Scala DynamicVariable 对 ThreadLocal 的替换

Question

看不懂DynamicVariable背后的代码和逻辑：

• 首先你创建一个带有默认值的DynamicVariable实例......一个默认值？？？你想要每个线程的值！这是否意味着您可能在所有线程之间共享相同的默认值？违背了目的……或者？
• 然后我在所有示例中看到 withValue 几乎每次都创建一个新实例，或者？

例如ThreadLocal 有一个有意义的经典案例 SimpleDateFormat 每次创建都非常昂贵，而且它不是线程安全的：

import java.text.SimpleDateFormat;

static ThreadLocal<SimpleDateFormat> dateFormatTl = new ThreadLocal<SimpleDateFormat>();
...
// many threads execute this, check if there already exists a 
// Thread-bound instance otherwise create a new one per-thread
if (dateFormatTl.get() == null) {
  dateFormatTl.set(new SimpleDateFormat("yyyy-dd-MM HH:mm:ss"));
}
// safely use the Thread-bound instance
SimpleDateFormat dateFormat = dateFormatTl.get();
dateFormat.format(new Date(java.util.Date.getTime()));

如何在 Scala 中使用 DynamicVariable 复制上述相同的功能？

// create with default instance ... 
// what for? I don't want the same instance shared across all Threads!
val dateFormatDv = new DynamicVariable[SimpleDateFormat](new SimpleDateFormat("yyyy-dd-MM HH:mm:ss"))

// many threads execute this ... 
// I see it is creating one new instance each time, and 
// that is not what I want
 dateFormatDv.withValue(new SimpleDateFormat("yyyy-dd-MM HH:mm:ss")) {
   // safely use the Thread-bound instance, but this is a new one each time arrrggggg
   dateFormatDv.value.format(new Date(java.util.Date.getTime()))
 }

Answer 1

你想做的可以这样做：

  Future {
    dateFormatDv.withValue(new SimpleDateFormat("yyyy-dd-MM HH:mm:ss")) {
      doStuffWithDateFormat(dateFormatDv.value)
      doMoreStuffWithTheSameFormatInstance(dateFormatDv.value)
    }          
  }

  Future {
    dateFormatDv.withValue(new SimpleDateFormat("yyyy-dd-MM HH:mm:ss")) {
      useADifferentInstanceOfDateFormat(dateFormat.value)
    }
  }

至于默认值，只是让你设置一下，方便你在当前线程中使用，不用.withValue

 doSomethingWithDefaultFormatInstance(dateFormat.value)

Answer 2

我猜 Scala 的 DynamicVariable 的 ThreadLocalification 会更像这样：

// do only once and put in static scope within some Object
val dateFormatDv = new DynamicVariable[Option[SimpleDateFormat]](None)

// this is executed by different threads
def formatTimestamp(timestamp: Long): String = {
  dateFormatDv.value match {
    case None => dateFormatDv.value_=(Some(new SimpleDateFormat("yyyy-dd-MM HH:mm:ss")))
  }
  // safely use the Thread-bound instance
  dateFormatDv.value.map(_.format(new Date(timestamp))).get
}