Scala 惰性值：性能损失？线程安全？

发布于 2024-09-30 06:07:47 字数 600 浏览 9 评论 0原文

可能的重复：
惰性 val 的（隐藏）成本是多少？（斯卡拉）

Scala 允许定义惰性值

lazy val maybeUnusedValue = someCostlyInitialization

，其中仅在第一次使用 maybeUnusedValue 时评估 someCostlyInitialization。也就是说，它最多被评估一次，如果从未使用过maybeUnusedValue，那么它也根本不会被评估。

这是线程安全的吗？这对性能有何影响？如果这是线程安全的，它必须使用某种同步/以某种方式使用 Java 易失性。不幸的是，Scala 语言规范对此没有任何说明。

原文

Possible Duplicate:
What's the (hidden) cost of lazy val? (Scala)

Scala allows the definition of lazy values

lazy val maybeUnusedValue = someCostlyInitialization

where someCostlyInitialization is evaluated only on the first use of maybeUnusedValue. That is, it is evaluated at most once, and if maybeUnusedValue is never used, it is also never evaluated at all.

Is this threadsafe? What are the performance implications of this?
If this is to be threadsafe, it has to use some kind of syncronization / use Java volatile in some way. Unfortunately the Scala language specification says nothing about this.

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走过海棠暮 2024-10-07 06:07:47

使用双重检查锁定使其成为线程安全的 http://code-o-matic.blogspot.com/2009/05/double-checked-locking-idiom-sweet-in.html 显然，这确实意味着访问惰性值比访问惰性值慢非懒惰者。

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肥爪爪 2024-10-07 06:07:47

更新：哎呀，正如瓦西尔指出的那样，这个问题是另一个线程的副本，而且碰巧的是，这个答案也是如此。

我参加了这门课程：

class Foo {
  lazy val test = "hi"
}

编译和反编译（使用 jd-gui）：

public class Foo
  implements ScalaObject
{
  private String test;
  public volatile int bitmap$0;

  public String test()
  {
    if (
      (this.bitmap$0 & 0x1) == 0);
    synchronized (this)
    {
      if (
        (this.bitmap$0 & 0x1) == 0) {
        this.test = "hi"; this.bitmap$0 |= 1; } return this.test;
    }
  }
}

正如您所看到的，它使用带有易失性变量的双重检查范例。所以我认为这是安全的

UPDATE: OOPS, as Vasil pointed out, the question is a copy of another thread, and as it happens, so is this answer.

I took this class:

class Foo {
  lazy val test = "hi"
}

Compiled and decompiled (with jd-gui):

public class Foo
  implements ScalaObject
{
  private String test;
  public volatile int bitmap$0;

  public String test()
  {
    if (
      (this.bitmap$0 & 0x1) == 0);
    synchronized (this)
    {
      if (
        (this.bitmap$0 & 0x1) == 0) {
        this.test = "hi"; this.bitmap$0 |= 1; } return this.test;
    }
  }
}

As you can see it is using the double check paradigm with a volatile variable. So I think it is safe

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