public boolean isValid(String s,
ConstraintValidatorContext constraintValidatorContext) {
if ( s == null ) {
return true;
}
...

Why would you ever return true (i.e. valid) for an input that is null? That doesn’t make sense until you realise that validation is a *combination* of all constraints.
This means that your validator should only return false for your specific test. If it fails for a chained validation, for example if input is null, it should return true and rely on the NotNull validation to report that error.
You still have to guard for null and empty inputs but return true if they occur rather then returning false.
Hide any required basic validations inside your custom constraint annotation. For example NotBlank does that:

...
@NotNull
public @interface NotBlank {
...

The post JSR 303: Combining custom and standard validators appeared first on The Story of David Kaspar.