Property values in bean class are null in constructor

Question

I'm trying to use values from my application.properties file in my service implementation class/bean. But when the bean is initialized through my config class, the property values are all null.

Config class:

@Configuration
public class AppConfig {
    @Bean AppServiceImpl appServiceImpl() {
        return new AppServiceImpl();
    }
}

Service class:

@Component
public class AppServiceImpl implements AppService {
    @Value("${value.one}")
    String value_one;

    @Value("${value.two}")
    String value_two;

    @Value("${value.three}")
    String value_three;

    //values are null here
    public AppServiceImpl() {
        functionOne(value_one, value_two, value_three);
    }
}

application.properties(under src/main/resources):

value.one=1
value.two=2
value.three=3

Doing some debugging i can see that the AppConfig class has found the properties file and if i try to declare @Value("${value.one}") String value_one; there it shows it has been given the value 1 as expected.

But in my AppServiceImpl class, all the values are null. What am I doing wrong here? How should this be done properly in Springboot? Or just Spring even.

Thanks.

Answer 1

If you use the values in the constructor they won't be available right away. Indeed they are injected on attribute. What's happening here is after spring created an instance then it will update the attribute value.

If you want to use those values in the constructor you should use constructor injection. Injection by constructor is a best practice.

public class AppServiceImpl implements AppService {
    String value_one;
    String value_two;
    String value_three;

    //values are null here
    public AppServiceImpl(String value1, String value2, String value3) {
        value_one = value1;
        value_two = value2;
        value_three = value3;
        functionOne(value_one, value_two, value_three);
    }
}

And your configuration class

@Configuration
public class AppConfig {
    @Bean
    AppServiceImpl appServiceImpl(@Value("${value.one}") String value1,  @Value("${value.two}") String value2,  @Value("${value.three}") String value3) {
        return new AppServiceImpl(value1, value2, value3);
    }
}

Answer 2

Those values are injected after the instance is created. So they are null in the constructor.

To execute a method after the values are injected use @javax.annotation.PostConstruct:

@PostConstruct
public void init(){ // method name doesn't matter
     functionOne(value_one, value_two, value_three);
}