Yes, this is an atomic operation, in the sense that there can be no reordering or time that will lead to the reading of a byte during partial writing. If the byte is reassigned while it is being read, it is guaranteed that the recipient will be returned a value before or after, but no other value even without volatile .
However, you must have volatile in double or long value to avoid getting incompatible reads that are neither old nor new:
For the purposes of the Java programming language memory model, one record in a non-volatile long or double value is considered as two separate records: one for each 32-bit half. This can lead to a situation where the stream sees the first 32 bits of a 64-bit value from one record and the second 32 bits from another record.
Implementations of the Java virtual machine are recommended to avoid splitting 64-bit values where possible. Programmers are advised to declare common 64-bit values as volatile or to synchronize their programs correctly in order to avoid possible complications.
Source: JLS8 Section 17.7