我们什么时候应该使用 scala.util.DynamicVariable?

问题描述

当我阅读scalatra的源代码时，我发现有一些代码如下:

When I read the source of scalatra, I found there are some code like:

protected val _response   = new DynamicVariable[HttpServletResponse](null)
protected val _request    = new DynamicVariable[HttpServletRequest](null)

有一个有趣的类，名为DynamicVariable.我看过这门课的文档，但我不知道我们什么时候以及为什么应该使用它?它有一个通常使用的 withValue().

There is an interesting class named DynamicVariable. I've looked at the doc of this class, but I don't know when and why we should use it? It has a withValue() which is usually be used.

如果我们不使用它，那么我们应该使用什么代码来解决它解决的问题?

If we don't use it, then what code we should use instead, to solve the problem it solved?

(我是 Scala 的新手，如果你能提供一些代码，那就太好了)

(I'm new to scala, if you can provide some code, that will be great)

推荐答案

DynamicVariable 是贷款和动态范围模式的实现.DynamicVariable 的用例与 Java 中的 ThreadLocal 非常相似(事实上，DynamicVariable 使用 InheritableThreadLocal> 在幕后) - 当您需要在封闭范围内进行计算时使用它，其中每个线程都有自己的变量值副本:

DynamicVariable is an implementation of the loan and dynamic scope patterns. Use-case of DynamicVariable is pretty much similar to ThreadLocal in Java (as a matter of fact, DynamicVariable uses InheritableThreadLocal behind the scenes) - it's used, when you need to do a computation within an enclosed scope, where every thread has it's own copy of the variable's value:

dynamicVariable.withValue(value){ valueInContext =>
  // value used in the context
}

鉴于 DynamicVariable 使用可继承的 ThreadLocal，变量的值被传递给上下文中产生的线程:

Given that DynamicVariable uses an inheritable ThreadLocal, value of the variable is passed to the threads spawned in the context:

dynamicVariable.withValue(value){ valueInContext =>
  spawn{
    // value is passed to the spawned thread
  }
}

DynamicVariable(和 ThreadLocal)在 Scalatra 中使用的原因与在许多其他框架(Lift、Spring、Struts 等)中使用的原因相同 - 它是非- 存储和传递上下文(线程)特定信息的侵入式方式.

DynamicVariable (and ThreadLocal) is used in Scalatra for the same reason it's used in many other frameworks (Lift, Spring, Struts, etc.) - it's a non-intrusive way to store and pass around context(thread)-specific information.

制作 HttpServletResponse 和 HttpServletRequest 动态变量(并因此绑定到处理请求的特定线程)只是在代码中的任何位置获取它们的最简单方法(不传递方法参数或以其他任何方式显式传递).

Making HttpServletResponse and HttpServletRequest dynamic variables (and, thus, binding to a specific thread that processes request) is just the easiest way to obtain them anywhere in the code (not passing through method arguments or anyhow else explicitly).

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