Binding to a collection passes in the current instance to the setter

In a @ConfigurationProperties bean if you bind to a collection Spring Boot binds the value and calls the setter (with the same instance). It’s a bit confusing and probably unexpected.

Failing test:

@RunWith(SpringRunner.class)
@SpringBootTest(properties = "app.values=foo")
public class BindingApplicationTests {

	@Autowired
	private ConfigProps props;

	@Test
	public void contextLoads() {
		assertThat(props.getValues()).contains("foo");
	}

}

@ConfigurationProperties("app")
@Component
class ConfigProps {
	private Set<String> values = new HashSet<>();

	public Set<String> getValues() {
		return values;
	}

	public void setValues(Set<String> values) {
		this.values.clear();
		this.values.addAll(values);
	}
}

Set a debug breakpoint in the setter to see what I mean.

[wilkinsona]

Changing the behaviour based on whether or not we can create a new collection sounds confusing to me.

FWIW, I think the setter in the test above is broken. It’s not a setter method as it doesn’t actually set the field’s value. I think it’s perfectly reasonable for the binder to expect that when it calls a setter, the passed in value will actually be set. It’s easy to describe and should be easy for users to implement.

[philwebb]

Perhaps if there’s a setter we should try to create a new collection. Then only mutate an existing one if that fails?