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Importing properties

You can use another mechanism to load your properties into a mapping interface. And this mechanism is to specify a Properties object programmatically when calling ConfigFactory.create():

public interface ImportConfig extends Config {

    String foo();

    String bar();

    String baz();


// then...

Properties props = new Properties();
props.setProperty("foo", "pineapple");
props.setProperty("bar", "lime");

ImportConfig cfg = ConfigFactory
    .create(ImportConfig.class, props); // props imported!

assertEquals("orange", cfg.baz());
Null keys and Null values are invalid.

A Properties or Map object accepts null key or null values, but that is obviously an error, so starting from version 1.0.10, an IllegalArgumentException is thrown.

The exception message also contains further information about the offending key, if applicable.

You can specify multiple properties to import on the same line:

ImportConfig cfg = ConfigFactory
    .create(ImportConfig.class, props1, props2, ...);

If there are prop1 and prop2 defining two different values for the same property key, the one specified first will prevail:

Properties p1 = new Properties();
p1.setProperty("foo", "pineapple");
p1.setProperty("bar", "lime");

Properties p2 = new Properties();
p2.setProperty("bar", "grapefruit");
p2.setProperty("baz", "blackberry");

ImportConfig cfg = ConfigFactory
    .create(ImportConfig.class, p1, p2); // props imported!


// p1 prevails, so this is lime and not grapefruit

assertEquals("blackberry", cfg.baz());

This is pretty handy if you want to reference system properties or environment variables:

interface SystemEnvProperties extends Config {
    String fileSeparator();

    String javaHome();

    String home();

    String user();

    void list(PrintStream out);

SystemEnvProperties cfg = ConfigFactory
assertEquals(File.separator, cfg.fileSeparator());
assertEquals(System.getProperty("java.home"), cfg.javaHome());
assertEquals(System.getenv().get("HOME"), cfg.home());
assertEquals(System.getenv().get("USER"), cfg.user());

Interactions with loading strategies

Notice that the “importing properties” feature is additional to the properties loading mechanism explained in chapter Loading strategies.

Properties imported programmatically have higher priority regarding the properties loaded from the @Sources attribute.

Imagine the scenario where the you define your configuration with @Sources annotation, but you want to allow the user to specify a configuration file at the command line.

interface MyConfig extends Config { 

public static void main(String[] args) {
    MyConfig cfg;
    if (args.lenght() > 0) {
        Properties props = new Properties();
        props.load(new FileInputStream(new File(args[0])));
        cfg = ConfigFactory.create(MyConfig.class, userProps);
    } else {
        cfg = ConfigFactory.create(MyConfig.class);

In the above example, the properties file specified by the user will override the properties loaded by @Sources if there is overlapping between the properties names. This approach is used by many command line tools, that allow the user to specify a configuration on the command line that overrides the default one.

This is true only with version and superior!

Be aware that in versions prior to imported properties have lower priority than others loaded properties. This behavior has been changed in version and it will be kept this way for future releases.