Pattern: Static DateFormat field
Issue: -
DateFormat objects should not be used as static fields. DateFormats are inherently unsafe for multi-threaded use. Sharing a single instance across thread boundaries without proper synchronization will result in erratic behavior of the application.
Under 1.4 problems seem to surface less often than under Java 5 where you will probably see random ArrayIndexOutOfBoundsException or IndexOutOfBoundsException in sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate(). You may also experience serialization problems. Using an instance field or a ThreadLocal is recommended.
Examples:
// Violations
class SomeClass {
static DateFormat dateFormat1
static java.text.DateFormat dateFormat2
static final DATE1 = DateFormat.getDateInstance(DateFormat.LONG, Locale.FRANCE)
static final def DATE2 = DateFormat.getDateInstance(DateFormat.LONG)
static Object date3 = DateFormat.getDateInstance()
static final DATETIME1 = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.SHORT, Locale.FRANCE)
static final def DATETIME2 = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.SHORT)
static final Object DATETIME3 = DateFormat.getDateTimeInstance()
static final TIME1 = DateFormat.getTimeInstance(DateFormat.LONG, Locale.FRANCE)
static final def TIME2 = DateFormat.getTimeInstance(DateFormat.LONG)
static final Object TIME3 = DateFormat.getTimeInstance()
}
// These usages are OK
class SomeCorrectClass {
private DateFormat calendar1
static ThreadLocal<DateFormat> calendar2
}