spring-017: spring-boot-property-loading-overview

Where does Spring look for application.properties?

Spring Boot searches for application.properties in the following order (highest priority wins):

🔹 Override with CLI args:

java -jar myapp.jar --spring.config.location=file:/mydir/application.properties

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How does Spring Boot load properties?

Spring Boot internally uses ConfigFileApplicationListener to load properties.

Key components:

• ConfigFileApplicationListener → Detects and loads application.properties
• PropertySourceLoader → Converts properties into key-value pairs
• YamlPropertySourceLoader → Handles application.yml

🔹 Spring Boot loads properties like this:

new ConfigFileApplicationListener().addPropertySources(environment, resourceLoader);

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What happens when a property is accessed?

When environment.getProperty("server.port") is called, Spring searches in this order:

1️⃣ Command-line arguments (--server.port=9090)

2️⃣ Environment variables (export SERVER_PORT=8081)

3️⃣ System properties (-Dserver.port=7070)

4️⃣ application.properties (server.port=6060)

5️⃣ Default value (server.port=8080)

🔹 Spring stops searching once a value is found!

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How to debug property loading?

1️⃣ Enable debug logs:

java -jar myapp.jar --debug

2️⃣ Log property loading in application.properties:

logging.level.org.springframework.boot.context.config=DEBUG

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What if application.properties is missing?

Spring Boot falls back to default values.