Jackson mixin is ignored on serialization and deserialization

For adding properties to an object when that object is serialized you can use @JsonAppend. For example:

@JsonAppend(attrs = {@JsonAppend.Attr(value = "nameTest")})
public class Level4Mixin {}

And the test:

@BeforeEach
void setUp() throws Exception {
    mapper = new ObjectMapper()
            .addMixIn(Level4Impl.class, Level4Mixin.class);
}

@Test
void test_ImplWrite() throws JsonProcessingException {
    Level4Impl lvl4 = new Level4Impl();
    lvl4.setId(1L);
    lvl4.setName("test");

    String json = mapper.writerFor(Level4Impl.class)
            .withAttribute("nameTest", "myValue")
            .writeValueAsString(lvl4);

    assertNotNull(json);
    assertTrue(json.contains("nameTest"));
    assertTrue(json.contains("myValue"));
}

The same works for test_InterfaceWrite.

The tests to deserialize a json into an object are not clear:

@Test
void test_ImplRead() {
    String json = "{\"id\":1,\"nameTest\":\"test\"}";
    assertDoesNotThrow(() -> {
        Level4Impl parsed = mapper.readValue(json, Level4Impl.class);
        assertNotNull(parsed);
    });
}

The class Level4Impl does not have the property nameTest so the deserialization fails. If you don't want to throw the exception you can configure the ObjectMapper to don't fail on unknown properties. For example:

@Test
void test_ImplRead() {
    String json = "{\"id\":1,\"nameTest\":\"test\"}";
    assertDoesNotThrow(() -> {
        Level4Impl parsed = new ObjectMapper()
                .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
                .readValue(json, Level4Impl.class);
        assertNotNull(parsed);
    });
}

First of all you have to let Jackson know which subclass of your interface it should instantiate. You do it by adding @JsonTypeInfo and/or @JsonSubTypes annotations to your mix-in class. For single subclass the following would suffice:

@JsonTypeInfo(use = Id.NAME, defaultImpl = Level4Impl.class)
public abstract class Level4Mixin {
}

For multiple sub-classes it will a bit more complex and will require additional field in JSON payload that will identify concrete type. See Jackson Polymorphic Deserialization for details. Also worth mentioning that adding type info will cause type ID field to be written to JSON. JFYI.

Adding new label would be as trivial as adding a pair of getter and setter for desired property. Obviously original name field will be written to JSON too in this case. To change that you may want to place @JsonIgnore on getter in subclass or in mix-in. In latter case name will be ignored for all sub-classes.

Last note: in this case you should register your mix-in with super-type only.

Here are the changes to your classes that satisfy your tests:

Level4Impl

public class Level4Impl implements Level4 {

    private Long id;
    private String name;

    @Override
    public Long getId() {
        return id;
    }

    @Override
    public void setId(Long id) {
        this.id = id;
    }

    @Override
    public String getName() {
        return name;
    }

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

    public String getNameTest() {
        return name;
    }

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

}

Mixin

@JsonTypeInfo(use = Id.NAME, defaultImpl = Level4Impl.class)
public interface Level4Mixin {

    @JsonIgnore
    String getName();
}

Level4MixinTest change

@BeforeEach
void setUp() throws Exception {
    mapper = new ObjectMapper();
    mapper.addMixIn(Level4.class, Level4Mixin.class);
    // remove 
    //mapper.addMixIn(Level4Impl.class, Level4Mixin.class);
}

In case you can't make it work by default (which was my case), try to modify existing MappingJackson2HttpMessageConverter converter, e.g. do it this way:

@Configuration
public class WebMvcConfig implements WebMvcConfigurer {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.stream()
            .filter(c -> c instanceof MappingJackson2HttpMessageConverter)
            .forEach(c -> {
                MappingJackson2HttpMessageConverter converter = (MappingJackson2HttpMessageConverter) c;
                converter.getObjectMapper().addMixIn(APIResponse.class, MixInAPIResponse.class);
            });
}

Where MixInAPIResponse is your configured MixIn class for target class ApiResponse.