问题描述

我想为一些静态方法使用日历并使用静态字段:

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

private static Calendar calendar = Calendar.getInstance();

现在我读到 java.util.Calendar 不是线程安全的.我怎样才能使这个线程安全(它应该是静态)?

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

推荐答案

如果它不是线程安全的，你就不能做一些线程安全的东西.在 Calendar 的情况下，即使从它读取数据也不是线程安全的，因为它可以更新内部数据结构.

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.

如果可能的话，我建议改用 Joda Time:

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

• 大多数类型是不可变的
• 不可变类型是线程安全的
• 无论如何，它通常是一个更好的 API

如果您绝对必须使用 Calendar，则可以创建一个锁定对象并通过锁定来进行所有访问.例如:

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

虽然很恶心.您可以只使用 一个 同步方法，该方法在每次需要时创建原始日历的克隆，当然...可以通过调用 computeFields 或 computeTime 当然，您可以使 后续 读取操作线程安全，但我个人不愿意尝试它.

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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