Configuration instances

Interface Config is the main access point to application configuration.

In it's simplest form, a Config instance can be created using a function that returns a String value for a given String key:

Map<String, String> values = ...;
Config config = values::get;

Interface's default methods provide additional functionality, like retrieving values as Boolean, Interger or Long.

String host = config.get("host");
Integer port = config.getInteger("port");
Boolean enabled = config.getBoolean("enabled");
Long timeout = config.getLong("timeout");
// Or with default values
String host = config.get("host", "localhost");
int port = config.getInteger("port", 8080);
boolean enabled = config.getBoolean("enabled", true);
long timeout = config.getLong("timeout", 3000L);

If the underlying configuration storage system supports it, it is possible to retrieve all property names available in the configuration:

Config config = ...;
config.isEmpty(); // check if configuration has any property
config.getKeys().forEach(key -> { ... }); // iterate all property names
config.getKeys(prefix).forEach(key -> { ... }); // iterate property names with given prefix
config.getKeys(predicate).forEach(key -> { ... }); // iterate property names matching given filter

Mutable configurations

Mutable configurations can be accesed through the MutableConfig interface, that extends Config with methods to set and remove properties.

MutableConfig config = ...;
config.set("host", "localhost");
config.set("port", 8080);
config.set("enabled", true);
config.set("timeout", 3000L);
config.remove("obsolete.property");

If the underlying configuration storage system supports it, listeners can be registered to be notified of changes to configuration properties. Those mutable configurations will implement the WatchableConfig interface, that extends MutableConfig with methods to add and remove change listeners:

WatchableConfig config = ...;
WatchableConfig.Listener listener = (instance, props) -> {
    System.out.printf("Configuration properties changed on %s%n", instance);
    System.out.prinln("New values:");
    for (String prop : props) {
        String value = instance.get(prop);
        System.out.printf(" - %s = %s%n", prop, value);
    }
};
config.addListener(listener);
// Make some changes to the configuration
config.removeListener(listener);

Configuration subtypes

Applications can provide extensions to the Config interface to provide direct access to specific configuration properties.

Subtypes must provide only default methods that access properties using the base Config, MutableConfig or WatchableConfig methods.

Proxies to underlying Config instance of subtypes can be created using Config.as() methods:

public interface DatabaseConfig extends Config {
    default String getUrl() {
        return get("db.url");
    }
    default String getUsername() {
        return get("db.username");
    }
    default String getPassword() {
        return get("db.password");
    }
}

Config config = ...;
DatabaseConfig dbConfig = config.as(DatabaseConfig.class);
String url = dbConfig.getUrl();
String username = dbConfig.getUsername();
String password = dbConfig.getPassword();

Configuration subsets

Configurations can provide subsets of properties using the Config.subset() method. The subset configuration will only provide access to properties that start with the given prefix.

Config config = ...;
Config dbConfig = config.subset("db.");
String url = dbConfig.get("url"); // actually gets "db.url"
String username = dbConfig.get("username"); // actually gets "db.username"
String password = dbConfig.get("password"); // actually gets "db.password"

Subsequent calls to subset() an as() is supported, allowing to create specific configuration subtypes for subsets of properties:

public interface SecurityConfig extends Config {
    default String getUrl() {
        return get("db.url");
    }
    default String getUsername() {
        return get("db.username");
    }
    default String getPassword() {
        return get("db.password");
    }
}

Config config = ...;
SecurityConfig dbConfig = config.subset("security.").as(SecurityConfig.class);
String url = dbConfig.getUrl(); // actually gets "security.db.url"
String username = dbConfig.getUsername(); // actually gets "security.db.username"
String password = dbConfig.getPassword(); // actually gets "security.db.password"