应如何“双重检查锁定”在Delphi中实现？ [英] How should "Double-Checked Locking" be implemented in Delphi?

问题描述

在C＃中，以下代码（来自此页面）可以用于懒惰以线程安全的方式实例化单例类：

  class Foo {
 private volatile Helper helper = null; 
 public Helper getHelper（）{
 if（helper == null）{
 lock（this）{
 if（helper == null）
 helper = new Helper （）; 
} 
} 
 return helper; 
} 
} 
  

什么是等效的线程安全的Delphi代码？ / p>

---

该文章还提到了Java中双重检查锁定的两个问题：

• 在辅助引用指向新创建的对象之前，新对象可能被构造，意味着创建两个对象


• 它有可能在对象仍在创建时帮助引用指向一个内存块，这意味着对不完整对象的引用将被返回

所以虽然上述文章中的C＃和Java版本的代码看起来几乎相同，但只有C＃版本可以按预期方式工作。这还有一个额外的问题，如果这两个问题也存在于Delphi版本的Double-Checked Locking？

解决方案

使用System.TMonitor以线程安全的方式锁定对象实例。

  function TFoo.GetHelper（）：THelper; 
 begin 
如果没有分配（FHelper）然后
 begin 
 System.MonitorEnter（Self）; 
尝试
如果没有分配（FHelper）然后
 FHelper：= THelper.Create（）; 
 finally 
 System.MonitorExit（Self）; 
结束
结束
结果：= FHelper; 
结束
  

如需进一步参考，请参阅锁定我的对象...，请！从 Allen Bauer 。其实，代表。我从这里收集应该去艾伦。

In C#, the following code (from this page) can be used to lazily instantiate a singleton class in a thread safe way:

  class Foo {
        private volatile Helper helper = null;
        public Helper getHelper() {
            if (helper == null) {
                lock(this) {
                    if (helper == null)
                        helper = new Helper();
                }
            }
            return helper;
        }
    }

What would be the equivalent thread safe Delphi code?

---

The article also mentions two problems with Double Checked Locking in Java:

• it is possible that the new object is constructed before the helper reference is made to point at the newly created object meaning that two objects are created
• it is possible that the helper reference is made to point at a block of memory while the object is still being created meaning that a reference to an incomplete object will be returned

So while the code of the C# and the Java version in the mentioned article look almost identical, only the C# version works as expected. Which leads to the additional question if these two problems also exist in a Delphi version of Double-Checked Locking?

解决方案

Use System.TMonitor to lock the object instance in a thread safe way.

function TFoo.GetHelper(): THelper;
begin
  if not Assigned(FHelper) then
  begin
    System.MonitorEnter(Self);
    try
      if not Assigned(FHelper) then
        FHelper := THelper.Create();
    finally
      System.MonitorExit(Self);
    end;
  end;
  Result := FHelper;
end;

For further reference look at Lock my object..., please! from Allen Bauer. In fact, the rep. I gather from this should go to Allen.

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