to stage failure: Task not serializable: java.io.NotSerializableException: Hadoop.Spark.basique.CharCounter
        at org.apache.spark.scheduler.DAGScheduler.org$apache$spark$scheduler$DAGScheduler$$failJobAndIndependentStages(DAGScheduler.scala:1033)
        at org.apache.spark.scheduler.DAGScheduler$$anonfun$abortStage$1.apply(DAGScheduler.scala:1017)
        at org.apache.spark.scheduler.DAGScheduler$$anonfun$abortStage$1.apply(DAGScheduler.scala:1015)
        at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
        at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:47)
        at org.apache.spark.scheduler.DAGScheduler.abortStage(DAGScheduler.scala:1015)
        at org.apache.spark.scheduler.DAGScheduler.org$apache$spark$scheduler$DAGScheduler$$submitMissingTasks(DAGScheduler.scala:770)
        at org.apache.spark.scheduler.DAGScheduler.org$apache$spark$scheduler$DAGScheduler$$submitStage(DAGScheduler.scala:713)
        at org.apache.spark.scheduler.DAGScheduler.handleJobSubmitted(DAGScheduler.scala:697)
        at org.apache.spark.scheduler.DAGSchedulerEventProcessActor$$anonfun$receive$2.applyOrElse(DAGScheduler.scala:1176)
        at akka.actor.ActorCell.receiveMessage(ActorCell.scala:498)
        at akka.actor.ActorCell.invoke(ActorCell.scala:456)
        at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:237)
        at akka.dispatch.Mailbox.run(Mailbox.scala:219)
        at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:386)
        at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
        at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
        at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
        at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

现在,我可以通过实现Function和Function2公共外部类来避免这个问题.然而,这是一个幸运的猜测而不是一个经过深思熟虑的决定.此外,由于我无法使文档示例工作,我想有些事情我不明白.

最后,我的问题是:

• 如何制作2.和3.工作？
• 为什么只有lambda工作？
• 还有其他使用方法functions吗？

Answer 1

该 stracktrace 的相关部分是：

Task not serializable: java.io.NotSerializableException: Hadoop.Spark.basique.CharCounter

当您将函数定义为内部类时，它们的封闭对象将被拉入函数闭包并序列化。如果此类是不可序列化的或包含不可序列化的字段，那么您将遇到此错误。

您在这里有几个选择：

• 将封闭对象的不可序列化字段标记为transient。
• 将您的函数定义为外部类。
• 将函数定义为静态嵌套类。