Static fields of type java.text.DateFormat or its descendants should be avoided because the class DateFormat is not thread-safe.

Use instance fields instead and synchronize access where necessary.

In the following example, DateFormattingThread declares a static field dateF of type DateFormat. When instances of DateFormattingThread are created and run by DateFormatThreadUnsafe, erroneous results are output because dateF is shared by all instances of DateFormattingThread.

In the following modification of the above example, DateFormattingThread declares an instance field dateF of type DateFormat. When instances of DateFormattingThread are created and run by DateFormatThreadUnsafeFix, correct results are output because there is a separate instance of dateF for each instance of DateFormattingThread.

Java API Documentation: java.text.DateFormat synchronization.