sameb edited this page Jul 7, 2014 · 1 revision

Bindings that are created automatically by Guice

Just-in-time Bindings

When the injector needs an instance of a type, it needs a binding. The bindings in a modules are called explicit bindings, and the injector uses them whenever they're available. If a type is needed but there isn't an explicit binding, the injector will attempt to create a Just-In-Time binding. These are also known as JIT bindings and implicit bindings.

Eligible Constructors

Guice can create bindings for concrete types by using the type's injectable constructor. This is either a non-private, no-arguments constructor, or a constructor with the @Inject annotation:

public class PayPalCreditCardProcessor implements CreditCardProcessor {
  private final String apiKey;

  public PayPalCreditCardProcessor(@Named("PayPal API key") String apiKey) {
    this.apiKey = apiKey;

Guice will not construct nested classes unless they have the static modifier. Inner classes have an implicit reference to their enclosing class that cannot be injected.


Annotate types tell the injector what their default implementation type is. The @ImplementedBy annotation acts like a linked binding, specifying the subtype to use when building a type.

public interface CreditCardProcessor {
  ChargeResult charge(String amount, CreditCard creditCard)
      throws UnreachableException;

The above annotation is equivalent to the following bind() statement:


If a type is in both a bind() statement (as the first argument) and has the @ImplementedBy annotation, the bind() statement is used. The annotation suggests a default implementation that can be overridden with a binding. Use @ImplementedBy carefully; it adds a compile-time dependency from the interface to its implementation.


@ProvidedBy tells the injector about a Provider class that produces instances:

public interface TransactionLog {
  void logConnectException(UnreachableException e);
  void logChargeResult(ChargeResult result);

The annotation is equivalent to a toProvider() binding:


Like @ImplementedBy, if the type is annotated and used in a bind() statement, the bind() statement will be used.

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