How to read a .properties settings file

Short answer

Suppose there is a file app.properties — lines of the form key=value, comments start with #:

# server settings
server.port=8080
app.name=Coffee Shop

The Properties class reads such a file with load():

void main() throws IOException {
    var props = new Properties();
    try (var reader = Files.newBufferedReader(Path.of("app.properties"))) {
        props.load(reader);
    }
    IO.println(props.getProperty("app.name"));
    IO.println(props.getProperty("server.port"));
}

Output:

Coffee Shop
8080

Defaults and numbers

The second argument of getProperty() is the value used when the key is missing. Parse numbers with parseInt():

void main() throws IOException {
    var props = new Properties();
    try (var reader = Files.newBufferedReader(Path.of("app.properties"))) {
        props.load(reader);
    }
    int port = Integer.parseInt(props.getProperty("server.port", "80"));
    String mode = props.getProperty("app.mode", "production");
 
    IO.println(port + 1);
    IO.println(mode);
}

Output:

8081
production

Environment variables

Secrets such as database passwords are better kept in environment variables rather than files. System.getenv() returns null if the variable is missing:

void main() {
    String url = Objects.requireNonNullElse(System.getenv("MY_APP_DB_URL"), "jdbc:h2:mem:test");
    IO.println(url);
}

Output:

jdbc:h2:mem:test

Common mistake

Loading the file with props.load(InputStream). This version reads the file as ISO-8859-1, and non-Latin text turns into garbage. Pass a Reader from Files.newBufferedReader() — it reads UTF-8.

Thanks for reading!

Short answer

Defaults and numbers

Environment variables

Common mistake

Functions used in this recipe

Thanks for reading!