Java utility to provide default value for a class.

View on GitHub


alt "Build status"

DefaultJ is a Java library to provide a default value of any class. It support multiple strategies of getting the default value. It can be use as a simple but powerful dependency injection utility. In that aspect, it injects value base on the class, very much similar in principle with Guice.

Quick example

	public class Car {
		public static Car newInstance() { return new Car(); }
		public String go() {
			return "Zoom!";
	public class Driver {
		private Car car;
		public Driver(Car car) { = car;
		public String drive() {
			return car.go();
	Car myCar = DefaultProvider.instance.get(Car.class);
	assertEquals("Zoom!", myCar.go());
	Driver myDriver = DefaultProvider.instance.get(Driver.class);

As you can see, DefaultJ can get default values of the Car and Driver. In case of Car, since there is @Default on a static method that return Car, DefaultJ use that to create an instance. In case of Driver, its has a constructor requiring a Car instance so DefaultJ create a Car instance and use that to create a Driver instance. Calling default constructor is one of the strategy DefaultJ uses. Factory method is another strategy. DefaultJ has 14 built-in strategies for getting default values. More detail about those strategies are discussed below.

It is important to note that DefaultJ does not require the annotation @Default to be from DefaultJ. The annotation can be from any package. It just has to have RUNTIME retention. This allow any code to specify default value providing strategy without depending on DefaultJ.

Getting the default

There are strategies DefaultJ deploys to obtain a default value. Calling the default constructor is just one of such strategies. With the help of a few annotations, DefaultJ will try different strategy to obtains the default value. The followings are the strategies in order that DefaultJ attempts. If the any strategy is not applicable, DefaultJ will skip it to the next one.

  1. Bindings – Explicit binding of how the value is obtained.
  2. @DefaultImplementation – specify a class name for the be used as default (only if in the classpath).
  3. @ImplementationBy – specify a class that is the default implementation.
  4. Enum value – default value for enum.
  5. @DefaultInterface – specify that all methods in the interface are default methods.
  6. Additional finder – additional strategies.
  7. Singleton field – a public static final field in the class that hold the default value.
  8. Factory method – a public static final method in the class that return the default value.
  9. @DefaultToNull – specify that the default for this class is null.
  10. @Default on constructor – specify the default constructor to use.
  11. Only constructor – use the only constructor.
  12. Default constructor – use the default constructor (the constructor with no argument).
  13. KnowNullValueNullableJ known null value.
  14. KnownNewValueNullableJ known new value.

Note on Annotation

As mentioned, DefaultJ check for annotations for more clue on how to provide default, there are a few points to be noted on annotation used with DefaultJ.

  1. All the annotation is defined in DefaultJ’s Annotations package.
  2. DefaultJ check the annotation by name so you don’t need to use the provided annotation. If desired, you can created and use yourown annotations. The name just have to match and the retention policy must be specified to Runtime (RetentionPolicy.RUNTIME). See many unit tests for example.
  3. Annotation processors to validate the use of annotation are available in DefaultJ’s Annotations package. Consult your IDE/build system on how to enable that. The supported validations are: 1) @Default on methods/fields must be for public static and final, 2) @DefaultInterface must be on an interface with all default methods, 3) @ImplementationBy must contains the class that exists and compatible with the annotated class.

Usage, Build, Issues and Contribute

Please refer to the main README page for the information on the usage, build, issues and contribution.