Format method in SimlpleDateFormat

[velmurugan]

Hi All,

Everybody knows that the SimpleDateFormat is not thread safe.

If the format of the date same for all the thread then format method in the SimpleDateFormat class is thread safe or not?

If the format of the data is same for all the thead then can we use a singel SimpleDateFormat instance for all the thread?

Does the below sample is thread safe (When 'dd/MM/yyyy' and 'MM/dd/yyyy' formats used) of not?
public class Helper {

private Static final SimpleDateFormat formatDDMMYY = new SimpleDateFormat("dd/MM/yyyy");

private Static final SimpleDateFormat formatMMDDYYYY = new SimpleDateFormat("MM/dd/yyyy");


public String formatDate(String format, Date date) {
SimpleDateFormat format = getFormater(format);
return format.format(date);
}

private SimpleDateFormat getFormater(String format) {
SimpleDateFormat format = null;
if(format.equals("dd/MM/yyyy")) {
format = formatDDMMYY ;
} else if (format.equals("MM/dd/yyyy")) ) {
format = formatMMDDYYYY ;
} else {
format = new SimpleDateFormat(format);
}
return format;
}


}

[Miklos Szeles]

I think it's thread safe in this way. If an object can't change(theoretically immutable) then it is safe to use it from different threads. You can make getFormatter and formatDate static, since they don't use any field from the object.

[Jesper de Jong]

SimpleDateFormat is indeed not thread-safe - this means you must not use the same SimpleDateFormat in different threads at the same time. It doesn't matter if you don't change the format; the SimpleDateFormat object seems to have some internal state that is modified when you call format().

Your code is not thread-safe.

Wrap your SimpleDateFormat object in a ThreadLocal:

public class Helper {
	private static final ThreadLocal<DateFormat> formatDDMMYY = new ThreadLocal<DateFormat>() {
		@Override
		protected DateFormat initialValue() {
			return new SimpleDateFormat("dd/MM/yyyy");
		}
	};

	private static final ThreadLocal<DateFormat> formatMMDDYYYY = new ThreadLocal<DateFormat>() {
		@Override
		protected DateFormat initialValue() {
			return new SimpleDateFormat("MM/dd/yyyy");
		}
	};

	public String formatDate(String format, Date date) {
		SimpleDateFormat format = getFormater(format); 
		return format.format(date); 
	}

	private SimpleDateFormat getFormater(String format) { 
		SimpleDateFormat format = null; 
		if (format.equals("dd/MM/yyyy")) { 
			format = formatDDMMYY.get();
		}
		else if (format.equals("MM/dd/yyyy"))) { 
			format = formatMMDDYYYY.get(); 
		}
		else { 
			format = new SimpleDateFormat(format); 
		} 

		return format; 
	} 
}

p.s. Please use code tags when you post source code.

[Miklos Szeles]

Whoops, you are right Jesper. Sorry for my missleading comment.

[velmurugan]

Thanks Jesper