Java : Formats are generally not synchronized.

From JavaDoc :

https://docs.oracle.com/javase/7/docs/api/java/text/Format.html

Formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

 It allows for formatting (date -> text), parsing (text -> date), and normalization

Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

Example : https://www.callicoder.com/java-simpledateformat-thread-safety-issues/

If using Java 8, it's better to use thread-safe

DateTimeFormatter