AvailabilityChangeEvent does not carry generic information

[alimate]

Synopsis

I was experimenting with the new Kubernetes probes support in Spring Actuator. The official documentation suggests that we can listen to changes in readiness or liveness by registering a bean like the following:

@Component
public class ReadinessStateExporter {

    @EventListener
    public void onStateChange(AvailabilityChangeEvent<ReadinessState> event) {
        switch (event.getState()) {
        case ACCEPTING_TRAFFIC:
            // create file /tmp/healthy
        break;
        case REFUSING_TRAFFIC:
            // remove file /tmp/healthy
        break;
        }
    }
}

However, running an application with a bean like this fails with:

java.lang.ClassCastException: org.springframework.boot.availability.LivenessState cannot be cast to org.springframework.boot.availability.ReadinessState

Details

This failure makes sense because during the application startup, Spring Boot fires both AvailabilityChangeEvent<ReadinessState> and AvailabilityChangeEvent<LivenessState> events.
Due to erasure, both of those events would e handled by the ReadinessStateExporter. Quite reasonably, the LivenessState change event should fail because we can't cast LivenessState to ReadinessState.
Hence the error:

java.lang.ClassCastException: org.springframework.boot.availability.LivenessState cannot be cast to org.springframework.boot.availability.ReadinessState
	at ReadinessChangedListener.onStateChange(ReadinessChangedListener.java:13) ~[classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.doInvoke(ApplicationListenerMethodAdapter.java:305) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.processEvent(ApplicationListenerMethodAdapter.java:190) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.onApplicationEvent(ApplicationListenerMethodAdapter.java:153) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:81) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:67) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.context.event.EventPublishingRunListener.started(EventPublishingRunListener.java:103) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.SpringApplicationRunListeners.started(SpringApplicationRunListeners.java:71) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:321) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
	at DemoApplication.main(DemoApplication.java:10) [classes/:na]

I guess we probably should change the documentation as the sample code makes the Spring App to fail at startup.
Also, we could listen to AvailabilityState :

@Component
public class ReadinessChangedListener {

    @EventListener
    public void onStateChange(AvailabilityChangeEvent<AvailabilityState> event) {
        // check if it's liveness or readiness or anything else
    }

}

Please kindly let me know what you think of this.
Cheers!

[philwebb]

I was under the impression that ApplicationListenerMethodAdapter would deal with generics correctly. Do you by any chance have a sample application that reproduces the problem?

[alimate]

Sure. This a simple sample application
https://github.com/alimate/boot-probes