斯卡拉懒惰的问题

Question

我有一个场景,我有一些对象需要互相参考.我可以编译的唯一方法是使用lazy

class A(b:B)
class B(a:A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)

我可以使用一些演员做同样的事情,并让它也编译

    abstract class Message
    case class Message1 extends Message
    case class Message2 extends Message

    class Actor1(otherActor:Actor) extends Actor {
        def act() {
            loop {
                react {
                    case Message1 =>
                        println("received message1")
                        otherActor ! Message2
                    case _ =>
                }
            }
        }
    }

    class Actor2(otherActor:Actor) extends Actor {
        def act() {
            loop {
                react {
                    case Message2 =>
                        println("received message2")
                        otherActor ! Message1
                    case _ =>
                }
            }
        }
    }

    lazy val actor1:Actor = new Actor1(actor2)
    lazy val actor2:Actor = new Actor2(actor1)

但是,当我添加以下内容时:

    actor1.start
    actor2.start
    actor1 ! Message1

我收到以下错误:

线程"main"中的异常java.lang.NoClassDefFoundError:com/fictitiousCompany/stackOverflowQuestion/Test由以下引起:java.lang.ClassNotFoundException:com.fictitiousCompany.stackOverflowQuestion.Test at java.net.URLClassLoader $ 1.run(URLClassLoader.java:202 )at java.security.AccessController.doPrivileged(Native Method)at java.net.URLClassLoader.findClass(URLClassLoader.java:190)at java.lang.ClassLoader.loadClass(ClassLoader.java:307)at sun.misc.Launcher $ java.lang.ClassLoader.loadClass上的AppClassLoader.loadClass(Launcher.java:301)(ClassLoader.java:248)

我正在使用Scala Eclipse Plugin 2.8.1.

Answer 1

请注意,即使是较小的示例也会出现问题(在REPL中):

{
class A(b:B)
class B(a:A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)
a
}
// causes stack overflow error

一旦a需要进行评估,它就需要B,这需要A.为了使其工作a或b必须完成构建.

使用by-name参数可以评估较小的示例.

{
class A(b: => B)
class B(a: => A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)
a
}

请注意,这是否也适用于您的演员示例.

编辑:按名称params在2.8.0本地工作.我用对象替换了case类来摆脱一些弃用警告,并在actor1,actor2上添加了启动方法,并用它来完成整个过程actor1 ! Message1.除此之外,我之前没有使用演员,所以我不能评论更多.这是我测试的:

import scala.actors._

abstract class Message
object Message1 extends Message
object Message2 extends Message

class Actor1(otherActor: => Actor) extends Actor {
def act() {
    loop {
    react {
        case Message1 =>
        println("received message1")
        otherActor ! Message2
        case _ =>
    }
    }
}
}

class Actor2(otherActor: => Actor) extends Actor {
def act() {
    loop {
    react {
        case Message2 =>
        println("received message2")
        otherActor ! Message1
        case _ =>
    }
    }
}
}

{
  lazy val actor1:Actor = new Actor1(actor2)
  lazy val actor2:Actor = new Actor2(actor1)
  actor1.start
  actor2.start
  actor1 ! Message1
}

打印一堆:

received message1
received message2