Scopes

Scopes

By default, Guice returns a new instance each time it supplies a value. This behaviour is configurable via scopes. Scopes allow you to reuse instances: for the lifetime of an application, a session, or a request. Guice comes with a built-in @Singleton (prefer javax.inject.Singleton over com.google.inject.Singleton) scope that reuses the same instance during the lifetime of an application. The servlet extension includes additional scopes for web apps such as @RequestScoped. Custom scopes can be written for other types of applications.

Applying Scopes

Guice uses annotations to identify scopes. Specify the scope for a type by applying the scope annotation to the implementation class. As well as being functional, this annotation also serves as documentation. For example, @Singleton indicates that the class is intended to be threadsafe.

@Singleton
public class InMemoryTransactionLog implements TransactionLog {
  /* everything here should be threadsafe! */
}

Scopes can also be configured in bind statements:

  bind(TransactionLog.class).to(InMemoryTransactionLog.class).in(Singleton.class);

And by annotating @Provides methods:

  @Provides @Singleton
  TransactionLog provideTransactionLog() {
    ...
  }

If there's conflicting scopes on a type and in a bind() statement, the bind() statement's scope will be used. If a type is annotated with a scope that you don't want, bind it to Scopes.NO_SCOPE.

In linked bindings, scopes apply to the binding source, not the binding target. Suppose we have a class Applebees that implements both Bar and Grill interfaces. These bindings allow for two instances of that type, one for Bars and another for Grills:

  bind(Bar.class).to(Applebees.class).in(Singleton.class);
  bind(Grill.class).to(Applebees.class).in(Singleton.class);

This is because the scopes apply to the bound type (Bar, Grill), not the type that satisfies that binding (Applebees). To allow only a single instance to be created, use a @Singleton annotation on the declaration for that class. Or add another binding:

  bind(Applebees.class).in(Singleton.class);

This binding makes the other two .in(Singleton.class) clauses above unnecessary.

The in() clause accepts either a scoping annotation like RequestScoped.class and also Scope instances like ServletScopes.REQUEST:

  bind(UserPreferences.class)
      .toProvider(UserPreferencesProvider.class)
      .in(ServletScopes.REQUEST);

The annotation is preferred because it allows the module to be reused in different types of applications. For example, an @RequestScoped object could be scoped to the HTTP request in a web app and to the RPC when it's in an API server.

Eager Singletons

Guice has special syntax to define singletons that can be constructed eagerly:

  bind(TransactionLog.class).to(InMemoryTransactionLog.class).asEagerSingleton();

Eager singletons reveal initialization problems sooner, and ensure end-users get a consistent, snappy experience. Lazy singletons enable a faster edit-compile-run development cycle. Use the Stage enum to specify which strategy should be used.

	PRODUCTION	DEVELOPMENT
.asEagerSingleton()	eager	eager
.in(Singleton.class)	eager	lazy
.in(Scopes.SINGLETON)	eager	lazy
@Singleton	eager*	lazy

* Guice will only eagerly build singletons for the types it knows about. These are the types mentioned in your modules, plus the transitive dependencies of those types.

Choosing a scope

If the object is stateful, the scoping should be obvious. Per-application is @Singleton, per-request is @RequestScoped, etc. If the object is stateless and inexpensive to create, scoping is unnecessary. Leave the binding unscoped and Guice will create new instances as they're required.

Singletons are popular in Java applications but they don't provide much value, especially when dependency injection is involved. Although singletons save object creation (and later garbage collection), initialization of the singleton requires synchronization; getting a handle to the single initialized instance only requires reading a volatile. Singletons are most useful for:

• stateful objects, such as configuration or counters
• objects that are expensive to construct or lookup
• objects that tie up resources, such as a database connection pool.

Scopes and Concurrency

Classes annotated @Singleton and @SessionScoped must be threadsafe. Everything that's injected into these classes must also be threadsafe. Minimize mutability to limit the amount of state that requires concurrency protection.

@RequestScoped objects do not need to be threadsafe. It is usually an error for a @Singleton or @SessionScoped object to depend on an @RequestScoped one. Should you require an object in a narrower scope, inject a Provider of that object.