NSDictionary *dict = @{
                           @"a":@"a"
                           };
 NSArray *arr = @[dict];
 NSError *parseError = nil;
 NSData *jsonData = [NSJSONSerialization dataWithJSONObject:arr options:NSJSONWritingPrettyPrinted  error:&parseError];
 NSString *jsonstr =[[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
 NSData *objectData = [jsonstr dataUsingEncoding:NSUTF8StringEncoding];
 NSDictionary *jsonDic = [NSJSONSerialization JSONObjectWithData:objectData
                             
                                                            options:NSJSONReadingMutableContainers
                             
                                                              error:&parseError];