ConstructorBinding default detection is considered with a private constructor

[snicoll]

I am upgrading a project to Spring Boot 3 and I am facing an issue that is summarized in this sample project: https://github.com/snicoll-scratches/demo-binder

I have a configuration properties with a nested object that has a constructor. The constructor is for convenience within the class as it allows me to set configurable default values. The nested object is used to signal that it is a @NestedConfigurationPropery. There's no setter.

If I run the annotation processor on this project, I can now see (after @mbhave told me!) that it defaults to constructor binding and the other POJO properties aren't detected.

I've added two tests. The first test sets such property (description). Binding works and the value is set using the setter. The second test try to set the value within the constructor. In that case, the context fails as follows:

 Caused by: org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'test' to com.example.demobinder.TestProperties
 	at org.springframework.boot.context.properties.bind.Binder.handleBindError(Binder.java:387)
 	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:347)
 	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:332)
 	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:262)
 	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:249)
 	at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:94)
 	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:89)
 	... 98 more
 Caused by: java.lang.IllegalStateException: No setter found for property: one
 	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:104)
 	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:83)
 	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:59)
 	at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:476)
 	at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:590)
 	at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:576)
 	at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:474)
 	at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:414)
 	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:343)
 	... 103 more

If the constructor is switched to private, the binder still tries to use it.

Now I know that this object is managed by constructor binding, the error message is helping. But without all that context, I find the upgrade path really confusing. It doesn't feel consistent that description is injected either (except if we have some sort of fallback handling I am not aware of?).

[philwebb]

It's a bit of a tricky one to know how to fix. We need some signal that the constructor for TestProperties.SimpleElement should not be used. We could introduce a @NoConstructorBinding annotation (with a better name), but I wonder if we can use a private constructor as a signal instead.

[philwebb]

I've gone with the private constructor for now. The sample app works if I change the SimpleElement. I suspect we might find another use-case in the future where a private constructor can't be used, but we should probably wait and see.