spring-projects/spring-framework · error · IllegalArgumentException

Not a Reactive Streams Publisher: {publisher}

Error message

Not a Reactive Streams Publisher: {publisher}

What it means

Thrown by CoroutinesUtils.asFlow (package-private, invoked by both CglibAopProxy.processReturnType and JdkDynamicAopProxy.invoke) when the return value of a Kotlin suspending function expected to produce a Flow is not a Reactive Streams Publisher. Spring's Kotlin coroutine bridge converts Publisher to a Kotlin Flow via ReactiveFlowKt.asFlow; passing anything else (a Mono for a non-flow path, a plain object, null-handled elsewhere) is treated as a contract violation. This is an internal invariant for the Flow-returning suspending-function branch.

Source

Thrown at spring-aop/src/main/java/org/springframework/aop/framework/CoroutinesUtils.java:39

import kotlinx.coroutines.reactor.MonoKt;
import org.jspecify.annotations.Nullable;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;

/**
 * Package-visible class designed to avoid a hard dependency on Kotlin and Coroutines dependency at runtime.
 *
 * @author Sebastien Deleuze
 * @since 6.1
 */
abstract class CoroutinesUtils {

	static Object asFlow(@Nullable Object publisher) {
		if (publisher instanceof Publisher<?> rsPublisher) {
			return ReactiveFlowKt.asFlow(rsPublisher);
		}
		else {
			throw new IllegalArgumentException("Not a Reactive Streams Publisher: " + publisher);
		}
	}

	@SuppressWarnings({"rawtypes", "unchecked"})
	static @Nullable Object awaitSingleOrNull(@Nullable Object value, Object continuation) {
		return MonoKt.awaitSingleOrNull(value instanceof Mono mono ? mono : Mono.justOrEmpty(value),
				(Continuation<Object>) continuation);
	}

}

View on GitHub (pinned to e8729d0438)

Solutions

1. Ensure the value returned for a Flow-returning Kotlin suspending function is a Reactive Streams Publisher (Flux/Mono from Reactor, or a kotlinx publisher).
2. If writing custom @Around advice for such methods, return the original proceed() result unchanged, or wrap your value in Flux.fromStream(...)/Mono.just(...) as appropriate.
3. Align kotlinx-coroutines-reactive / kotlinx-coroutines-reactor versions with Spring's expected 6.1+ contracts.
4. Avoid advising Flow-returning Kotlin functions with generic interceptors that do not understand coroutine return-type semantics.

Example fix

// before
@Around("kotlinSuspendFlowPointcut()")
public Object around(ProceedingJoinPoint pjp) {
  return List.of(1, 2, 3);  // not a Publisher -> throws in Flow branch
}

// after
@Around("kotlinSuspendFlowPointcut()")
public Object around(ProceedingJoinPoint pjp) throws Throwable {
  return Flux.fromIterable(List.of(1, 2, 3));  // Reactive Streams Publisher
}

Object ret = ...; // value to convert for a Flow-returning Kotlin suspending fn
if (!(ret instanceof org.reactivestreams.Publisher<?>)) {
  throw new IllegalArgumentException("Expected a Reactive Streams Publisher for Flow conversion, got " + ret);
}

static boolean isPublisherForFlow(Object o) {
  return o instanceof org.reactivestreams.Publisher<?>;
}

When it happens

Trigger: A Kotlin suspending function whose last parameter type is a Flow and whose returned value is not a Publisher (e.g. a coroutine framework returns a Deferred, a raw value, or an unexpected wrapper); AOP advice that replaces the return value of such a method with a non-Publisher; version mismatch between kotlinx-coroutines/reactor bindings producing unexpected types.

Common situations: Returning the wrong reactive type from advice wrapping a Flow-returning Kotlin suspending function; mixing Reactor/Kotlin coroutine versions; a custom Around interceptor that substitutes null or a scalar for a Flow position.

Related errors

• MethodInvocation is not a Spring ProxyMethodInvocation: {}
• 'argumentNames' property of AbstractAspectJAdvice contains a
• Only afterReturning advice can be used to bind a return valu
• Returning name '{}' is neither a valid argument name nor the
• Only afterThrowing advice can be used to bind a thrown excep

AI-assisted analysis of spring-projects/spring-framework@e8729d0438 (2026-08-04). Data as JSON: /data/errors/9418fb359e7a41c7.json. Report an issue: GitHub.