Extend ApplicationContextRunner to accept simple bean definitions

[snicoll]

Inspired by ProjectTester in Spring Initializr, I'd like us to extend ApplicationContextRunner to allow registering beans in addition to Configurations.

Lots of tests are declaring a test @Configuration class with one or two bean definitions to export additional beans in the context to test. This requires quite a lot of code in the end and a muscle memory to remember what the inner class at the end of the test does.

Consider this:

@Test
public void cacheManagerBackOff() {
	this.contextRunner.withUserConfiguration(CustomCacheManagerConfiguration.class)
			.run((context) -> assertThat(
					getCacheManager(context, ConcurrentMapCacheManager.class)
							.getCacheNames()).containsOnly("custom1"));
}

To be replaced with this proposal by

@Test
public void cacheManagerBackOff() {
	this.contextRunner
			.withBean(CacheManager.class,
					() -> new ConcurrentMapCacheManager("custom1"))
			.run((context) -> assertThat(
					getCacheManager(context, ConcurrentMapCacheManager.class)
							.getCacheNames()).containsOnly("custom1"));
}

[wilkinsona]

I like that a lot, particularly as it helps to make the test more self-contained.

[snicoll]

Considering calling that withUserBean to make it crystal clear it's not related with auto-configuration at all?

[wilkinsona]

I think withBean is fine. Unlike with configuration, I can’t see a need to offer a way to add user beans and auto-configured beans. I prefer the more concise name and the javadoc should be able to remove any ambiguity about what it does.

[snicoll]

Unfortunately it is not that easy as the method to register beans that way is part of GenericApplicationContext and some of the contexts we use are not inheriting from that base class.

[snicoll]

This issue is blocked by #16096