Spring Boot应用程序需要在@Configuration类中为使用@Value定义默认bean？

Question

Recently, I have encountered a pretty strange situation.

Please, consider a simple Spring Boot application:

@SpringBootApplication
public class SampleApp implements WebMvcConfigurer {

    public static void main(String[] args) {
            SpringApplication.run(SampleApp.class, args);
    }
}

Inside this SampleApp, we do have a Configuration class such as:

@Configuration
public class Config {

    //Some @Bean definitions, but without things such as String
}

And then we do have some class:

@Service
@RequiredArgsConstructor //Lombok
public class SomeService {

    @Value("${value.to.be.injected}")
    private final String someValue;
}

With the current state of things, the application refuses to launch:

***************************
APPLICATION FAILED TO START
***************************
Description:

Parameter 0 of constructor in com.sampleapp required a bean of type 'java.lang.String' that could not be found.

Action:

Consider defining a bean of type 'java.lang.String' in your configuration.

However, should we define a @Bean in our configuration such as:

@Configuration
public class Config {

    @Bean
    public String string() {
        return new String();
    }
}

The application launches correctly, and even more, injects the value correctly. Why does that happen, and why does Spring act like this?

Answer 1

My guess is that you have added that "someValue" in the constructor and it tries to create the instance but cannot call the constructor without passing a value. That's why it breaks. Then if it finds a String value (a empty String bean which is matched as the only one that fits by the dependency injection) it uses it for the creation of the object and then sets the value because of the @Value annotation.

You can try by removing the constructor of the bean (and @RequiredArgsConstructor) and it should work as expected because then all the properties will be injected using reflection and it will call the default no args constructor.

Another cool solution that would look nice and in a more "Spring" way would be to add the bean with the real value that you want to inject in the configuration and not have the @Value annotation - just use the constructor for initialization of the beans. Currently you are mixing it - part of the stuff comes from constructor and the other part is set later on.

And probably the best way (Which I should have written first maybe) is to add that value annotation to the constructor field. Then it will use the value instead of searching for a bean.

@Service
@RequiredArgsConstructor //Lombok
public class SomeService {

   private final String someValue;
   public SomeService(@Value("${value.to.be.injected}") String someValue, other stuff....) { 
     this.someValue=someValue;
   }
}