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What is Properties?

Java .properties files have been part of the Java platform since JDK 1.0 in 1996, providing a simple key=value format for externalizing application configuration and internationalization (i18n) strings. The java.util.Properties class loads these files with built-in support for Unicode escapes, continuation lines with backslash, and both = and : as key-value separators. Properties files are fundamental to the Java ecosystem: Spring Boot's application.properties (and its profile-specific variants like application-prod.properties) configures everything from server ports and database connections to logging levels and security settings.

The ResourceBundle system uses .properties files for localization — separate files like messages_en.properties, messages_fr.properties, and messages_ja.properties provide translated strings keyed by message identifiers. This i18n pattern means that large applications may maintain dozens of properties files across supported languages. Gradle uses gradle.properties for build configuration, and Apache projects like Kafka, Tomcat, and Log4j rely on .properties for runtime configuration.

The format's flat key-value structure (optionally organized with dot-notation prefixes like spring.datasource.url) makes it straightforward to parse and diff, though the lack of nesting means complex configurations use long, dot-separated key paths. Properties files support comments with # or ! prefixes, and values can span multiple lines using trailing backslashes. While YAML has gained popularity as an alternative in Spring Boot, .properties files remain the more portable and less error-prone choice, avoiding YAML's indentation sensitivity and implicit type coercion issues.