Today I faced a very common problem and I did not find out how to solve it for some time. I am using Java Beans Validation (JSR 380), so this means that I can use annotations in my class and at the time of validation, my framework (in this case Springboot) is automatically trying to validate the values of the class.
Example:

public class ClassToValidate {
  @NotNull
  private String email;
}

This works fine, and I can add annotations like:

• @NotNull
• @Size(min = 2, max = 64)
• etc…

But my problem was that I wanted to have a special kind of validation:

• If the field is null, this is ok for me.
• But if the field is not null, then I want to add special validation (like checking that the minimal and maximal length of the string is validated and is between 2 and 64 chars).

The Java Beans validation provides a very helpful way how to create custom annotations, and this is what is needed to have such a conditional (OR) validation logic. Just create your own annotations, and combine them using OR:

public class ClassToValidate {
    @ConstraintComposition(CompositionType.OR)
    @Null
    @Size(min =2, max=256)
    @Target({ElementType.METHOD, ElementType.FIELD})
    @Retention(RetentionPolicy.RUNTIME)
    @Constraint(validatedBy = {})
    public @interface EmailOrNull {
        String message() default "Not valid";

        Class[] groups() default {};

        Class[] payload() default {};
    }

    @EmailOrNull
    private String email;
}

I found some interesting information here:
JBOSS Hibernate validator documentation