Author: limpbizkit
Date: Thu Jan 1 17:52:16 2009
New Revision: 781
Added:
wiki/FrequentlyAskedQuestions.wiki
Log:
Created wiki page through web user interface.
Added: wiki/FrequentlyAskedQuestions.wiki
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+=Frequently Asked Questions=
+
+===How do I inject configuration parameters?===
+You need a binding annotation to identify your parameter. Create an
annotation class that defines the parameter:
+{{{
+/**
+ * Annotates the URL of the foo server.
+ */
+...@retention(RetentionPolicy.RUNTIME)
+...@target({ElementType.FIELD, ElementType.PARAMETER})
+...@bindingannotation
+public @interface FooServerAddress {}
+}}}
+Bind the annotation to its value value in your module:
+{{{
+public class FooModule {
+ private final String fooServerAddress;
+
+ /**
+ * @param fooServerAddress the URL of the foo server.
+ */
+ public FooModule(String fooServerAddress) {
+ this.fooServerAddress = fooServerAddress;
+ }
+
+ @Override public void configure() {
+
bindConstant().annotatedWith(FooServerAddress.class).to(fooServerAddress);
+ ...
+ }
+}
+}}}
+Finally, inject it into your class:
+{{{
+public class FooClient {
+
+ @Inject
+ FooClient(@FooServerAddress String fooServerAddress) {
+ ...
+ }
+}}}
+You may save some keystrokes by using Guice's built-in `...@named` binding
annotation rather than creating your own.
+
+===How do I load configuration properties?===
+Use
[http://google-guice.googlecode.com/svn/trunk/latest-javadoc/com/google/inject/name/Names.html#bindProperties(com.google.inject.Binder,%20java.util.Properties)
Names.bindProperties()] to create bindings for each of the properties in a
configuration file.
+
+===How do I pass a parameter when creating an object via Guice?===
+You can't directly pass a parameter into an injected value. But you can
use Guice to create a `Factory`, and use that factory to create your object.
+{{{
+public class Thing {
+ // note: no @Inject annotation here
+ private Thing(A a, B b) {
+ ...
+ }
+
+ public static class Factory {
+ @Inject
+ public Factory(A a) { ... }
+ public Thing make(B b) { ... }
+ }
+}
+}}}
+
+{{{
+public class Example {
+ @Inject
+ public Example(Thing.Factory factory) { ... }
+}
+}}}
+See [AssistedInject], which can be used to remove the factory boilerplate.
+
+===How do I build two similar but slightly different trees of objects?===
+This is commonly called the "robot legs" problem: How to create a robot
with a two `Leg` objects, the left one injected with a `LeftFoot`, and the
right one with a `RightFoot`. But only one `Leg` class that's reused in
both contexts.
+
+There's a [http://docs.google.com/Doc?id=dhfm3hw2_51d2tmv6pc
PrivateModules solution]. It uses two separate private modules, a `...@left`
one and an `...@right` one. Each has a binding for the unannotated
`Foot.class` and `Leg.class`, and exposes a binding for the annotated
`Leg.class`:
+{{{
+class LegModule extends PrivateModule {
+ private final Class<? extends Annotation> annotation;
+
+ LegModule(Class<? extends Annotation> annotation) {
+ this.annotation = annotation;
+ }
+
+ @Override protected void configurePrivateBindings() {
+ bind(Leg.class).annotatedWith(annotation).to(Leg.class);
+ expose(Leg.class).annotatedWith(annotation);
+
+ bindFoot();
+ }
+
+ abstract void bindFoot();
+}
+}}}
+{{{
+ public static void main(String[] args) {
+ Injector injector = Guice.createInjector(new LegModule(Left.class) {
+ @Override void bindFoot() {
+ bind(Foot.class).toInstance(new Foot("leftie"));
+ }
+ }, new LegModule(Right.class) {
+ @Override void bindFoot() {
+ bind(Foot.class).toInstance(new Foot("righty"));
+ }
+ });
+ }
+}}}
+
+===How can I inject an inner class?===
+Guice doesn't support this. However, you can inject a _nested_ class
(sometimes called a "static inner class"):
+{{{
+class Outer {
+ static class Nested {
+ ...
+ }
+}
+}}}
+
+=== How to inject class with generic type? ===
+You may need to inject a class with a parameterized type, like
`List<String>`:
+{{{
+class Example {
+ @Inject
+ void setList(List<String> list) {
+ ...
+ }
+}
+}}}
+You need to use
[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/TypeLiteral.html
TypeLiteral] to create the binding. `TypeLiteral` is a special class that
allows you to specify a full parameterized type.
+{{{
+ @Override public void configure() {
+ bind(new TypeLiteral<List<String>>() {}).toInstance(new
ArrayList<String>());
+ }
+}}}
+
+===How can I get other questions answered?===
+Please post to the [http://groups.google.com/group/google-guice
google-guice] discussion group.
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