Clarify when BootstrapContext get methods may return null rather than throwing an exception or calling the fallback supplier

[tschut]

Hey,

Note: this is on 4.0.0-RC1

I noticed with this commit 4ebf09a the BootstrapContext::getOrElseThrow is now marked as @Nullable:

	<T, X extends Throwable> @Nullable T getOrElseThrow(Class<T> type, Supplier<? extends X> exceptionSupplier)
			throws X;

I'm not sure if this is intentional or not, but it does feel pretty unexpected from a user perspective. It's also not in the javadoc, which seems to also be conflicting on what exception is to be thrown:

	 * @return the instance managed by the context
	 * @throws X if the type has not been registered
	 * @throws IllegalStateException if the type has not been registered

As a user, I expect a method named getOrElseThrow to either return the thing or throw, and not return null in any case.

[wilkinsona]

The nullability looks correct to me as you can register a type with a supplier that produces null. There's a test for this:

spring-boot/core/spring-boot/src/test/java/org/springframework/boot/bootstrap/DefaultBootstrapContextTests.java

Lines 239 to 243 in f11e3f3

	@Test
	void getOrElseThrowWhenRegisteredAsNullReturnsNull() {
	this.context.register(Number.class, InstanceSupplier.of(null));
	assertThat(this.context.getOrElseThrow(Number.class, RuntimeException::new)).isNull();
	}

We should update the javadoc to clarify the behavior when a registered supplier produces null. The only time when an exception will be thrown or when the fallback supplier will be called is when isRegistered(Class) returns false.

[tschut]

Thanks for the quick fix! 👍

[tschut]

I think there is a minor thing that could still be improved here:

	 * @throws X if the type has not been registered
	 * @throws IllegalStateException if the type has not been registered

This seems ambiguous.