Why I can't extract server's port value from application.yml file?

[Stefan Jankovic]

I have application.yml file with following properties:

server:
  port: 8050

When I run my app, it shows application is running on port 8050.

And then I wanted to "extract" that value using some @Configuration annotated class:

@Configuration
public class GeneralConfig {
    @Value("${server.port}")
    private int portNum;

    @Value("${server.port}")
    private String portString;

    {
        System.out.println(portNum);
        System.out.println(portString);
    }

}

What I get is:

0
null

Can someone explain this weird behavior? I have done reading from yml files several times and it worked every single time. Well, except with this case. What am I missing?

[Rob Spoor]

Stefan Jankovic wrote:Can someone explain this weird behavior? I have done reading from yml files several times and it worked every single time. Well, except with this case. What am I missing?

I think you're running into an order issue. The printing is done in an initializer block. That runs after any call to a parent constructor (super(...) inside a constructor) but before any constructor body. After the constructor is done (in your case, you only have a default one) your object is initialized. It's only then that Spring can perform its magic and inject the values for these fields. That means that the injection is done after the printing.

Spring provides some techniques that allow you to inject these fields in a way you want. I can think of the following:

1) Constructor injection. Spring will call a constructor with the injected values if you annotate the constructor arguments with @Value.

@Configuration
public class GeneralConfig {
    private final int portNum;
    private final String portString;

    public GeneralConfig(@Value("${server.port}") int portNum, @Value("${server.port}") String portString) {
        this.portNum = portNum;
        this.portString = portString;

        System.out.println(portNum);
        System.out.println(portString);
    }
}

2) Any method annotated with @PostConstruct is called after any injection is performed.

@Configuration
public class GeneralConfig {
    @Value("${server.port}")
    private int portNum;

    @Value("${server.port}")
    private String portString;

    @PostConstruct
    void init() {
        System.out.println(portNum);
        System.out.println(portString);
    }
}

3) Implement InitializingBean. Its afterPropertiesSet method is called after the properties are injected (duh).

@Configuration
public class GeneralConfig implements InitializingBean {
    @Value("${server.port}")
    private int portNum;

    @Value("${server.port}")
    private String portString;

    @Override
    public void afterPropertiesSet() { // no need to add "throws Exception"
        System.out.println(portNum);
        System.out.println(portString);
    }
}

Note that you can probably also inject ServerProperties if you need more options. It still needs to be injected before you can access it like the separate properties.

[Stefan Jankovic]

Thanks for the very good reply. Everything is clear now

[Rob Spoor]

You're welcome.