Question about spring data jdbc and kotlin code gen

[ichenhe]

I have successfully run a project with Kotlin + gradle kts + spring data jpa + querydsl 4.4.

However, now I want to replace JPA with spring-data-jdbc which is more concise. Meanwhile, v5.0 released and brings me confusion.

1. How to complete the integration with data-jdbc? There seems to be only jpa relevant documents.
2. What is kotlin-codegen, and how to use it? Now I'm using kapt("com.querydsl:querydsl-apt:4.4.0:jpa"), Is it any different from the kotlin-codegen?

Any hints will be appreciated.

[jwgmeligmeyling]

@F43nd1r could you explain the kotlin-codegen introduced in v5?

[F43nd1r]

querydsl-kotlin-codegen does exactly one thing: It replaces all java code generation in querydsl-apt with kotlin code. It's only used in addition to the apt module:

 kapt("com.querydsl:querydsl-apt:5.0.0:jpa")
 kapt("com.querydsl:querydsl-kotlin-codegen:5.0.0")

You don't need this just because your project is using kotlin, but it brings some advantages:

• no need for java compilation improves build performance (only if you have no other (generated) java sources)
• lazy QClass fields allow for arbitrarily deep field access without any configuration even with circular entity dependencies
• nullsafety (this is just a general advantage of kotlin over java, no specific problem case known to me)

I developed this plugin because I had issues with cross-compilation of kotlin and java, which was solved by this as well, but that's mostly anecdotal.

[F43nd1r]

Here's an example of a Java QClass and a Kotlin QClass for comparison:

@Generated("com.querydsl.codegen.DefaultEntitySerializer")
public class QVersion extends EntityPathBase<Version> {

    private static final long serialVersionUID = -738061112L;

    private static final PathInits INITS = PathInits.DIRECT2;

    public static final QVersion version = new QVersion("version");

    public final QApp app;

    public final NumberPath<Integer> code = createNumber("code", Integer.class);

    public final NumberPath<Integer> id = createNumber("id", Integer.class);

    public final StringPath mappings = createString("mappings");

    public final StringPath name = createString("name");

    public QVersion(String variable) {
        this(Version.class, forVariable(variable), INITS);
    }

    public QVersion(Path<Version> path) {
        this(path.getType(), path.getMetadata(), PathInits.getFor(path.getMetadata(), INITS));
    }

    public QVersion(PathMetadata metadata) {
        this(metadata, PathInits.getFor(metadata, INITS));
    }

    public QVersion(PathMetadata metadata, PathInits inits) {
        this(Version.class, metadata, inits);
    }

    public QVersion(Class<? extends Version> type, PathMetadata metadata, PathInits inits) {
        super(type, metadata, inits);
        this.app = inits.isInitialized("app") ? new QApp(forProperty("app"), inits.get("app")) : null;
    }

}

@Generated("com.querydsl.kotlin.codegen.KotlinEntitySerializer")
public class QVersion : EntityPathBase<Version> {
  public val app: QApp by lazy {
    QApp(forProperty("app"))
  }


  public val code: NumberPath<Int> = createNumber("code", Int::class.java)

  public val id: NumberPath<Int> = createNumber("id", Int::class.java)

  public val mappings: StringPath = createString("mappings")

  public val name: StringPath = createString("name")

  public constructor(variable: String) : super(Version::class.java, forVariable(variable))

  public constructor(path: Path<Version>) : super(path.type, path.metadata)

  public constructor(metadata: PathMetadata) : super(Version::class.java, metadata)

  public constructor(type: Class<out Version>, metadata: PathMetadata) : super(type, metadata)

  public companion object {
    private const val serialVersionUID: Long = -738061112

    public val version: QVersion = QVersion("version")
  }
}

As you can see, in java we have PathInits.DIRECT2 by default, which limits our deep field access to two levels. In kotlin we don't use PathInits at all. Instead entity fields are lazy, which means we don't need to worry about how deep our field access goes.

[F43nd1r]

@jwgmeligmeyling should we maybe document this somewhere? And if so, where would you put it?

[ichenhe]

Great explanation, thank you. I think it should be documented in case other people are confused.

[jwgmeligmeyling]

I think the best part to store this documentation is in the "Querydsl Reference Guide", specificially as a subchapter under "3.3. Code generation".

That page is maintained under https://github.com/querydsl/querydsl/blob/master/querydsl-docs/src/main/docbook/en-US/content/general/codegen.xml .

The documentation is currently written in DocBook, so there is yet another legacy hurdle to overcome before even starting to update the documentation. I am planning to rewrite major portions of the reference guide at the end of the year anyway, as portions of it have become outdated. So I am open to switching to Markdown or Asciidoc as well.

[ichenhe]

In my opinion, Markdown is great. It's simple, but it's good enough, and many document generators are compatible with it.

[alvistar]

Hi,
I am writing here before raising this as an issue.

I have a project running with Spring Boot 2.5.5 and Spring Data. Gradle version 7.2

In build.kts:
implementation("com.querydsl:querydsl-jpa:5.0.0")
implementation("com.querydsl:querydsl-core:5.0.0")
kapt("com.querydsl:querydsl-apt:5.0.0:jpa")

Everything works, but if I add:

kapt("com.querydsl:querydsl-kotlin-codegen:5.0.0")

Spring will throw this exception:
Caused by: java.lang.IllegalStateException: Could not access method or field: class org.springframework.util.ReflectionUtils cannot access a member of class manta_backend.entities.QAccount with modifiers "private static final"

Any help? I cannot understand if this is more an issue to be raise on Spring Data.

Thanks in advance,
Alessandro

[F43nd1r]

querydsl-kotlin-codegen is currently incompatible with spring data. Fix is in #3076

[alvistar]

"jwgmeligmeyling closed this 3 hours ago".

Realtime!

Thanks

[ArchieLabs]

is this fix pushed into 5.0.0 ? I'm having the same issue and I cannot use QueryDSL in a kotlin project.

Could not access method or field: class org.springframework.util.ReflectionUtils cannot access a member of class com.test.entities.QAccount with modifiers "private static final"

[xmlking]

still seams like issue with Spring Boot 3.0.2 as stated by @alvistar above

[GourdErwa]

still seams like issue with Spring Boot 3.1.1 as stated by @alvistar above

Caused by: java.lang.IllegalStateException: Could not access method or field: class org.springframework.util.ReflectionUtils cannot access a member of class io.norn.examples.mongodb.customer.QCustomer with modifiers "private static final"
	at org.springframework.util.ReflectionUtils.handleReflectionException(ReflectionUtils.java:107) ~[spring-core-6.0.10.jar:6.0.10]
	at org.springframework.util.ReflectionUtils.getField(ReflectionUtils.java:658) ~[spring-core-6.0.10.jar:6.0.10]
	at org.springframework.data.querydsl.SimpleEntityPathResolver.lambda$createPath$0(SimpleEntityPathResolver.java:74) ~[spring-data-commons-3.1.1.jar:3.1.1]