ConfigurationPropertiesBean detects class-level @ConstructorBinding unexpectedly

[snicoll]

Consider the following:

@ConfigurationProperties("test")
@ConstructorBinding
public class TestProperties {

    public TestProperties(String property, int counter) { ... }

}

And the current code:

spring-boot/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/ConfigurationPropertiesBean.java

Lines 247 to 248 in d254c62

	Constructor<?> bindConstructor = BindMethod.findBindConstructor(type);
	BindMethod bindMethod = (bindConstructor != null) ? BindMethod.VALUE_OBJECT : BindMethod.forClass(type);

BindMethod.findBindConstructor(TestProperties.class) will return null as it does not test the fact @ConstructorBinding is specified at class level. However, BindMethod.forClass(type) will work as it has the extra assertion.

The current code arrangement feels a bit weird considering we have a dedicated method to retrieve the relevant constructor and that returns null whereas it shouldn't.

[snicoll]

An additional note that the binder takes any constructor when built in such a mode (i.e. I don't see an immediate check that only one relevant constructor is present).

Something else that I find a bit odd is that that check detects the Constructor to use but doesn't seem to store that anywhere. I think it would be worthwhile to store the result of the search so that the binder can call exactly what was detected.

[snicoll]

At this point, I don't understand why we store a Predicate<Constructor<?>> rather than the Constructor<?> to use effectively as this information is statically defined and we have access to that.

[mbhave]

I think this can be refactored as part of #18670. There are a few bugs covered by these tests which might involve touching a lot of that code. We can leave the issue open so that we don't forget about it. Assigning myself to avoid any duplication of effort.