Binding to collection fails with unbound elements error if collection overridden in another property source

[mschallar]

Hello,

after migrated from Spring Boot 1.5.3.RELEASE to 2.1.3.RELEASE we've encountered a bug(?) with property binding.

The example code can be found here. It's a simple application with one property class that receives values from the application.yml via the @ConfigurationProperties annotation.

Starting the DemoApplication works fine.

But starting the simple DemoApplicationTests does not work, as the demo uses "test" profile and therefore the property class receives it's values from "application-test.yml".

The excepton does NOT occur if "ignoreUnknownFields" is NOT present (i.e., set to true).

Caused by: org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'demo-properties' to com.example.demo.DemoApplication$DemoProperties
	at org.springframework.boot.context.properties.bind.Binder.handleBindError(Binder.java:249) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:225) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:208) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:190) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:83) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:107) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	... 43 common frames omitted
Caused by: org.springframework.boot.context.properties.bind.UnboundConfigurationPropertiesException: The elements [demo-properties.data[2]] were left unbound.
	at org.springframework.boot.context.properties.bind.handler.NoUnboundElementsBindHandler.checkNoUnboundElements(NoUnboundElementsBindHandler.java:88) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.handler.NoUnboundElementsBindHandler.onFinish(NoUnboundElementsBindHandler.java:73) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.handleBindResult(Binder.java:235) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:222) ~[spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	... 47 common frames omitted

The bug does only appear when the list in application-test.yml is SMALLER than the list in application.yml.

Regards,
Manuel

[snicoll]

Thanks for the sample, that is very useful.

We've tighten the mechanism to merge complex types, see the documentation.

I am not sure if we intended to throw an exception if you combine that with ignoreUnknownFields. If I add a string6 in the test profile, the exception goes away as the full list has been replaced. This shouldn't be necessary IMO.

[mbhave]

I think it's a bug and we shouldn't throw an exception since the list is completely overridden.

[rhamedy]

I would be happy to work on this issue should the team decide that it should be fixed 🙂

[mbhave]

Thanks for the offer @rhamedy but I'm already looking into this.