Classes proxied by CGLIB produces NPE with coroutine async job on Springboot 3.2.0

[FelipeNathan]

Hi, I'm about to upgrade the version of springboot framework of my kotlin with coroutine (and webflux) project to 3.2.0

id("org.springframework.boot") version "3.2.0"

And now, I'm facing a problem with classes proxied by cglib when I'm trying to make a coroutine async code and return some null value

Yon ca reproduce it making a simple endpoint and annotate one dependency class with @RefreshScope, like this:

@RestController
@RequestMapping("/ping")
class PingController(val someCGLIBClass: SomeCGLIBClass) {
    @GetMapping
    suspend fun ping() = coroutineScope {
        // some method that can run a code asynchronously
        val job = async { someCGLIBClass.execute() }
        job.await()
    }
}

@RefreshScope
@Component
class SomeCGLIBClass {
    // some method with some rule that can return null
    suspend fun execute() = null
}

The result will produce the following error:

java.lang.NullPointerException: value
	at java.base/java.util.Objects.requireNonNull(Objects.java:233) ~[na:na]
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
	*__checkpoint ⇢ Handler com.my.application.controller.PingController#ping(Continuation) [DispatcherHandler]
	*__checkpoint ⇢ com.my.application.config.MyProfileWebFilter$$SpringCGLIB$$0 [DefaultWebFilterChain]
	*__checkpoint ⇢ HTTP GET "/ping" [ExceptionHandlingWebHandler]
Original Stack Trace:
		at java.base/java.util.Objects.requireNonNull(Objects.java:233) ~[na:na]
		at reactor.core.publisher.MonoJust.<init>(MonoJust.java:35) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.Mono.just(Mono.java:753) ~[reactor-core-3.6.0.jar:3.6.0]
		at org.springframework.aop.framework.CoroutinesUtils.awaitSingleOrNull(CoroutinesUtils.java:42) ~[spring-aop-6.1.1.jar:6.1.1]
		at org.springframework.aop.framework.CglibAopProxy.processReturnType(CglibAopProxy.java:427) ~[spring-aop-6.1.1.jar:6.1.1]
		at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:719) ~[spring-aop-6.1.1.jar:6.1.1]
		at com.my.application.controller.SomeCGLIBClass$$SpringCGLIB$$0.execute(<generated>) ~[main/:na]
		at com.my.application.controller.PingController$ping$2$job$1.invokeSuspend(PingController.kt:18) ~[main/:na]
		at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) ~[kotlin-stdlib-1.9.20.jar:1.9.255-SNAPSHOT]
		at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:108) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:68) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.internal.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:375) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:30) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable$default(Cancellable.kt:25) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:110) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:126) ~[kotlinx-coroutines-core-jvm-1.7.3.jar:na]
		at kotlinx.coroutines.reactor.MonoKt.monoInternal$lambda$2(Mono.kt:92) ~[kotlinx-coroutines-reactor-1.7.3.jar:na]
		at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:85) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:57) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:146) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:200) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:293) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:188) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:297) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:478) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onNext(MonoPeekTerminal.java:180) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2571) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:139) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip$ZipInner.onSubscribe(MonoZip.java:470) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:152) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:55) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip$ZipCoordinator.request(MonoZip.java:220) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onSubscribe(MonoIgnoreThen.java:135) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip.subscribe(MonoZip.java:129) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:53) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:241) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onComplete(MonoIgnoreThen.java:204) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onComplete(MonoFlatMap.java:189) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.Operators.complete(Operators.java:137) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoZip.subscribe(MonoZip.java:121) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:264) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:51) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.Mono.subscribe(Mono.java:4512) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:126) ~[reactor-core-3.6.0.jar:3.6.0]
		at reactor.core.scheduler.ExecutorScheduler$ExecutorTrackedRunnable.run(ExecutorScheduler.java:192) ~[reactor-core-3.6.0.jar:3.6.0]
		at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
		at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
		at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

Debuggin the code, I was lead to a new class called CoroutineUtils and that have this code:

package org.springframework.aop.framework;

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

Shouldn't the "else" part of the ternary be Mono.justOrEmpty(value)?