System.getenv()

Examples

The variable is set when the program starts. On Linux and macOS: APP_MODE=production java Main.java, in PowerShell first run $env:APP_MODE = "production":

void main() {
    String mode = System.getenv("APP_MODE");
    IO.println("Mode: " + mode);
}

Output:

Mode: production

If the variable is missing, the method returns null — substitute a default value:

void main() {
    String port = System.getenv("APP_PORT");
    IO.println(port);
 
    int value = port != null ? Integer.parseInt(port) : 8080;
    IO.println(value);
}

Output:

null
8080

Notes

Environment variables are set by the system or by whoever starts the program; Java can only read them. On Windows the names are case-insensitive, on Linux and macOS they are case-sensitive. Without an argument System.getenv() returns an unmodifiable Map with all variables. A short way to give a default is Objects.requireNonNullElse(System.getenv("APP_PORT"), "8080"). Passwords and API keys are usually passed through environment variables rather than written in the code.