What methods are used to store external configuration data in applications?

In Java applications, external configuration data are usually stored and loaded using the following methods:

1. Properties files (.properties) — a classic method where configuration is stored in a text format key=value. Loaded using Properties.

2. YAML or JSON files — more structured formats, often used with libraries like Jackson or SnakeYAML.

3. Environment variables — convenient for container and cloud configurations.

4. Command-line arguments — passed during application startup.

5. Spring Framework — uses application.properties or application.yml, supports profiles, external configurations, and automatic binding to POJOs.

6. Configuration servers — e.g., Spring Cloud Config, for centralized management.

Example of loading from a properties file:

Properties props = new Properties();
try (InputStream input = new FileInputStream("config.properties")) {
    props.load(input);
    String url = props.getProperty("db.url");
} catch (IOException ex) {
    ex.printStackTrace();
}

The choice of method depends on the application's requirements for structure, dynamism, and environment.