java - Making DateFormat Threadsafe. What to use, synchronized or Thread local

Question

I want to make following code thread safe. What is the best way to achieve it?

private static final DateFormat DATE_FORMAT = DateFormat.getDateTimeInstance();

public static final String eventTypeToDateTimeString(long timestamp)
{
   return DATE_FORMAT.format(new Date(timestamp));
}

Answer 1

Avoid the legacy date-time classes

The troublesome old date-time classes bundled with the earliest versions of Java have been supplanted by the java.time classes. The java.time classes are thread-safe and use immutable objects.

java.time

Replace your formatter and date types with java.time types to automatically get thread-safety.

Define your DateTimeFormatter globally if so desired. That class can automatically localize the string being generated, or you can specify a certain format.

• Specify a FormatStyle to determine length of abbreviation.
• Specify a Locale to determine (a) the human language for translation of name of day, name of month, and such, and (b) the cultural norms deciding issues of abbreviation, capitalization, punctuation, and such.
• Specify a ZoneId for a time zone in which to adjust the moment.

The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds. The ZonedDateTime class adjusts an Instant into a particular time zone.

Your code, translated to java.time classes. In real work I would break this out into multiple lines, and would trap for exceptions.

private static final DateTimeFormatter DATE_TIME_FORMATTER = DateTimeFormatter.ofLocalizedDateTime( FormatStyle.FULL ).withLocale( Locale.CANADA_FRENCH ) ;
private static final ZoneId ZONE_ID = ZoneId.of( "America/Montreal" );

public static final String eventTypeToDateTimeString(long timestamp)
{
   return Instant.ofEpochMilli( timestamp ).atZone( ZONE_ID ).format( DATE_TIME_FORMATTER );
}

Do not track date-time as count-from-epoch

I do not advise passing around a long as a way of representing date-time values. Makes debugging and logging difficulty as a human cannot discern the meaning of the date-time value. Instead, pass around the java.time types such as Instant. Using the java.time types provides type-safety and makes your code more self-documenting.

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.