Skip gRPC port initializer when spring-grpc is absent

[r0h1tb]

See gh-50825

Problem

GrpcPortInfoApplicationContextInitializer is registered unconditionally via META-INF/spring.factories, and its initialize() unconditionally adds a listener declared as ApplicationListener<GrpcServerStartedEvent>.

With spring-boot-grpc-test on the classpath but spring-grpc absent, Spring resolves that listener's generic interface to decide whether to deliver an event, and Class.getGenericInterfaces() throws — which is the stack trace in the report:

java.lang.TypeNotPresentException: Type org.springframework.grpc.server.lifecycle.GrpcServerStartedEvent not present
	at java.base/java.lang.Class.getGenericInterfaces(Class.java:1278)
	at org.springframework.core.ResolvableType.getInterfaces(ResolvableType.java:544)

Fix

Guard registration on the event type being present:

private static final String GRPC_SERVER_STARTED_EVENT =
    "org.springframework.grpc.server.lifecycle.GrpcServerStartedEvent";

@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
    if (!ClassUtils.isPresent(GRPC_SERVER_STARTED_EVENT, applicationContext.getClassLoader())) {
        return;
    }
    applicationContext.addApplicationListener(new Listener(applicationContext));
}

ClassUtils.isPresent rather than a try/catch because that's what LogbackLoggingSystem.isBridgeHandlerAvailable() already does for the SLF4J bridge handler — same situation, optional class reached through a factories-registered component.

The classloader comes from the context rather than the class, so the guard sees the same classpath the listener resolution will.

Scope

Deliberately narrow: only the initializer is guarded. I did not touch the spring.factories registration itself, since the initializer being constructed is harmless — it's only the listener whose generic type can't resolve. Happy to take a different approach if you'd rather condition the registration.

Tests

Two added to GrpcPortInfoApplicationContextInitializerTests, using the existing FilteredClassLoader:

• whenGrpcServerStartedEventIsNotOnTheClasspathRegistersNoListener — hides GrpcServerStartedEvent, asserts no listener is registered
• whenGrpcServerStartedEventIsOnTheClasspathRegistersListener — control, so the guard can't silently disable the feature

With the source change stashed but both tests kept, the first fails:

GrpcPortInfoApplicationContextInitializerTests > whenGrpcServerStartedEventIsNotOnTheClasspathRegistersNoListener() FAILED

java.lang.AssertionError:
Expected size: 0 but was: 1 in:
[org.springframework.boot.grpc.test.autoconfigure.GrpcPortInfoApplicationContextInitializer$Listener@2407a36c]

Verification

./gradlew :module:spring-boot-grpc-test:test

	tests	failures	errors
baseline (main, unmodified)	16	0	0
this branch	18	0	0
tests kept, source change stashed	18	1	0

The third row is what shows the tests pin the reported bug rather than restating the fix.

I ran the affected module's suite rather than the full build. Happy to run anything wider you'd like to see.