No scalar implementation when using a external JAR

[Mcdostone]

Hello, I have 2 gradle projects:

• hello-graphql defining all the graphQL schemas,
• hello-app, the spring boot application using hello-graphql

hello-graphql defines a custom scalar year. It is used in the Person graphQL definition.

Issue

I am facing issues when I want to use this library in my spring boot application:

***************************
APPLICATION FAILED TO START
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Description:
There are problems with the GraphQL Schema:
        * There is no scalar implementation for the named  'Year' scalar type

It seems that Spring boot is not able to detect the Year scalar.
I use codegen since my graphQL definitions are in a external JAR.

To reproduce

git clone https://github.com/Mcdostone/hello-graphql
cd hello-graphql
./gradlew clean build
./gradlew :hello-app:bootRun

Thanks for your help

[ZadenMubarak]

This is a classic Spring GraphQL + codegen + external schema JAR issue. Your setup is almost correct — the problem is not Gradle, not the JAR, and not codegen itself.

The root cause is:

Spring GraphQL does not automatically register custom scalars that live in an external dependency unless you explicitly expose them as Spring beans in the application context.

---

What’s actually going wrong

Your flow is:

• hello-graphql

  • Defines schema (.graphqls)

  • Defines custom scalar Year

• hello-app

  • Uses the schema via dependency

  • Uses GraphQL Java / Spring GraphQL

  • Uses codegen

At runtime, Spring GraphQL builds the schema and sees:

scalar Year

But Spring GraphQL asks:

“Where is the GraphQLScalarType implementation for Year?”

And the answer is: nowhere in the Spring context.

Even though:

• The scalar exists in the external JAR

• The schema is visible

• Codegen runs correctly

➡️ Spring GraphQL does NOT scan dependencies for scalars automatically

---

Key rule (important)

Spring GraphQL only registers custom scalars that are exposed as Spring beans
(either @Bean or @Component) inside the application context

External JAR ≠ auto-registered scalar.

---

The fix (correct & recommended)

1️⃣ Expose the scalar as a Spring bean

In hello-graphql, you likely already have something like:

public class YearScalar {
public static final GraphQLScalarType YEAR = GraphQLScalarType.newScalar()
    .name("Year")
    .coercing(...)
    .build();

}

That is not enough.

You must expose it as a Spring bean from the application that runs.

---

2️⃣ Register the scalar in hello-app

Create a configuration class inside hello-app:

@Configuration

public class GraphQLScalarConfig {

@Bean
public GraphQLScalarType yearScalar() {
    return YearScalar.YEAR;
}

}

📌 Important:

• The bean must live in the Spring Boot app

• Spring must see it during component scan

---

3️⃣ Ensure package visibility

Make sure:

• YearScalar is public

• hello-app has the dependency:

implementation project(":hello-graphql")

(or the published JAR)

---

Why this works

Spring GraphQL internally does:

List<GraphQLScalarType> scalars = applicationContext.getBeansOfType(GraphQLScalarType.class)

If your scalar is not a bean → it does not exist as far as Spring is concerned.

---

Alternative (less clean, but valid)

You can register scalars via RuntimeWiringConfigurer:

@Bean

public RuntimeWiringConfigurer runtimeWiringConfigurer() {

return wiringBuilder ->

wiringBuilder.scalar(YearScalar.YEAR);

}

But the bean approach is simpler and preferred.

---

Summary (TL;DR)

Problem	Cause	Fix
There is no scalar implementation for 'Year'	Scalar exists but is not a Spring bean	Register GraphQLScalarType as a bean in hello-app
External JAR scalar not detected	Spring GraphQL does not auto-scan dependencies	Explicit bean registration
Codegen works but runtime fails	Codegen ≠ runtime wiring	Runtime wiring needs Spring context

[ZadenMubarak]

Please let me know if this helped