How Spring Boot Decides Which Config Value Wins When Sources Conflict

Spring Boot uses a concept called externalized configuration, which keeps app settings like database URLs, ports, and API keys outside compiled code in files, environment variables, or command-line arguments. All these settings are loaded at startup into a unified key-value store called the Environment, which any part of the application can query. Developers can access these values using the @Value annotation or by directly interacting with the Environment object. When the same setting is defined in multiple places — for example, in an application.properties file and as a command-line flag — Spring follows a strict priority order to determine which value takes effect. In this order, command-line arguments rank higher than property files, so passing --server.port=9000 at launch will override a file-based setting of 8080.