問(wèn)題描述

我想為一些靜態(tài)方法使用日歷并使用靜態(tài)字段:

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

private static Calendar calendar = Calendar.getInstance();

現(xiàn)在我讀到 java.util.Calendar 不是線程安全的.我怎樣才能使這個(gè)線程安全(它應(yīng)該是靜態(tài))?

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

推薦答案

如果它不是線程安全的，你就不能做一些線程安全的東西.在 Calendar 的情況下，即使從它讀取數(shù)據(jù)也不是線程安全的，因?yàn)樗梢愿聝?nèi)部數(shù)據(jù)結(jié)構(gòu).

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.

如果可能的話，我建議改用 Joda Time:

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

• 大多數(shù)類型是不可變的
• 不可變類型是線程安全的
• 無(wú)論如何，它通常是一個(gè)更好的 API

如果您絕對(duì)必須使用 Calendar，則可以創(chuàng)建一個(gè)鎖定對(duì)象并通過(guò)鎖定來(lái)進(jìn)行所有訪問(wèn).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

雖然很惡心.您可以只使用 一個(gè) 同步方法，該方法在每次需要時(shí)創(chuàng)建原始日歷的克隆，當(dāng)然...可以通過(guò)調(diào)用 computeFields 或 computeTime 當(dāng)然，您可以使 后續(xù) 讀取操作線程安全，但我個(gè)人不愿意嘗試它.

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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