从外部配置文件读取值将返回空值

Question

I have a configuration file config.properties that contains the following content:

myFirstName='John'  
myLastName='Doe'

I also have a very simple config class that simply outputs the values of the config.properties file:

@Configuration
@ComponentScan(basePackages = {"com.boot.Training.*"})
@PropertySource("classpath:config.properties")
public class AppConfig {

    @Value("${myFirstName}")
    private static String myFirstName;

    @Value("${myLastName}")
    private static String myLastName;

    public static void showVariables() {
        System.out.println("firstName: " + myFirstName);
        System.out.println("lastName: " + myLastName);
    }

}

The problem is, when the values are output to the console, they appear null, even though they are obviously given a value in the config.properties file.:

2018-09-08 10:52:46.334  INFO 2787 --- [           main] c.b.Training.EnvironmentVariables.App    : Started App in 2.542 seconds (JVM running for 3.084)
firstName: null
lastName: null

What am I missing here?

Answer 1

Your problem is about the static keyword. Because static fields initialization happens in many processor-cycles before Spring containers starts up . So remove the static keyword from your class variable ;

@Value("${myFirstName}")
private String myFirstName;

@Value("${myLastName}")
private String myLastName;

EDIT : I added my code which is working correctly. And my main class which is starting spring context like this ;

public class AppMain {

    public static void main(String args[]){
        AbstractApplicationContext  context = new AnnotationConfigApplicationContext(AppConfig.class);
        context.close();
    }

}

So In my AppConfig.class like this;

@Configuration
@ComponentScan(basePackages = "com.example.spring")
@PropertySource(value = {"classpath:application.properties"})
public class AppConfig implements InitializingBean {

    @Value("${firstKey}")
    private String myFirstName;

    @Value("${secondKey}")
    private String myLastName;

    public void showVariables() {
        System.out.println("firstName: " + myFirstName);
        System.out.println("lastName: " + myLastName);
    }

    public void afterPropertiesSet() throws Exception {
        showVariables();
    }
}

Dont care about implementing InitializingBean . It provide us what do u want when bean was created. Like a postConstruct as well.

Also my application.configuration is ;

firstKey = firsKeyValue
secondKey = secondKeyValue

Now when i am starting the app , i see this output ;

org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@37bba400: startup date [Sun Sep 09 11:37:35 EET 2018]; root of context hierarchy
firstName: firsKeyValue
lastName: secondKeyValue

It's working i think. Apply your requirements and try once.