Scala 中的多线程——只处理不变性 [英] Multi-threading in Scala -- dealing only with immutability

问题描述

我有Scala的代码

class MyClass {
  private val myData: Map[String, MyClass2] = new HashMap[String, MyClass2]()

  def someMethod = {
      synchronized(myData) {
        val id = getSomeId
        if (myData.containsKey(id)) myData = myData.remove(id)
        else Log.d("123", "Not found!")
      }
  }

  def getSomeId = //....
}

我想知道，是否有可能在不使用 synchronized 并且不涉及其他一些库(例如 Akka 或任何其他库)的情况下保持此代码线程安全strong>(类)甚至内置在 Java 或 Scala 中?

I wonder, is it possible to remain this code thread-safe without using synchronized and without involving some other libraries such Akka or any other libraries (classes) even built-in in Java or Scala?

理想情况下，我只想通过使用不变性的概念(Java 中的 final，如果您愿意)来使线程安全.

Ideally, I'd like to make thread-safe only by using the conception of immutability (final in Java, if you will).

更新:

class MyClass(myData: Map[String, MyClass2] = new HashMap[String, MyClass2]()) {

  def someMethod = {
      synchronized(myData) {
        val id = getSomeId
        if (myData.containsKey(id)) new MyClass(myData.remove(id))
        else {
           Log.d("123", "Not found!")
           this
         }
      }
  }

  def getSomeId = //....
}

推荐答案

只有让 MyClass 也是不可变的(并且让它只使用不可变的数据结构)，你才能解决不变性问题.原因很简单:如果MyClass是可变的，那么你必须通过并发线程同步修改.

You can solve the problem with immutability only if you make MyClass immutable too (and let it use only immutable data structures as well). The reason is simple: If MyClass is mutable, then you have to synchronize modifications by concurrent threads.

这需要不同的设计 - 每个导致 MyClass 实例更改"的操作都将返回一个(可能)修改过的实例.

This requires a different design - every operation that causes an instance of MyClass to "change" will instead return a (possibly) modified instance.

import collection.immutable._

class MyClass2 {
  // ...
}

// We can make the default constructor private, if we want to manage the
// map ourselves instead of allowing users to pass arbitrary maps
// (depends on your use case):
class MyClass private (val myData: Map[String,MyClass2]) {
  // A public constructor:
  def this() = this(new HashMap[String,MyClass2]())

  def someMethod(id: String): MyClass = {
    if (myData.contains(id))
      new MyClass(myData - id) // create a new, updated instance
    else {
      println("Not found: " + id)
      this // no modification, we can return the current
            // unmodified instance
    }
  }

  // other methods for adding something to the map
  // ...
}

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