Reactive find() with entity mapping fails on Java 21 / Spring Boot 3.5.x due to CGLIB CodeGenerationException

[trinetra-innovate]

When using Spring Boot 3.5.x (Spring Data MongoDB 4.6.x) with Java 21 and reactive MongoDB, reactive queries such as mongoOperations.find(query, Message.class) fail with a CodeGenerationException caused by CGLIB attempting to subclass lambda-generated classes in Reactor.

This happens even if:

Entities are simple POJOs

Auditing annotations (@createdby, @LastModifiedDate) are removed

Mapping to raw Document.class is attempted

The pipeline is a simple Flux → DTO mapping

The issue appears to be inside Spring’s reactive entity instantiation / reactive pipeline instrumentation, not the user entity.

Steps to Reproduce:

Java 21

Spring Boot 3.5.6 (Spring Data MongoDB 4.6.x)

Reactive MongoDB configuration

Define entity:
@DaTa
@NoArgsConstructor
@AllArgsConstructor
@document(collection = "i18n_messages")
public class Message {
private String language;
private String description;
private List messages;
}

@DaTa
@NoArgsConstructor
@AllArgsConstructor
public class MessageItem {
private String messageCode;
private String display;
}
Call a reactive query:
Flux messages = mongoOperations.find(new Query(), Message.class);
messages.subscribe(System.out::println)

Expected behavior:

Flux emits mapped entities without exception.

Actual behavior:

Application fails with:

java.lang.IllegalStateException:
org.springframework.cglib.core.CodeGenerationException:
java.lang.NoClassDefFoundError-->IllegalName: reactor.core.publisher.Operators$LiftFunction$$Lambda/0x00000165e681fcd8_Accessor_99oe3o

Occurs even when mapping to Document.class and manually converting to DTO.

Works on Spring Boot 3.4.4 / Java 21.

Environment:

Java 21

Spring Boot 3.5.6

Spring Data MongoDB 4.6.3

ReactiveMongoOperations

Maven or Gradle build

No DevTools

Workarounds Tested:

Removing all auditing annotations (@createdby, @LastModifiedDate) — does not fix

Mapping raw Document → DTO manually — does not fix

Setting spring.mongodb.instantiator.simple=true — does not fix

Downgrading to Spring Boot 3.4.4 — works

[christophstrobl]

Thank you for getting in touch - If you'd like us to spend some time investigating, please take the time to provide a complete minimal sample (something that we can unzip or git clone, build, and deploy) that reproduces the problem.

[trinetra-innovate]

public record GridResults(List content, long totalElements, int pageNumber, int pageSize)
{
}

@OverRide
public Mono<GridResults> seekList(final int page, final int size)
{
Pageable pageable = getDefaultPageable(page, size);

    Query query = new Query().addCriteria(defaultFilter()).with(pageable);

    //Get total count (for pagination info)
    Mono<Long> count = mongoOperations.count(new Query().addCriteria(tenantFilter()), getEntityClass());

    //Get page content
    **Flux<E> contentFlux = mongoOperations.find(query, getEntityClass());**

    //Combine count + content into Grid Results with exception handling
    return contentFlux.collectList()
            .zipWith(count)
            .map(tuple -> new GridResults<>(
                    tuple.getT1(),        // list of records
                    tuple.getT2(),        // total count
                    page,
                    size
            ))
            .flatMap(pageResult ->
            {
                if (pageResult.content().isEmpty()) {
                    return Mono.error(new NoDataFoundException());
                }
                return Mono.just(pageResult);
            });
}