/kotlin-tooling-java-to-kotlin

Quarkus Conversion Guide

When This Applies

This guide applies when the Java source contains imports matching io.quarkus.*, javax.enterprise.*, or jakarta.enterprise.*. This covers Quarkus REST, Quarkus CDI, and Panache ORM.

Key Rules

1. CDI beans need a no-arg constructor

The CDI specification requires beans to have a no-arg constructor (package-private or public). In Kotlin, satisfy this by giving all constructor parameters default values, or by adding a secondary no-arg constructor.

2. Scope annotations

@ApplicationScoped, @RequestScoped, @Dependent — preserve these exactly. Beans with these scopes must have a no-arg constructor accessible to CDI.

3. @Inject field injection → constructor injection

Replace @Inject on fields with an @Inject-annotated primary constructor in Kotlin. CDI requires the @Inject annotation on the constructor when multiple constructors exist. With a single constructor, Quarkus discovers it automatically.

4. REST endpoint annotations

@Path, @GET, @POST, @PUT, @DELETE, @Produces, @Consumes — preserve these exactly. No annotation site target is needed.

5. Panache entities

Panache entities must remain open — do NOT use data class. Extend PanacheEntity (auto-generated Long ID) or PanacheEntityBase (custom ID type). Keep fields as open mutable properties because Panache enhances field access at build time.

6. @ConfigProperty

Use on constructor parameters with a default value to satisfy CDI's no-arg constructor requirement:

@ConfigProperty(name = "app.greeting") val greeting: String = ""

Examples

Example 1: REST Resource with CDI Injection

Java:

package com.acme.web;

import jakarta.enterprise.context.ApplicationScoped;
import jakarta.inject.Inject;
import jakarta.ws.rs.GET;
import jakarta.ws.rs.Path;
import jakarta.ws.rs.PathParam;
import jakarta.ws.rs.Produces;
import jakarta.ws.rs.core.MediaType;
import java.util.List;

@Path("/api/products")
@ApplicationScoped
@Produces(MediaType.APPLICATION_JSON)
public class ProductResource {

    @Inject
    ProductService productService;

    @Inject
    PricingService pricingService;

    @GET
    public List<ProductDto> listProducts() {
        return productService.findAll();
    }

    @GET
    @Path("/{id}")
    public ProductDto getProduct(@PathParam("id") Long id) {
        return productService.findById(id);
    }
}

Kotlin:

package com.acme.web

import jakarta.enterprise.context.ApplicationScoped
import jakarta.inject.Inject
import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.PathParam
import jakarta.ws.rs.Produces
import jakarta.ws.rs.core.MediaType

@Path("/api/products")
@ApplicationScoped
@Produces(MediaType.APPLICATION_JSON)
class ProductResource @Inject constructor(
    private val productService: ProductService,
    private val pricingService: PricingService
) {

    // No-arg constructor required by CDI — default values satisfy this
    constructor() : this(
        productService = ProductService(),
        pricingService = PricingService()
    )

    @GET
    fun listProducts(): List<ProductDto> {
        return productService.findAll()
    }

    @GET
    @Path("/{id}")
    fun getProduct(@PathParam("id") id: Long): ProductDto? {
        return productService.findById(id)
    }
}

Key changes:

• @Inject field injection is replaced by an @Inject-annotated primary constructor.
• A secondary no-arg constructor is added to satisfy the CDI specification. In practice, CDI will use the @Inject constructor — the no-arg constructor exists only to pass validation.
• Constructor parameters become private val in the primary constructor.
• Return type ProductDto becomes ProductDto? where the service may return null.