Instead of receiving a JsonPatch directly from the client, define a DTO to handle the validation and then you will later convert the DTO instance to a JsonPatch.

Say you want to update a user of instance User.class, you can define a DTO such as:

public class UserDTO {

    @Email(message = "The provided email is invalid")
    private String username;

    @Size(min = 2, max = 10, message = "firstname should have at least 2 and a maximum of 10 characters")
    private String firstName;

    @Size(min = 2, max = 10, message = "firstname should have at least 2 and a maximum of 10 characters")
    private String lastName;

    @Override
    public String toString() {
        return new Gson().toJson(this);
    }

//getters and setters
}

The custom toString method ensures that fields that are not included in the update request are not prefilled with null values.

Your PATCH request can be as follows(For simplicity, I didn't cater for Exceptions)

@PatchMapping("/{id}")
    ResponseEntity<Object> updateUser(@RequestBody @Valid UserDTO request,
                                      @PathVariable String id) throws ParseException, IOException, JsonPatchException {
        User oldUser = userRepository.findById(id);
        String detailsToUpdate = request.toString();
        User newUser = applyPatchToUser(detailsToUpdate, oldUser);
        userRepository.save(newUser);
        return userService.updateUser(request, id);
    }

The following method returns the patched User which is updated above in the controller.

private User applyPatchToUser(String detailsToUpdate, User oldUser) throws IOException, JsonPatchException {
        ObjectMapper objectMapper = new ObjectMapper();
        // Parse the patch to JsonNode
        JsonNode patchNode = objectMapper.readTree(detailsToUpdate);
        // Create the patch
        JsonMergePatch patch = JsonMergePatch.fromJson(patchNode);
        // Convert the original object to JsonNode
        JsonNode originalObjNode = objectMapper.valueToTree(oldUser);
        // Apply the patch
        TreeNode patchedObjNode = patch.apply(originalObjNode);
        // Convert the patched node to an updated obj
        return objectMapper.treeToValue(patchedObjNode, User.class);
    }