Scala 惰性值:性能损失?线程安全? [英] Scala lazy values : performance penalty? Threadsafe?

问题描述

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

Scala allows the definition of lazy values

lazy val maybeUnusedValue = someCostlyInitialization

其中 someCostlyInitialization 仅在第一次使用 maybeUnusedValue 时进行评估.也就是说，它最多被评估一次，如果 maybeUnusedValue 从未被使用过，它也永远不会被评估.

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.

这个线程安全吗?这对性能有什么影响?如果这是线程安全的，它必须以某种方式使用某种同步/使用 Java volatile.不幸的是，Scala 语言规范对此一无所知.

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.

推荐答案

使用双重检查锁定使线程安全 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 val 比非惰性 val 慢.

It is made thread-safe using double-checked locking http://code-o-matic.blogspot.com/2009/05/double-checked-locking-idiom-sweet-in.html Obviously this does mean that accessing lazy vals is slower than non-lazy ones.

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