properties

Properties

Spring supports externalized configuration in a flexible way.

@ConfigurationProperties

@ConfigurationProperties is a class-level annotation that allows you to bind external properties to a class. As we bind properties to a class, we can use them as regular Java objects. It makes the code to be more readable and maintainable.

How to define

We can bind properties to a class by using @ConfigurationProperties annotation.

@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private String name;
    private String version;
    private String description;
}

But there are many ways to bind property values to a class.

@EnableConfigurationProperties

@EnableConfigurationProperties is a class-level annotation that allows you to register @ConfigurationProperties. It is used to register @ConfigurationProperties annotated classes. Usually, it is used in the main class.

@EnableConfigurationProperties(AppProperties.class)
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

However, it has drawback. If we have more classes with properties, we need to register all of them in the main class. It will bother us.

Meta-annotation

A meta-annotation like @Configuration allows a bean to be registered in the Spring context.

@Configuration
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    // ...
    
    // needs getter and setter
}

This is not ideal for immutable classes because a setter could potentially alter our properties at any time.

@ConfigurationPropertiesScan

@ConfigurationPropertiesScan is similar to @ComponentScan. It scans the package for @ConfigurationProperties classes and registers them in the Spring context. Therefore, we don't need to update the main class if we add a new one.

@SpringBootApplication
@ConfigurationPropertiesScan()
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

In addition, it is possible to make a properties class immutable using final or record.

@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private final String name;
    private final String version;
    private final String description;
    
    // @ConstructorBinding // it is required for more than one constructor to specify which one to use.
    public AppProperties(String name, String version, String description) {
        this.name = name;
        this.version = version;
        this.description = description;
    }
}

@ConfigurationProperties(prefix = "app")
public record AppProperties(String name, String version, String description) {
}

Therefore, I recommend using @ConfigurationPropertiesScan and record to make properties class immutable.

References

• Guide to @ConfigurationProperties in Spring Boot - Baeldung