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2.5 Environment Variables and Configuration Properties (application.yml, @ConfigurationProperties)

Spring Boot provides various ways to easily separate external configurations from the application code. In this chapter, we will look into how to utilize application.yml and @ConfigurationProperties which provides type safety.

1. application.properties vs application.yml​

Although Spring Boot generates an application.properties file by default, the YAML format is overwhelmingly preferred in practice because it allows for hierarchical configuration.

application.yml Example:

server:
  port: 8080

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/mydb
    username: root
    password: password

2. Injecting Single Values using @Value​

The simplest way to inject property values is using the @Value annotation.

@Service
public class EmailService {
    
    @Value("${spring.mail.host}")
    private String mailHost;

    public void sendEmail() {
        System.out.println("Host: " + mailHost);
    }
}
warning

While @Value is simple, typos or missing values can cause runtime errors and it lacks type safety.

3. Type-Safe Mapping using @ConfigurationProperties​

In production, it is highly recommended to logically group configuration values into a single Java Bean object using @ConfigurationProperties.

@Getter
@Setter
@Component
@ConfigurationProperties(prefix = "app.aws.s3")
public class AwsS3Properties {
    private String accessKey;
    private String secretKey;
    private String bucketName;
}

To activate these properties, add @ConfigurationPropertiesScan to the main class, or simply attach @Component to the class itself.

# application.yml
app:
  aws:
    s3:
      access-key: AKIA...
      secret-key: aBcd...
      bucket-name: my-asset-bucket

Advantages of @ConfigurationProperties​

  1. Robust IDE auto-completion support preventing simple typos.
  2. Detects type mismatch errors before runtime at application startup.
  3. Can be validated continuously by combining with JSR-303 (@Validated).
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