Master Spring Boot 3: Real‑World Config File Tricks and Advanced Use Cases

1. Introduction

Spring Boot supports flexible externalized configuration via application.properties or application.yml. It can load multiple files from the classpath or the current directory, merge their contents, and prioritize values based on location order, making it easy to switch between development, testing, and production environments.

2. Practical Scenarios

2.1 Default configuration loading order

When the application starts, Spring Boot searches for configuration files in the following order:

Classpath root

Classpath /config package

Current directory

Current directory /config sub‑directory

Sub‑directories of /config Values from later entries override earlier ones, and the files are merged rather than exclusive.

2.2 Changing the default file name

The default name application can be changed with the spring.config.name environment property, e.g.:

java -jar springboot-configfile-1.0.0.jar \
  --spring.config.name=pack

2.3 Modifying the search locations

Use spring.config.location to specify explicit locations (comma‑separated). The optional: prefix prevents startup failure if a file is missing.

java -jar springboot-configfile-1.0.0.jar \
  --spring.config.location=optional:classpath:/pack.yml,optional:file:f:/

2.4 Adding extra configuration files

Additional files can be loaded without replacing the defaults via spring.config.additional-location. Their properties override those from earlier locations.

java -jar springboot-configfile-1.0.0.jar \
  --spring.config.location=optional:classpath:/pack.yml \
  --spring.config.name=pack \
  --spring.config.additional-location=optional:file:f:/pack-akf.yml

2.5 Optional locations and wildcard support

Appending optional: allows non‑existent paths to be ignored. Wildcards (e.g., config/*/) expand to all matching sub‑directories, enabling batch loading of related configuration sets such as Redis and MySQL files.

java -jar springboot-configfile-1.0.0.jar \
  --spring.config.location=optional:classpath:/pack.yml \
  --spring.config.name=pack \
  --spring.config.additional-location=file:f:/sb-config/*/ \
  --spring.config.on-not-found=ignore

2.6 Profile‑specific files

When a profile (e.g., prod or live) is active, Spring Boot also loads application‑{profile}.yml. Later profiles override earlier ones.

spring:
  profiles:
    active:
    - prod
    - live

With spring.config.name=pack, pack‑prod.yml and pack‑live.yml are loaded, and the live values win.

2.7 Importing other resources

The spring.config.import property can pull in extra files (including those without extensions) at any point in the configuration hierarchy. Extension hints are supplied in brackets when importing extension‑less files.

spring:
  config:
    import:
    - file:f:/ack[.yml]

2.8 Handling missing files gracefully

Set spring.config.on-not-found=ignore to continue startup even if a configured location does not exist.

java -jar springboot-configfile-1.0.0.jar \
  --spring.config.location=optional:classpath:/pack.yml \
  --spring.config.name=pack \
  --spring.config.additional-location=file:f:/pack-akf1.yml \
  --spring.config.on-not-found=ignore

3. Code Examples

Injecting values from merged configuration files:

@Value("${pack.name}")
private String name;

@Value("${pack.age}")
private Integer age;

Reading profile‑specific properties:

@Value("${redis.name:}")
private String redisName;

@Value("${mysql.name:}")
private String mysqlName;

4. Summary

The article demonstrates how Spring Boot 3’s configuration mechanism can be customized extensively: rename the base file, relocate it, add optional and wildcard locations, import additional resources, and manage profile‑specific overrides, all while ensuring the application starts reliably even when some files are absent.