.properties File — Java Properties File

Properties files are the standard configuration format for Java applications, storing key-value pairs as plain text with a simple key=value or key:value syntax. PROPERTIES files support comments (lines starting with # or !), Unicode escape sequences for international characters, multi-line values using trailing backslashes, and whitespace around the separator. The java.util.Properties class provides native read/write support, making properties files the simplest configuration mechanism in the Java ecosystem. Properties files are used for application settings, internationalization (resource bundles with locale-specific .properties files), logging configuration (log4j.properties), database connection parameters, and Spring Boot external configuration (application.properties). The format is intentionally simple — no nesting, sections, or data types beyond strings — which makes it easy to parse, edit, and manage. For more complex configuration needs, Java applications often use XML, YAML, or JSON formats. Spring Boot popularized using properties files alongside YAML as alternative configuration formats with identical capabilities. Properties files are commonly stored in src/main/resources in Maven/Gradle projects and loaded from the classpath at runtime.