@Nullable
    private Object defaultOrException() {
        if (!shouldExist) {
            // May return 'null' if no default value is specified.
            if (elementType.isPrimitive() && defaultValue == null) {
                throw new IllegalArgumentException("Cannot set null to primitive type parameter: " +
                                                   httpElementName);
            }
            return defaultValue;
        }
        throw new IllegalArgumentException("Mandatory parameter/header/attribute is missing: " +
                                           httpElementName);
    }
Returns an attribute resolver which retrieves a value specified by {@code attrKeys}
from the {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java