DemoController.java

interface DemoInterface {
    HelloMsg sayHelloImpl(AddresseeRequest request);

    @RequestMapping(value = "/hello")
    default HelloMsg sayHello(@RequestBody AddresseeRequest request) {
        return sayHelloImpl(request);
    }
}

@RestController
public class DemoController implements DemoInterface {

    private final String message = "Hello, ";

    @Override
    public HelloMsg sayHelloImpl(AddresseeRequest request) {
        final HelloMsg msg = new HelloMsg();
        msg.addressee = request.getName();
        msg.message = message + ", " + request.getName();
        return msg;
    }
}

public class AddresseeRequest {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}

public class HelloMsg {
    public String message;
    public String addressee;
}