了解 play/scala 错误处理

Question

我在 Scala/Play 中有一个 Web 应用程序，使用 Cassandra 作为数据库。我在测试潜在错误时运行遇到以下问题。

下面是我项目的组成部分

Application.scala -> 基本控制器
Model.scala -> 持有业务逻辑
CassandraClient.scala -> 逻辑连接 cassandra 和运行查询 cassandra

CassandraClient.scala 如下所示

object CassandraClient {

private val cluster = Cluster.builder()
    .withLoadBalancingPolicy(
      new WhiteListPolicy(new RoundRobinPolicy(), nodes))
    .withSocketOptions(socketOptions)
    .addContactPointsWithPorts(nodes)
    .withCredentials(CASSANDRA_USERNAME, CASSANDRA_PWD).build()
}

 private val session = cluster.connect()

def getValueFromCassandraTable(token:String) = {
 var query = QueryBuilder.select.all()...
 seesion.execute(query)
}

Cassandra连接是在我第一次调用getValueFromCassandraTable时建立的。由于 CassandraClient 是一个对象，连接到 Cassandra 的逻辑只被调用一次。

Model.Scala 有一些代码来处理 session.execute 返回的 future。

例如：查看下面 Model.scala 中的示例代码。

    def getTitles(titles: String)(implicit ctxt: ExecutionContext): Future[List[<Sometype>]] = {
        try{
            CassandraClient.getValueFromCassandraTable(token).toScalaFuture.map { rows =>
                  rows.map(row => row("value").toList
                }.recover { case e: Exception =>List()}
              }
        } catch{case e:Exception => 
           Logger.info("THIS DOES NOT EXECUTE??")
           Future{List()}}
    }

现在，当上面的示例代码第一次执行时，session.execute() 将被执行。然后执行 getValueFromCassandraTable。所以我认为执行流程是 Models.scala -> session.execute() -> getValueFromCassandraTable()

中的代码片段

因此，如果 session.execute 失败，我应该能够在 try catch 异常块中捕获它。但令我惊讶的是，当 session.execute 失败时，catch 块没有被执行。而是播放框架抛出异常。谁能解释一下这种行为。

异常堆栈

Caused by: java.lang.RuntimeException: java.lang.ExceptionInInitializerError
    at play.api.mvc.ActionBuilder$$anon.apply(Action.scala:498) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$$anonfun$apply$$anonfun$apply$$anonfun$apply.apply(Action.scala:105) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$$anonfun$apply$$anonfun$apply$$anonfun$apply.apply(Action.scala:105) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.utils.Threads$.withContextClassLoader(Threads.scala:21) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$$anonfun$apply$$anonfun$apply.apply(Action.scala:104) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$$anonfun$apply$$anonfun$apply.apply(Action.scala:103) ~[play_2.10-2.4.2.jar:2.4.2]
    at scala.Option.map(Option.scala:145) ~[scala-library-2.10.5.jar:na]
    at play.api.mvc.Action$$anonfun$apply.apply(Action.scala:103) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$$anonfun$apply.apply(Action.scala:96) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.libs.iteratee.DoneIteratee$$anonfun$mapM.apply(Iteratee.scala:741) ~[play-iteratees_2.10-2.4.2.jar:2.4.2]
Caused by: java.lang.ExceptionInInitializerError: null
    at models.Model$.getTitles(Model.scala:121) ~[classes/:na]
    at models.Model$.getMatchingTitles(Model.scala:48) ~[classes/:na]
    at models.Model$.getMatchingTitles(Model.scala:56) ~[classes/:na]
    at controllers.Application$$anonfun$searchTitle.apply(Application.scala:15) ~[classes/:na]
    at controllers.Application$$anonfun$searchTitle.apply(Application.scala:15) ~[classes/:na]
    at play.api.mvc.ActionBuilder$$anonfun$async.apply(Action.scala:456) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.ActionBuilder$$anonfun$async.apply(Action.scala:456) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$.invokeBlock(Action.scala:533) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.Action$.invokeBlock(Action.scala:530) ~[play_2.10-2.4.2.jar:2.4.2]
    at play.api.mvc.ActionBuilder$$anon.apply(Action.scala:493) ~[play_2.10-2.4.2.jar:2.4.2]
Caused by: com.datastax.driver.core.exceptions.AuthenticationException: Authentication error on host /10.65.5.44:9042: Username and/or password are incorrect
    at com.datastax.driver.core.Connection.apply(Connection.java:368) ~[cassandra-driver-core-2.1.6.jar:na]
    at com.datastax.driver.core.Connection.apply(Connection.java:338) ~[cassandra-driver-core-2.1.6.jar:na]
    at com.google.common.util.concurrent.Futures$ChainingListenableFuture.run(Futures.java:861) ~[guava-16.0.1.jar:na]
    at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:297) ~[guava-16.0.1.jar:na]
    at com.google.common.util.concurrent.ExecutionList.executeListener(ExecutionList.java:156) ~[guava-16.0.1.jar:na]
    at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:145) ~[guava-16.0.1.jar:na]
    at com.google.common.util.concurrent.AbstractFuture.set(AbstractFuture.java:185) ~[guava-16.0.1.jar:na]
    at com.datastax.driver.core.Connection$Future.onSet(Connection.java:1170) ~[cassandra-driver-core-2.1.6.jar:na]
    at com.datastax.driver.core.Connection$Dispatcher.channelRead0(Connection.java:1000) ~[cassandra-driver-core-2.1.6.jar:na]
    at com.datastax.driver.core.Connection$Dispatcher.channelRead0(Connection.java:922) ~[cassandra-driver-core-2.1.6.jar:na]

