getDefaultSharedPreferences(Context) - any context?

问题

I have a helper class for dealing with the default shared preferences. I retrieve the preferences once and I wrap all the SP methods I need, providing the same cached instance. It goes like:

public final class AccessPreferences {

    private static SharedPreferences prefs; // cache

    private static SharedPreferences getPrefs(Context ctx) {
        // synchronized is really needed or volatile is all I need (visibility)
        SharedPreferences result = prefs;
        if (result == null)
            synchronized (AccessPreferences.class) {
                result = prefs;
                if (result == null) {
                    result = prefs = PreferenceManager
                        .getDefaultSharedPreferences(ctx);
                }
            }
        return result;
    }

    public static boolean contains(Context ctx, String key) {
            if (key == null)
                throw new NullPointerException("Null keys are not permitted");
            return getPrefs(ctx).contains(key);
    }
    //etc
}

I have two questions I want to be absolutely sure about:

• Do I need synchronization as I do or a simple volatile would suffice ? Of course this helper class is accessed by different threads (the UI, Intent services etc).
• Do I need to call ctx.getApplicationContext() or not when retrieving the shared preferences ?

I am interested in Froyo and up

回答1:

Use synchronization, but I would suggest using something like a ReentrantLock (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantLock.html) instead of "synchronized". It is generally more performant.

Any context will work, so long as it's not null of course. I would just use the application context here, which will save your callers from having to supply it. Expose a static reference to it, as seen here: Static way to get 'Context' on Android?

来源：https://stackoverflow.com/questions/20846812/getdefaultsharedpreferencescontext-any-context