Creating GraphQL Custom Scalars with SmallRye GraphQL

While by default MicroProfile GraphQL specification doesn't provide direct support for creating custom scalars for GraphQL literal types of String, Int and Float, SmallRye GraphQL has implemented this feature based on user feedback. To implement a custom scalar with SmallRye GraphQL, you can annotate your custom scalar class with @CustomScalar in addition to following the pattern below of including a public constructor that takes one of String|BigDecimal|BigInteger depending on the scalar type:

  @CustomScalar("BigDecimalString")
  public class BigDecimalString implements CustomStringScalar {
    public BigDecimalString(String stringValue) {
        ...
    }
    @Override
    public String stringValueForSerialization() {
        ...
  }
}

In this example, BigDecimalString implements the CustomStringScalar which is used to identify the proper (de) serialization for BigDecimalString. BigDecimalString also provides a single argument constructor which takes a String. Finally, BigDecimalString implements stringValueForSerialization() which provides the String representation to be used during serialization.

[NOTE] If the user wants to create a literal for GraphQL Int or Float, they would implement either CustomIntScalar with the intValueForSerialization method or CustomFloatScalar with the floatValueForSerialization method respectively.