No primary or single unique constructor found for interface com.example.demo.models.IPerson

Question

I was working on trying to make a call to my backend and have set up a simple controller like this.

@RestController
public class HelloController {

    @RequestMapping("/hello")
    public String Hello(IPerson person) {
        return person.sayHello();
    }
}

IPerson is an interface as follows

package com.example.demo.models;

public interface IPerson {

    default String sayHello() {
        return "Hey There";
    }

}

And I have implemented the interface as

package com.example.demo.models;

import lombok.Builder;

@Builder
public class Person implements IPerson{

    String name;
    String age;

}

The call works when I change the parameter in the controller to the implementation of the interface, i.e

@RestController
public class HelloController {

    @RequestMapping("/hello")
    public String Hello(Person person) {
        return person.sayHello();
    }
}

On calling the function with IPerson as parameter I get the following error.

java.lang.IllegalStateException: No primary or single unique constructor found for interface com.example.demo.models.IPerson
    at org.springframework.beans.BeanUtils.getResolvableConstructor(BeanUtils.java:267) ~[spring-beans-5.3.12.jar:5.3.12]
    at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.createAttribute(ModelAttributeMethodProcessor.java:219) ~[spring-web-5.3.12.jar:5.3.12]
    at org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.createAttribute(ServletModelAttributeMethodProcessor.java:85) ~[spring-webmvc-5.3.12.jar:5.3.12]
    at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:147) ~[spring-web-5.3.12.jar:5.3.12]
    at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121) ~[spring-web-5.3.12.jar:5.3.12]
    at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:179) ~[spring-web-5.3.12.jar:5.3.12]

What I think might be happening is it expects an implementation of IPerson but is not able to find one but I am unsure if this kind of call is even possible and if it is then what configuration I might be missing.

Answer 1

Spring uses reflection to instantiate a model attribute in a request handling method. It will try to find a default constructor and call that.

By design in java you cannot create an instance of an interface (nor abstract class) but only for concrete classes. Hence your IPerson will fail as that is an interface and your Person will succeed as that is a class with a constructor.

So in short you cannot use interfaces to bind variables to a model object, only concrete classes.