Injection Points

Constructor Selection

When Guice instantiates a type using its constructor, it decides which constructor to invoke by following these rules:

1. Find and return an @Inject-annotated constructor
• If any are @Inject(optional=true) constructors, record an error. Constructors may not be optional.
• If there are multiple @Inject-annotated constructors, record an error. There may be at most one @Inject-annotated constructor.
• If this constructor has binding annotations (like @Named), record an error. Binding annotations on the constructor's parameters are okay, but not on the constructor itself.
2. Find and return a no-arguments constructor
• If this constructor is private but the class is not private, record an error. Private constructors on non-private classes are not injectable without the @Inject annotation.
• If this constructor itself has binding annotations (like @Named), record an error. Binding annotations on the constructor's parameters are okay, but not on the constructor itself.
3. No suitable constructor was found. Record an error.

Injecting Fields and Methods

Injections are performed in a specific order. All fields are injected and then all methods. Within the fields, supertype fields are injected before subtype fields. Similarly, supertype methods are injected before subtype methods.

What Gets Injected

Guice injects instance methods and fields of:

• All values that are bound using toInstance(). These are injected at injector-creation time.
• All providers that are bound using toProvider(Provider). These are injected at injector-creation time.
• All instances that are registered for injection using requestInjection. These are injected at injector-creation time.
• The arguments to Injector.injectMembers when that method is invoked.
• All values instantiated by Guice via its injectable constructor, immediately after it is instantiated. Regardless of how the value is scoped, it is only injected once.

Guice injects static methods and fields of:

• All classes that are registered for static injection using requestStaticInjection.

How Guice Injects

To inject a field, Guice will:

• Lookup the binding, creating a just-in-time binding if necessary. If no just-in-time binding can be created, an error is reported.
• The binding is exercised to provide a value. How this works depends on the type of the binding.
• The binding's value is set into the field. Injecting final fields is not recommended because the injected value may not be visible to other threads.

To inject a method, Guice will:

• Lookup bindings for all of the parameters, creating just-in-time bindings if necessary. If any just-in-time binding cannot be created, an error is reported.
• The bindings are each exercised to provide a value. How this works depends on the types of the bindings.
• The method is called, using the bindings' values as arguments.