A module whose configuration information is hidden from its environment by default. Only bindings that are explicitly exposed will be available to other modules and to the users of the injector. This module may expose the bindings it creates and the bindings of the modules it installs.

A private module can be nested within a regular module or within another private module using install(). Its bindings live in a new environment that inherits bindings, type converters, scopes, and interceptors from the surrounding ("parent") environment. When you nest multiple private modules, the result is a tree of environments where the injector's environment is the root.

Guice EDSL bindings can be exposed with expose(). @Provides bindings can be exposed with the @Exposed annotation:

public class FooBarBazModule extends PrivateModule {
  protected void configure() {
    bind(Foo.class).to(RealFoo.class);
    expose(Foo.class);

    install(new TransactionalBarModule());
    expose(Bar.class).annotatedWith(Transactional.class);

    bind(SomeImplementationDetail.class);
    install(new MoreImplementationDetailsModule());
  }

  @Provides @Exposed
  public Baz provideBaz() {
    return new SuperBaz();
  }
}

Private modules are implemented using parent injectors. When it can satisfy their dependencies, just-in-time bindings will be created in the root environment. Such bindings are shared among all environments in the tree. See the note in createChildInjector about how hierarchical injectors change the importance of otherwise unnecessary binding statements (such as bind(ServiceImpl.class);).

The scope of a binding is constrained to its environment. A singleton bound in a private module will be unique to its environment. But a binding for the same type in a different private module will yield a different instance.

A shared binding that injects the Injector gets the root injector, which only has access to bindings in the root environment. An explicit binding that injects the Injector gets access to all bindings in the child environment.

To promote a just-in-time binding to an explicit binding, bind it:

  bind(FooImpl.class);