Environment and property sources

Definition

The Environment is where Spring looks up settings. It reads several property sources in order (command line, environment variables, files, defaults), and the first source that has the key wins. @Value injects a setting, with an optional default.

Example code

Reading settings, with a default and an override

import java.util.Map;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.core.env.MapPropertySource;
import org.springframework.stereotype.Component;

@Component
class MailSettings {
    @Value("${mail.host:localhost}")                          // default used when the setting is missing
    String host;

    @Value("${mail.port:25}")
    int port;
}

@ComponentScan
class Demo {
    public static void main(String[] args) {
        var context = new AnnotationConfigApplicationContext();
        context.getEnvironment().getPropertySources().addFirst(new MapPropertySource("overrides", Map.of("mail.port", "587")));
        context.register(Demo.class);
        context.refresh();

        MailSettings settings = context.getBean(MailSettings.class);
        System.out.println(settings.host + ":" + settings.port);          // host from the default, port overridden
        System.out.println(context.getEnvironment().getProperty("mail.port"));
        context.close();
    }
}

// Output:
// localhost:587
// 587