scala - 什么是 Scala 中的盒装错误？

Question

当我运行我的应用程序时，浏览器显示

[ExecutionException: Boxed Error]

它没有说明行号等。

在控制台中，我有以下内容

! @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]

Answer 1

“盒装错误”是 Scala 对 Error 的回应。被扔在 Future 内.在 Java 和 Scala 中，Error 类型的子类具有 fatal error 的特殊含义。见 Differences between Exception and Error .简而言之，javadoc 说:

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.

不像扔其他 Throwable s 在 future ，当 Error 的子类被抛出，默认的 Scala 解析器将结束 Error在 java.util.concurrent.ExecutionException ，带有消息字符串“Boxed Error”，并以失败完成您的 promise 。

引用 future 文档 http://docs.scala-lang.org/overviews/core/futures.html w.r.t. Error被抛出:

[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.

如果你想对Failure做一些特别的事情，原文 Error可以通过 ExecutionException#getCause() 提取抛出的内容(但不是以特别适合模式匹配的方式)