What is a Boxed Error in Scala?

Question

When I run my application, the browser shows

[ExecutionException: Boxed Error]

It doesn't say anything about the line number, etc.

In the console, I have the following

! @6elaah0c8 - Internal server error, for (GET) [/testlearn] ->

play.api.Application$$anon$1: Execution exception[[ExecutionException: Boxed Error]]
    at play.api.Application$class.handleError(Application.scala:289) ~[play_2.10.jar:2.1.1]
    at play.api.DefaultApplication.handleError(Application.scala:383) [play_2.10.jar:2.1.1]
    at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$17$$anonfun$apply$24.apply(PlayDefaultUpstreamHandler.scala:326) [play_2.10.jar:2.1.1]
    at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$17$$anonfun$apply$24.apply(PlayDefaultUpstreamHandler.scala:324) [play_2.10.jar:2.1.1]
    at play.api.libs.concurrent.PlayPromise$$anonfun$extend1$1.apply(Promise.scala:113) [play_2.10.jar:2.1.1]
    at play.api.libs.concurrent.PlayPromise$$anonfun$extend1$1.apply(Promise.scala:113) [play_2.10.jar:2.1.1]
java.util.concurrent.ExecutionException: Boxed Error
    at scala.concurrent.impl.Promise$.resolver(Promise.scala:52) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at scala.concurrent.impl.Promise$.scala$concurrent$impl$Promise$$resolveTry(Promise.scala:44) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at scala.concurrent.impl.Promise$DefaultPromise.tryComplete(Promise.scala:116) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at scala.concurrent.Promise$class.complete(Promise.scala:55) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at scala.concurrent.impl.Promise$DefaultPromise.complete(Promise.scala:58) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at scala.concurrent.impl.Future$PromiseCompletingRunnable.run(Future.scala:23) [factorie-1.0.0-M4-jar-with-dependencies.jar:na]
Caused by: java.lang.AssertionError: assertion failed
    at scala.Predef$.assert(Predef.scala:165) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at cc.factorie.util.TraversableExtras$class.max2ByDouble(TraversableExtras.scala:95) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at cc.factorie.package$$anon$2.max2ByDouble(package.scala:148) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at cc.factorie.optimize.SampleRankExample.accumulateExampleInto(SampleRank.scala:31) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at cc.factorie.optimize.OnlineTrainer$$anonfun$processExamples$3.apply(Trainer.scala:72) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]
    at cc.factorie.optimize.OnlineTrainer$$anonfun$processExamples$3.apply(Trainer.scala:63) ~[factorie-1.0.0-M4-jar-with-dependencies.jar:na]

score 50 · Accepted Answer · edited May 23 '17 at 11:46

"Boxed Error" is Scala's response to an Error being thrown within a Future. In Java, and hence Scala, subclasses of type Error have a special meaning as Fatal errors. See Differences between Exception and Error. In short, the javadoc says:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Unlike throwing other Throwables within a future, when a subclass of Error is thrown, the default Scala resolver will wrap up the Error in a java.util.concurrent.ExecutionException, with the message string "Boxed Error", and complete your promise with a failure.

To quote the futures documentation http://docs.scala-lang.org/overviews/core/futures.html w.r.t. Error being thrown:

[Error] exceptions are rethrown in the thread executing the failed asynchronous computation. The rationale behind this is to prevent propagation of critical and control-flow related exceptions normally not handled by the client code and at the same time inform the client in which future the computation failed.

If you want to do something special with the Failure, the original Error that was thrown can be extracted (but not in a way particularly amenable to pattern matching), by ExecutionException#getCause()

I'm kinda missing why anything throwable "leaks" from the Future in the first place, shouldn't be the original AssertionError just caught and future's oncomplete callback be passed a Failure(ex) ? — lisak, Nov 20 '14 at 08:53
I see Future handling NonFatal exception, but AssertionError is actually a NonFatal exception, so it should be caught and passed as a value — lisak, Nov 20 '14 at 08:57

score 3 · Answer 2 · answered Jun 24 '13 at 07:35

I don't know that's a Boxed Error, but according to your stacktrace, the root problem comes from the factorie lib, from the max2Double method at line 95.

Extract from the source code:

/**Returns both the maximum element and the second-to-max element */
  def max2ByDouble(extractor: A => Double): (A, A) = {
    val s1 = t.toSeq
    assert(s1.length > 1)   // <<<== HERE
    var best1 = Double.NegativeInfinity
    ...

It seems that your Traversable is empty.

What is a Boxed Error in Scala?

2 Answers2