这是双重检查锁定吗？ [英] Is this broken double checked locking?

问题描述

Checkstyle将此代码报告为双重检查锁定习惯用法已损坏，但我认为我的代码实际上并未受到双重检查锁定问题的影响。

Checkstyle reports this code as "The double-checked locking idiom is broken", but I don't think that my code actually is affected by the problems with double-checked locking.

如果具有该id的行不存在，则代码应该在数据库中创建一行。它在多线程环境中运行，我想避免主键存在的SQL异常。

The code is supposed to create a row in a database if a row with that id doesn't exist. It runs in a multi-threaded environment and I want to avoid the primary-key-exists SQL-exceptions.

伪代码：

private void createRow(int id) {
  Row row = dao().fetch(id);
  if (row == null) {
     synchronized (TestClass.class) {
        row = dao().fetch(id);
        if (row == null) {
           dao().create(id);
        }
     }
  }
}

I可以同意它看起来像双重检查锁定，但我没有使用静态变量，fetch（）和create（）中的代码可能太复杂而无法内联并且无序。

I can agree that it looks like double-checked locking, but I am not using static variables and the code in fetch() and create() is probably too complex to be inlined and put out of order.

我错了还是格式？ ：）

Am I wrong or checkstyle? :)

推荐答案

假设您想要读取最内线：

Assuming you want that innermost line to read:

row = dao().create(id);

假设 dao（），这不是经典的双重检查锁定问题。 与create方法正确互斥。

It's not a classic double-checked lock problem assuming dao().fetch is properly mutexed from the create method.

编辑 :(代码已更新）

双重检查锁的经典问题是在初始化发生之前分配了一个值，其中两个线程正在访问相同的值。

The classic problem of a double-checked lock is having a value assigned before initialization occurs where two threads are accessing the same value.

假设DAO正确同步并且不会返回部分初始化的值，这不会受到双重检查锁定习惯的缺陷的影响。

Assuming the DAO is properly synchronized and will not return a partially initialized value, this doesn't suffer from the flaws of the double-checked lock idiom.

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