Converting java.sql.timestamp to millisecond format - spring-boot

Question

I have a private java.sql.Timestamp myDate in some model (POJO) class like below
private String isActive;
private Date dob;
private Timestamp createdDate;
private Timestamp lastMktPrefUpdateAt;
We were using spring boot version to 1.5.10.RELEASE and REST API response for timestamp field was in millisecond format like below -
{
"isActive": "y",
"lastMktPrefUpdateAt": 1632195609787,
"dob": "08/12/1991",
"createdDate": 1632195609788
}
We have recently upgraded the spring boot version to 2.3.0.RELEASE and this started sending timestamp field in response to some ISO format like below -
{
"isActive": "y",
"lastMktPrefUpdateAt": "2021-09-20T22:10:09.787+0000",
"dob": "07/12/1991",
"createdDate": "2021-09-20T22:10:09.788+0000"
}
Can someone please help answering, how can we format Timestamp back to millisecond format?
We need to format Timestamp at global level meaning without changing all the POJO classes.

Answer 1

Try this in your properties file
spring.jackson.serialization.write-dates-as-timestamps:true
OR
Use #JsonFormat(shape = JsonFormat.Shape.NUMBER) above the property that you want.
Update
If you want to add millis to your timestamp, try to pass the long value to the Timestamp constructor and call the getTime() method to receive a 13-digits timestamp.
long now = System.currentTimeMillis();
Timestamp timeStamp= new Timestamp(now);
System.out.println(timeStamp.getTime());

Answer 2

You could convert that string to a ZonedDateTime object which has a .toInstant().toEpochMilli() method.
Something like:
long millis = ZonedDateTime.of(LocalDateTime.parse(str), ZoneId.of("Etc/UTC")).toInstant().toEpochMilli();
see:
How to get milliseconds from LocalDateTime in Java 8
https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html
https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html
However, I would recommend refactoring the system to use the immutable LocalDatTime object at a later point.
(There may be an appropriate annotation you can put on that timestamp field to parse it in a specific way, but not one that I am aware of.)