SimpleDateFormatThreadSafe

gistfile1.java

package com.foocoders.text;

import java.text.AttributedCharacterIterator;
import java.text.DateFormatSymbols;
import java.text.FieldPosition;
import java.text.NumberFormat;
import java.text.ParseException;
import java.text.ParsePosition;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

public class SimpleDateFormatThreadSafe extends SimpleDateFormat {

	private static final long serialVersionUID = 5448371898056188202L;
	ThreadLocal<SimpleDateFormat> localSimpleDateFormat;

	public SimpleDateFormatThreadSafe() {
		super();
		localSimpleDateFormat = new ThreadLocal<SimpleDateFormat>() {
			protected SimpleDateFormat initialValue() {
				return new SimpleDateFormat();
			}
		};
	}

	public SimpleDateFormatThreadSafe(final String pattern) {
		super(pattern);
		localSimpleDateFormat = new ThreadLocal<SimpleDateFormat>() {
			protected SimpleDateFormat initialValue() {
				return new SimpleDateFormat(pattern);
			}
		};
	}

	public SimpleDateFormatThreadSafe(final String pattern, final DateFormatSymbols formatSymbols) {
		super(pattern, formatSymbols);
		localSimpleDateFormat = new ThreadLocal<SimpleDateFormat>() {
			protected SimpleDateFormat initialValue() {
				return new SimpleDateFormat(pattern, formatSymbols);
			}
		};
	}

	public SimpleDateFormatThreadSafe(final String pattern, final Locale locale) {
		super(pattern, locale);
		localSimpleDateFormat = new ThreadLocal<SimpleDateFormat>() {
			protected SimpleDateFormat initialValue() {
				return new SimpleDateFormat(pattern, locale);
			}
		};
	}

	public Object parseObject(String source) throws ParseException {
		return localSimpleDateFormat.get().parseObject(source);
	}

	public String toString() {
		return localSimpleDateFormat.get().toString();
	}

	public Date parse(String source) throws ParseException {
		return localSimpleDateFormat.get().parse(source);
	}

	public Object parseObject(String source, ParsePosition pos) {
		return localSimpleDateFormat.get().parseObject(source, pos);
	}

	public void setCalendar(Calendar newCalendar) {
		localSimpleDateFormat.get().setCalendar(newCalendar);
	}

	public Calendar getCalendar() {
		return localSimpleDateFormat.get().getCalendar();
	}

	public void setNumberFormat(NumberFormat newNumberFormat) {
		localSimpleDateFormat.get().setNumberFormat(newNumberFormat);
	}

	public NumberFormat getNumberFormat() {
		return localSimpleDateFormat.get().getNumberFormat();
	}

	public void setTimeZone(TimeZone zone) {
		localSimpleDateFormat.get().setTimeZone(zone);
	}

	public TimeZone getTimeZone() {
		return localSimpleDateFormat.get().getTimeZone();
	}

	public void setLenient(boolean lenient) {
		localSimpleDateFormat.get().setLenient(lenient);
	}

	public boolean isLenient() {
		return localSimpleDateFormat.get().isLenient();
	}

	public void set2DigitYearStart(Date startDate) {
		localSimpleDateFormat.get().set2DigitYearStart(startDate);
	}

	public Date get2DigitYearStart() {
		return localSimpleDateFormat.get().get2DigitYearStart();
	}

	public StringBuffer format(Date date, StringBuffer toAppendTo, FieldPosition pos) {
		return localSimpleDateFormat.get().format(date, toAppendTo, pos);
	}

	public AttributedCharacterIterator formatToCharacterIterator(Object obj) {
		return localSimpleDateFormat.get().formatToCharacterIterator(obj);
	}

	public Date parse(String text, ParsePosition pos) {
		return localSimpleDateFormat.get().parse(text, pos);
	}

	public String toPattern() {
		return localSimpleDateFormat.get().toPattern();
	}

	public String toLocalizedPattern() {
		return localSimpleDateFormat.get().toLocalizedPattern();
	}

	public void applyPattern(String pattern) {
		localSimpleDateFormat.get().applyPattern(pattern);
	}

	public void applyLocalizedPattern(String pattern) {
		localSimpleDateFormat.get().applyLocalizedPattern(pattern);
	}

	public DateFormatSymbols getDateFormatSymbols() {
		return localSimpleDateFormat.get().getDateFormatSymbols();
	}

	public void setDateFormatSymbols(DateFormatSymbols newFormatSymbols) {
		localSimpleDateFormat.get().setDateFormatSymbols(newFormatSymbols);
	}

	public Object clone() {
		return localSimpleDateFormat.get().clone();
	}

	public int hashCode() {
		return localSimpleDateFormat.get().hashCode();
	}

	public boolean equals(Object obj) {
		return localSimpleDateFormat.get().equals(obj);
	}

}

The idea is not bad but you shoud not extend from SimpleDateFormat and you should remove methods which can manipulate the SimpleDateFormat. Otherwise the SimpleDateFormat can be manipulated and you will have other states in different threads!

final SimpleDateFormatThreadSafe formatter = new SimpleDateFormatThreadSafe("yyyy-MM-dd'T'HH:mm:ssZ");

Thread thread = new Thread(new Runnable() {
    public void run() {
        System.out.println(formatter.format(new Date())); // 2021-05-26T16:55:07+0200
        formatter.applyPattern("yyyy-MM-dd");
        System.out.println(formatter.format(new Date())); // 2021-05-26
    }
});

thread.start();
thread.join();

thread = new Thread(new Runnable() {
    public void run() {
        System.out.println(formatter.format(new Date())); // 2021-05-26T16:55:07+0200 <- The `applyPattern` method has no effect in this thread! inconsistent ! not thread safe
    }
});

thread.start();
thread.join();