Author: limpbizkit
Date: Mon Mar 2 21:04:08 2009
New Revision: 890
Added:
wiki/BindingAnnotations.wiki
Log:
Created wiki page through web user interface.
Added: wiki/BindingAnnotations.wiki
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+=Binding Annotations=
+Occasionally you'll want multiple bindings for a same type. For example,
you might want both a PayPal credit card processor and a Google Checkout
processor. To enable this, bindings support an optional *binding
annotation*. The annotation and type together uniquely identify a binding.
This pair is called a *key*.
+
+Defining a binding annotation requires two lines of code plus several
imports. Put this in its own `.java` file or inside the type that it
annotates.
+{{{
+package example.pizza;
+
+import com.google.inject.BindingAnnotation;
+import java.lang.annotation.Target;
+import java.lang.annotation.Retention;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+
+...@bindingannotation @Target({ FIELD, PARAMETER, METHOD })
@Retention(RUNTIME)
+public @interface PayPal {}
+}}}
+You don't need to understand all of these meta-annotations, but if you're
curious:
+ * `...@bindingannotation` tells Guice that this is a binding annotation.
Guice will produce an error if ever multiple binding annotations apply to
the same member.
+ * `...@target({FIELD, PARAMETER, METHOD})` is a courtesy to your users. It
prevents `...@paypal` from being accidentally being applied where it serves no
purpose.
+ * `...@retention(RUNTIME)` makes the annotation available at runtime.
+
+To depend on the annotated binding, apply the annotation to the injected
parameter:
+{{{
+public class RealBillingService implements BillingService {
+
+ @Inject
+ public RealBillingService(@PayPal CreditCardProcessor processor,
+ TransactionLog transactionLog) {
+ ...
+ }
+}}}
+Lastly we create a binding that uses the annotation. This uses the
optional `annotatedWith` clause in the `bind()` statement:
+{{{
+ bind(CreditCardProcessor.class)
+ .annotatedWith(PayPal.class)
+ .to(PayPalCreditCardProcessor.class);
+}}}
+
+
+...@named==
+Guice comes with a built-in binding annotation `...@named` that uses a string:
+{{{
+public class RealBillingService implements BillingService {
+
+ @Inject
+ public RealBillingService(@Named("Checkout") CreditCardProcessor
processor,
+ TransactionLog transactionLog) {
+ ...
+ }
+}}}
+To bind a specific name, use `Names.named()` to create an instance to pass
to `annotatedWith`:
+{{{
+ bind(CreditCardProcessor.class)
+ .annotatedWith(Names.named("Checkout"))
+ .to(PayPalCreditCardProcessor.class);
+}}}
+Since the compiler can't check the string, we recommend using `...@named`
sparingly.
+
+
+==Binding Annotations with Attributes==
+Guice supports binding annotations that have attribute values. In the rare
case that you need such an annotation:
+ # Create the annotation `...@interface`.
+ # Create a class that implements the annotation interface. Follow the
guidelines for `equals()` and `hashCode()` specified in the
[http://java.sun.com/javase/6/docs/api/java/lang/annotation/Annotation.html
Annotation Javadoc]. Pass an instance of this to the `annotatedWith()`
binding clause.
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