Question

我正在编写一段代码，当缓冲区（列表）增长到一定大小时，它会填充mongoDB集合。

import scala.actors.Actor
import com.mongodb.casbah.Imports._
import scala.collection.mutable.ListBuffer

class PopulateDB extends Actor {
  val buffer = new ListBuffer[DBObject]
  val mongoConn = MongoConnection()
  val mongoCol = mongoConn("casbah_test")("logs")

  def add(info: DBObject = null) {
    if (info != null) buffer += info

    if (buffer.size > 0 && (info == null || buffer.length >= 1000)) {
      mongoCol.insert(buffer.toList)
      buffer.clear
      println("adding a batch")
    }
  }

  def act() {
    loop {
      react {
        case info: DBObject => add(info)

        case msg if msg == "closeConnection" =>
          println("Close connection")
          add()
          mongoConn.close
      }
    }
  }
}

但是，当我运行以下代码时，scala偶尔会在“mongoCol.insert（buffer.toList）”行中抛出“ConcurrentModificationException”。我很确定它与“mongoCol.insert”有关。我想知道代码是否有任何根本性的错误。或者我应该使用类似Akka的“atomic {...}”来避免这个问题。

这是完整的堆栈跟踪：

PopulateDB@7e859a68: caught java.util.ConcurrentModificationException
java.util.ConcurrentModificationException
    at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:373)
    at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:392)
    at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:391)
    at org.bson.BSONEncoder.putObject(BSONEncoder.java:113)
    at org.bson.BSONEncoder.putObject(BSONEncoder.java:67)
    at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:215)
    at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:180)
    at com.mongodb.DBCollection.insert(DBCollection.java:85)
    at com.mongodb.casbah.MongoCollectionBase$class.insert(MongoCollection.scala:561)
    at com.mongodb.casbah.MongoCollection.insert(MongoCollection.scala:864)
    at PopulateDB.add(PopulateDB.scala:14)
    at PopulateDB$$anonfun$act$1$$anonfun$apply$1.apply(PopulateDB.scala:26)
    at PopulateDB$$anonfun$act$1$$anonfun$apply$1.apply(PopulateDB.scala:25)
    at scala.actors.ReactorTask.run(ReactorTask.scala:34)
    at scala.actors.Reactor$class.resumeReceiver(Reactor.scala:129)
    at PopulateDB.scala$actors$ReplyReactor$$super$resumeReceiver(PopulateDB.scala:5)
    at scala.actors.ReplyReactor$class.resumeReceiver(ReplyReactor.scala:69)
    at PopulateDB.resumeReceiver(PopulateDB.scala:5)
    at scala.actors.Actor$class.searchMailbox(Actor.scala:478)
    at PopulateDB.searchMailbox(PopulateDB.scala:5)
    at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114)
    at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114)
    at scala.actors.ReactorTask.run(ReactorTask.scala:36)
    at scala.concurrent.forkjoin.ForkJoinPool$AdaptedRunnable.exec(ForkJoinPool.java:611)
    at scala.concurrent.forkjoin.ForkJoinTask.quietlyExec(ForkJoinTask.java:422)
    at scala.concurrent.forkjoin.ForkJoinWorkerThread.mainLoop(ForkJoinWorkerThread.java:340)
    at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:325)

谢谢，德里克

Answer 1

DBObject 不线程安全;您正在使用您的演员消息发送DBObject。它可能会在以后再次修改，这将导致并发修改问题。

我建议首先尝试在DBObject进入actor时使用clone()，并将其放入缓冲区。它只是一个浅层副本，但至少应该足以导致LinkedHashMap上的并发修改问题，它支持DBObject上的键（由于LHM而保持有序）。

我试试：

  def act() {
    loop {
      react {
        case info: DBObject => add(info.clone())

        case msg if msg == "closeConnection" =>
          println("Close connection")
          add()
          mongoConn.close
      }
    }
  }

如果这不起作用，请在将DBObject发送到Actor之后查看正在修改DBObject的任何其他位置。

Answer 2

下面为什么class？

class PopulateDB extends Actor

你有多个PupulateDB演员吗？我期待object PopulateDB extends Actor，这样一个演员就可以集中精力完成这项任务。

除此之外，问题似乎是在casbah或mongodb本身内部。

使用Scala的Actor时如何避免竞争条件

2 个答案: