如何使一个静态日历线程安全 [英] How to make a static Calendar thread safe

问题描述

我想对某些静态方法使用日历，并使用静态字段：

 私人静态日历calendar = Calendar.getInstance（）; 
  

现在我读了java.util.Calendar不是线程安全的。如何使此线程安全（应为静态）？

解决方案

使事情线程安全，如果不是。在 Calendar 的情况下，甚至从其读取数据不是线程安全的，因为它可以更新内部数据结构。

如果可能的话，建议您使用 Joda Time 代替：

• 大多数类型都是不可变的

$ b

如果你绝对有使用日历，您可以创建一个锁定对象，并通过锁定所有的访问。例如：

  private static final Calendar calendar = Calendar.getInstance（）; 
 private static final Object calendarLock = new Object（）; 
 
 public static int getYear（）
 {
 synchronized（calendarLock）
 {
 return calendar.get（Calendar.YEAR）; 
} 
} 
 
 //同上其他方法
  

这是非常讨厌的。您可以只使用一种同步方法，在每次需要时创建原始日历的克隆，当然...可能通过调用 computeFields 或 computeTime ，您可以使后续读取操作线程安全，当然，但我个人不喜欢尝试。 / p>

I'd like to use a Calendar for some static methods and use a static field:

private static Calendar calendar = Calendar.getInstance();

Now I read java.util.Calendar isn't thread safe. How can I make this thread safe (it should be static)?

解决方案

You can't make something thread-safe if it isn't. In the case of Calendar, even reading data from it isn't thread-safe, as it can update internal data structures.

If at all possible, I'd suggest using Joda Time instead:

• Most of the types are immutable
• The immutable types are thread-safe
• It's a generally much better API anyway

If you absolutely have to use a Calendar, you could create a locking object and put all the access through a lock. For example:

private static final Calendar calendar = Calendar.getInstance();
private static final Object calendarLock = new Object();

public static int getYear()
{
    synchronized(calendarLock)
    {
        return calendar.get(Calendar.YEAR);
    }
}

// Ditto for other methods

It's pretty nasty though. You could have just one synchronized method which created a clone of the original calendar each time it was needed, of course... it's possible that by calling computeFields or computeTime you could make subsequent read-operations thread-safe, of course, but personally I'd be loathe to try it.

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