In this post, we will externalize the properties used in an application in a property file and will use PropertyPlaceHolderConfigurer to resolve the placeholder at startup time.

Java Configuration for PropertyPlaceHolderConfigurer

@Configuration
public class AppConfig {

    @Bean
    public PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
        PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer = new PropertySourcesPlaceholderConfigurer();
        propertySourcesPlaceholderConfigurer.setLocations(new ClassPathResource("application-db.properties"));
        //propertySourcesPlaceholderConfigurer.setIgnoreUnresolvablePlaceholders(true);
        //propertySourcesPlaceholderConfigurer.setIgnoreResourceNotFound(true);
        return propertySourcesPlaceholderConfigurer;
    }
}

We created an object of PropertySourcesPlaceholderConfigurer and set the locations to search. In this example, we used ClassPathResource to resolve the properties file from the classpath. You can use a file-based resource, which needs the absolute path of the file.

DBProperties File

@Configuration
public class DBProperties {

  @Value("${db.username}")
  private String userName;

  @Value("${db.password}")
  private String password;

  @Value("${db.url}")
  private String url;

  //getters for instance fields
}

We used the @Value annotation to resolve the placeholders.

Testing the Configuration

public class Main {
    private static final Logger logger = Logger.getLogger(Main.class.getName());

    public static void main(String[] args) {
        try (ConfigurableApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class, DBProperties.class);) {
          DBProperties dbProperties = context.getBean(DBProperties.class);
          logger.info("This is dbProperties: " + dbProperties.toString());
        }
    }
}

For testing, we created an object of AnnotationConfigApplicationContext and got theDBProperties bean from it and logged it using Logger. This is a simple way to externalize the configuration properties from framework configuration. You can also get the full example code from GitHub.