当我阅读 scalatra 的源代码时，我发现有一些代码如下：
protected val _response = new DynamicVariable[HttpServletResponse](null)
protected val _request = new DynamicVariable[HttpServletRequest](null)
有一个有趣的类名为DynamicVariable. 我看过这个类的文档，但我不知道什么时候以及为什么我们应该使用它？它有一个withValue()通常被使用的。
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？
（我是scala的新手，如果你能提供一些代码，那就太好了）
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
}
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 (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.
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).
Vasil 很好地回答了这个问题，但我将添加一个额外的简单示例，可能会进一步帮助理解。
假设我们必须使用一些使用 println() 的代码来写整个标准输出。我们希望此输出转到日志文件，但我们无权访问源。
println()用途Console.println()Console.println()（幸运的是）基于默认DynamicVariable[PrintStream]为java.lang.System.outConsole定义withOut只转发到动态变量的withValue我们可以使用它来简单地解决我们的问题：
def noisy() { println("robot human robot human") }
noisy() // prints to stdout
val ps = new java.io.PrintStream("/tmp/mylog")
scala.Console.withOut(ps) { noisy() } // output now goes to /tmp/mylog file
这是一个最小的片段：
val dyn = new DynamicVariable[String]("withoutValue")
def print=println(dyn.value)
print
dyn.withValue("withValue") {
print
}
print
输出将是：
withoutValue
withValue
withoutValue