Unexpected constructor-based initialization of @ConfigurationProperties leads to inconsistent behavior

Consider the following test case:

    @RunWith(SpringRunner.class)
    @SpringBootTest(classes = { BindingTest.Config.class })
    @EnableConfigurationProperties
    @TestPropertySource
    public class BindingTest {

        @Autowired FooProperties fooProperties;
    
        @Test
        public void shouldInjectBar() {
            assertThat(fooProperties.getBar()).isEqualTo("baz");
        }
    
        @Configuration
        public static class Config {
            @Bean
            public FooProperties fooProperties() {
                return new FooProperties();
            }
        }
    
        @ConfigurationProperties("foo")
        @Validated
        public static class FooProperties {
            @NotNull @Getter @Setter
            private String bar;
        
            public FooProperties() {}
        
            public FooProperties(String bar) { // (1)
                 this.bar = bar;
            }
        }
    }

BindingTest.properties:

foo.bar=baz
foo=unrelated # (2)

I expected fooProperties.bar to be populated, or, at least, @NotNull validation error to be thrown.

However, in presence of single argument constructor (1) and conflicting property (2) (it may be, for example, a totally unrelated system environment variable), I got the following:

• fooProperties.bar is null
• no validation error is thrown

This happens because Binder decides to use ObjectToObjectConverter to initialize FooProperties using its single argument constructor (despite the fact that FooProperties is explicitly created in @Bean method), creates new instance of FooProperties and applies validation to it, but ConfigurationPropertiesBindingPostProcessor ignores that new instance.

This applies to Spring Boot 2.0.x, 2.1.x, 2.2.x.

[mbhave]

@Raheela1024 As @wilkinsona mentioned here, we want to ignore any property that matches the prefix of the @ConfigurationProperties bean that we’re binding to. For example, we want to bind foo.* to something with a foo prefix, but to ignore a foo property if one is set. This would be a change in the Binder class. It looks like the change is a bit more involved than we originally thought as quite a few tests would need to be updated. We really appreciate your offer to help but we’d like to tackle this one ourselves.

[Raheela1024]

@mbhave Sorry i was busy but now looking into will update shortly.