static String stringValue(JsonNode jsonNode, String field) {
        final JsonNode value = jsonNode.get(field);
        if (value == null) {
            throw new IllegalArgumentException(field + " is missing!");
        }
        return value.textValue();
    }
Converts an {@link HttpRequest} to a {@link BlogPost}.
examples/tutorials/rest-api-annotated-service/src/main/java/example/armeria/server/blog/BlogPostRequestConverter.java