NSDateFormatter is not thread safe

NSDateFormatter's right usage

// test code that generate the error
for (int i = 0; i < 1000; i++) {
    NSDate *date= [NSDate date];
    NSString *queueName = [NSString stringWithFormat:@"queue%i", i];
    dispatch_queue_t testqueue = dispatch_queue_create([queueName cStringUsingEncoding:NSUTF8StringEncoding], NULL);
    dispatch_async(testqueue, ^{
        NSDateFormatter *ndf = [BXAppDelegate dateFormatter];
        DDLogInfo(@"%@", [ndf stringFromDate:date]);
    });
}


// Try 1  code wrong
+ (NSDateFormatter *)dateFormatter
{
    static NSDateFormatter *dateFormatter = nil ;
    if (dateFormatter == nil)
    {
        dateFormatter = [[NSDateFormatter alloc] init] ;
        [dateFormatter setLocale: [[NSLocale alloc] initWithLocaleIdentifier: @"en_GB"] ] ;
        [dateFormatter setDateFormat: @"YYYY-MM-dd HH:mm:ss ZZZ"] ;
    }
    return dateFormatter ;
}

// right try
+ (NSDateFormatter *)dateFormatter2
{
    NSMutableDictionary *threadDic = [[NSThread currentThread] threadDictionary];
    NSDateFormatter *dateFormatter = threadDic[@"BXMyDateFormatter"];
    if (dateFormatter == nil) {
        dateFormatter = [[NSDateFormatter alloc] init];
        [dateFormatter setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_GB"]];
        [dateFormatter setDateFormat:@"YYYY-MM-dd HH:mm:ss ZZZ"];
        [threadDic setObject:dateFormatter forKey:@"BXMyDateFormatter"];
    }
    return dateFormatter;
}