Validating Optional Field

I have a following request json

{
 "ParameterA":"value",
 "ParameterB":[
  {
    "key":"",
  "value":""
  },
  {
    "key":"",
  "value":""
  }
    ]
}

I am trying to validate this json , my requirement is as such that if ParameterB is present then a list shd be present inside for sure, else ParameterB is optional.Such as if ParameterB itself only not present there shd nt be a problem.

I am looking for an java validation annotation for the same. i have used @NotNull on key and value but not able to decide what to use on ParameterB since it is the array inside which list is present

Since it has a database connectivity as well , I am using JPA and making this REST call.

I tried @NotEmpty on ParameterB but that doesnt help. I want an annotation so that if ParameterB itself is not present it shd nt affect the actual flow. Following Case

{
 "ParameterA":"value"
}

any suggestion or help will be appreciated.

The validation your are looking for is non standard one. I don't know about the existing annotation that implements this kind of validation.

Fortunately for you javax.validation constraints can be easily extended with custom validation rules:

a) Create a constraint annotation

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import javax.validation.Constraint;
import javax.validation.Payload;

@Target({METHOD, FIELD, ANNOTATION_TYPE})
@Retention(RUNTIME)
@Constraint(validatedBy = NullOrNotEmptyValidator.class)
@Documented
public @interface NullOrNotEmpty {

    String message() default "{com.mycompany.constraints.nullornotempty}";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

}

b) Implement a validator

import java.lang.reflect.Array;
import java.util.Map;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class NullOrNotEmptyValidator implements ConstraintValidator<NullOrNotEmpty, Object> {

    @Override
    public void initialize(NullOrNotEmpty constraintAnnotation) {
    }

    @Override
    public boolean isValid(Object object, ConstraintValidatorContext constraintContext) {

        if (object == null) {
            return true;
        }

        final Class<?> fieldClass = object.getClass();
        if (fieldClass.isArray()) {
            return Array.getLength(object) > 0;
        }

        if (Iterable.class.isAssignableFrom(fieldClass)) {
            return ((Iterable) object).iterator().hasNext();
        }

        if (Map.class.isAssignableFrom(fieldClass)) {
            return !((Map) object).isEmpty();
        }

        return false;
    }

}

c) Define a default error message

This step depends on validation engine you are using

Article with examples for Hibernate Validator: Chapter 3. Creating custom constraints