Cannot initialize a map inside @ConfigurationProperties

This fails

@SpringBootTest(properties = { "bar.foos.alice.name=alice", "bar.foos.alice.age=20",
        "bar.foos.bob.age=21" }, classes = PropertiesTests.Bar.class)
public class PropertiesTests {

    @Autowired
    private Bar bar;

    @Test
    public void test() {
        assertThat(bar.getFoos().get("alice").getAge()).isEqualTo(20);
        assertThat(bar.getFoos().get("alice").getName()).isEqualTo("alice");
        assertThat(bar.getFoos().get("bob").getAge()).isEqualTo(21);
        assertThat(bar.getFoos().get("bob").getName()).isEqualTo("bob");
    }

    @TestConfiguration
    @ConfigurationProperties("bar")
    static class Bar {
        private Map<String, Foo> foos = new HashMap<>(Map.of("bob", new Foo("bob")));

        public Map<String, Foo> getFoos() {
            return foos;
        }
    }

    static class Foo {
        private String name;
        private int age;

        Foo() {
        }

        public Foo(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }
}

Bob’s name is null.

It means you can't initialize a map if you are going to bind to it as well (there's no point as it will just get overwritten). That seems like a bug to me.

Use the supplied SpringBootTest reproduction and start at the @ConfigurationProperties binding of Bar.foos. Investigate why binding the map replaces the initialized Foo value's name, then verify the behavior against the shown assertions. Done means initialized values such as Bob's name are preserved while configured map properties still bind correctly.