Modle.scala中的Lin#121是调用CassandrClient.getValueFromCassandraTable的那个。我通过传递连接到 cassandra 的错误密码来模拟异常，因此获得 AuthroizationException。我希望在 Try catch 中捕获此异常。但它没有被抓住。

（请注意，这与 Future 无关，因为 session.executAysnc 甚至在异常发生时都没有被调用。所以 futures 还没有发挥作用。）

--编辑--

看起来 Play 正在吞下 Exception 并抛出 Error 对象。不确定为什么会这样。

Answer 1

也许你有InterruptedException

例如，如果您不使用 Await，则代码不会打印 "InterruptedException test"。

 try {
   val exception = Future {
     throw new InterruptedException("exception")
   }.recover { case e: Exception => "ok" }

  exception.onComplete {
    case Success(a) => println(a)
    case Failure(err) => println(err)
  }
  //Await.result(exception, 1 seconds) 

 } catch {
   case e: Exception => println("InterruptedException test")
 }

Answer 2

session.execute() 可能会失败并出现类似 IOError 的错误。而你只捕获异常。

检查你是否没有错误，只是微不足道的异常。如果您将堆栈跟踪添加到您的问题，那就太好了。

你可以尝试捕获任何Throwable（实际项目中不要这样做，你可以只为了调试而尝试）

try {
  .... your code ...
} catch {
  case _ => errorHandler(e)
}

只有在 Future 中才会捕获错误。

编辑：

我的 cassandra 数据库上的实例。如果在连接到 cassandra 期间出现异常，它会显示用户列表或 "something going wrong" 字符串。例如错误的节点或 cassandra 正在关闭。

    package controllers

    import com.datastax.driver.core.Cluster
    import com.datastax.driver.core.Session
    import scala.collection.JavaConversions._

    import play.api._
    import play.api.mvc._

    class Application extends Controller {

      def index = Action {
        try{
          var b = new StringBuilder
          for (row <- CassandraClient.getValueFromCassandraTable()) {
            b ++= row.getString("user_id")
            b ++= "\n"
          }
          Ok(b.toString())
        } catch {
          case _ => InternalServerError("something going wrong")
        }
      }

    }

    object CassandraClient {
       private val cluster = Cluster.builder()
        .addContactPoint("localhost")
        .withPort(9042)
        .build()

      val session = cluster.connect()

      def getValueFromCassandraTable() = {
        session.execute("SELECT * FROM mykeyspace.users")
      }
    }

了解 play/scala 错误处理

Understanding play/scala error handling

error-handling

scala

try-catch

playframework-2.0