Guice Dependency Injection Tutorial: Modules, Bindings, Scopes, and Assisted Injection

Guice is a Google‑open‑source dependency injection (DI) library that is smaller and faster than Spring IoC, widely used in projects like Elasticsearch. It reduces the need for factory methods and new calls, making code easier to test and reuse.

Learning Objectives include understanding Guice basics, quickly starting with examples, mastering core concepts such as binding, scope, and injection, and applying official best‑practice recommendations.

Core Concepts are illustrated with a simple billing service example. Interfaces like BillingService and TransactionLog are bound to concrete implementations ( RealBillingService , DatabaseTransactionLog ) in a BillingModule that extends AbstractModule and overrides configure() to call bind(...).to(...) .

Various binding styles are covered:

Linked Binding : bind(TransactionLog.class).to(DatabaseTransactionLog.class);

Annotated Binding with custom annotations or @Named to differentiate multiple implementations.

Instance Binding : bind(String.class).annotatedWith(Names.named("JDBC URL")).toInstance("jdbc:mysql://localhost/pizza");

@Provides Method Binding : a method annotated with @Provides returns the object to bind, allowing custom construction logic.

Provider Binding : implement Provider<TransactionLog> for complex creation logic.

Constructor Binding (Guice 3.0): bind a class to a specific constructor via toConstructor(...) .

Scope : default is no‑scope (new instance each injection); you can apply @Singleton , custom scopes, or eager singletons with asEagerSingleton() .

Injection methods include constructor injection (the preferred way), method injection, and field injection, with examples showing @Inject on constructors and setters. Optional injection is demonstrated using @Inject(optional = true) .

Assisted injection is explained as a way to inject parameters that are not managed by Guice, using @Assisted and a factory interface, or the newer FactoryModuleBuilder approach.

The article also contains practical notes on testing with mock implementations, handling multiple implementations via annotation binding, and using Guice’s built‑in @Named for simple qualifiers.