4

我想玩scala.ref.WeakReference。但是，在尝试实现大事之前，我想尝试检查 scala 控制台中的行为。我尝试了一些事情，但我无法获得要取消引用的对象。这是我的尝试之一：

> class A
defined class A

> class B(var value: A)
defined class B

> new B(new A)
res0: B = B@c8aeb3

> new scala.ref.WeakReference(res0.value)
res1: scala.ref.WeakReference[A] = scala.ref.WeakReferenceWithWrapper@16a5d72

> res0.value = new A

> res1.get // Here I hope to get None
res3: Option[A] = Some(A@135707c)

下面的 oxbow_lakes 给出了另一种尝试。

我还尝试徒劳地显式运行垃圾收集器（调用java.lang.System.gc）。

有没有办法取消引用的内容res1？

4

4 回答 4

5 
Welcome to Scala version 2.8.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class A  
defined class A

scala> class B(var value: A)
defined class B

scala> new B(new A)
res0: B = B@4223d9b

scala> new scala.ref.WeakReference(res0.value)
res1: scala.ref.WeakReference[A] = scala.ref.WeakReferenceWithWrapper@20eb607d

scala> res0.value = new A

scala> System gc

scala> res1 get
res3: Option[A] = None

顺便说一句，如果我将它作为没有显式的脚本运行System gc，它也不会删除引用。所以对我来说，这不是 REPL 的问题，而只是弱引用和垃圾收集器的工作方式。

于 2010-12-09T15:39:38.197 回答
1

运行您的代码，scala -Xprint:parser您将看到var即使在重新分配引用后，它仍然保留了旧值。

我将在这里简化一些事情，只需运行两行代码：

var b=1
b=2

这就是 Scala 打印的内容：

scala> var b=1
[[syntax trees at end of parser]]// Scala source: <console>
package <empty> {
  object line2$object extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    object $iw extends scala.ScalaObject {
      def <init>() = {
        super.<init>();
        ()
      };
      object $iw extends scala.ScalaObject {
        def <init>() = {
          super.<init>();
          ()
        };
        var b = 1                                ///// YOUR CODE HERE
      }
    }
  }
}

[[syntax trees at end of parser]]// Scala source: <console>
package <empty> {                                /////THIS IS AN object
                                                 /////SO PRESUMABLY IT CAN'T BE GC'ED
  object RequestResult$line2$object extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    lazy val scala_repl_value = {                 /////THIS LAZY VAL
      scala_repl_result;                          /////WILL REFERENCE THE OLD VALUE
      line2$object.$iw.$iw.b                      /////EVEN AFTER YOU REASSIGN THE var
    };
    val scala_repl_result: String = {
      line2$object.$iw.$iw;
      "".$plus("b: Int = ").$plus(scala.runtime.ScalaRunTime.stringOf(line2$object.$iw.$iw.b))
    }
  }
}

b: Int = 1

scala> b=2
[[syntax trees at end of parser]]// Scala source: <console>
package <empty> {
  object line3$object extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    object $iw extends scala.ScalaObject {
      def <init>() = {
        super.<init>();
        ()
      };
      import line2$object.$iw.$iw.b;              ///// I DON'T THINK THIS (ORDINARILY ILLEGAL)
                                                  ///// import CONTRIBUTES TO THE PROBLEM
      object $iw extends scala.ScalaObject {
        def <init>() = {
          super.<init>();
          ()
        };
        b = 2;                                    /////YOUR CODE HERE
        val synthvar$0 = b
      }
    }
  }
}

[[syntax trees at end of parser]]// Scala source: <console>
package <empty> {
  object RequestResult$line3$object extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    lazy val scala_repl_value = {
      scala_repl_result;
      line3$object.$iw.$iw.synthvar$0
    };
    val scala_repl_result: String = {
      line3$object.$iw.$iw;
      "".$plus("b: Int = ").$plus(line3$object.$iw.$iw.synthvar$0).$plus("\012")
    }
  }
}

b: Int = 2

编辑：要添加到Debilski 的答案，我认为以下解决方案将允许您根据需要多次重新分配变量，而 REPL 不会保留对旧值的引用：

class A
class B{
   var _value:A = new A
   def value = _value
   def pleaseUpdate( closure: B => Unit ) = closure(this)
}

将您的容器对象定义为：

val b=new B

每当您想更新其中的变量时：

b.pleaseUpdate( _._value = new A )
于 2010-12-09T16:17:21.673 回答
0

var我的想法是明确使用 a并设置为null：

scala> var b = new B(new A)
b: B = B@45033fb5

scala> new scala.ref.WeakReference(b.value)
res0: scala.ref.WeakReference[A] = scala.ref.WeakReferenceWithWrapper@6a7be687

scala> b = null
b: B = null

scala> res0.get
res1: Option[A] = Some(A@79f71773)

但是，它仍然不起作用：REPL 可能正在做一些事情，以保持参考。因此，我不建议使用它来测试References 的使用。

于 2010-12-09T13:44:56.983 回答
0

来自 Java API 文档：

调用 gc 方法表明 Java 虚拟机花费精力回收未使用的对象，以便使它们当前占用的内存可用于快速重用。当控制从方法调用返回时，Java 虚拟机已尽最大努力从所有丢弃的对象中回收空间。

在我的轶事经验中，这意味着它很少触发真正完整的集合，尤其是在可用内存方面没有压力的情况下。如果你对它施加压力，它就会被收集起来。我认为您期望 GC 提供比它提供的更多确定性行为。

Welcome to Scala version 2.8.0.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> var s = new String("Hello, world!")         
s: java.lang.String = Hello, world!

scala> import scala.ref.WeakReference
import scala.ref.WeakReference

scala> val w = new WeakReference(s)  
w: scala.ref.WeakReference[java.lang.String] = scala.ref.WeakReferenceWithWrapper@663f3fbd

scala> s = null
s: java.lang.String = null

scala> Array.ofDim[Byte](1024*1024)
res1: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> Array.ofDim[Byte](1024*1024)
res2: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> Array.ofDim[Byte](1024*1024)
res3: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> Array.ofDim[Byte](1024*1024)
res4: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> Array.ofDim[Byte](1024*1024)
res5: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> Array.ofDim[Byte](1024*1024)
res6: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
scala> System.gc

scala> var g = w.get
g: Option[java.lang.String] = None
于 2010-12-09T15:47:19.000 回答