Avoid using synchronization on any value-based classes

This rule flags any synchronization on instances of value-based classes. Synchronization on the following classes within your application is discouraged:

• java.lang.Byte
• java.lang.Short
• java.lang.Integer
• java.lang.Long
• java.lang.Float
• java.lang.Double
• java.lang.Boolean
• java.lang.Character
• java.lang.Runtime.Version
• java.util.Optional
• java.util.OptionalInt
• java.util.OptionalLong
• java.util.OptionalDouble
• java.time.Instant
• java.time.LocalDate
• java.time.LocalTime
• java.time.LocalDateTime
• java.time.ZonedDateTime
• java.time.ZoneId
• java.time.OffsetTime
• java.time.OffsetDateTime
• java.time.ZoneOffset
• java.time.Duration
• java.time.Period
• java.time.Year
• java.time.YearMonth
• java.time.MonthDay
• java.time.chrono.MinguoDate
• java.time.chrono.HijrahDate
• java.time.chrono.JapaneseDate
• java.time.chrono.ThaiBuddhistDate

Also, synchronization should be avoided on the interface java.lang.ProcessHandle, its implementation classes, and the implementation classes of the collection factories in java.util: List.of, List.copyOf, Set.of, Set.copyOf, Map.of, Map.copyOf, Map.ofEntries, and Map.entry.

For more information on these jdk changes, see What's New in JDK 16.

For more information on the feature change, see Warnings for Value-Based Classes.