.properties
Also known as: properties file, Java properties, config properties
A .properties file is a plain-text configuration format, most associated with Java, that stores settings as simple key=value pairs. Each line sets one property, making it an easy, human-readable way to configure an application.
- Plain-text key=value configuration format
- Classic for Java apps and localization strings
- Flatter and simpler than YAML or JSON
How properties files work
Each line is a setting written as key=value (or key: value), with lines starting in # treated as comments. There is no nesting — just a flat list of named values an app reads at startup.
The format is the classic way to configure Java applications and frameworks, holding things like database URLs, feature flags, and localization strings. Its simplicity is the point.
Properties vs other config formats
Compared with YAML or JSON, .properties is flatter and simpler, with no native support for nested structures or lists beyond conventions. That makes it readable but less expressive for complex configuration.
These are tiny text files you edit in any editor. They cost no real storage and are meant to be human-edited.