Guice does "just in time" binding if it encounters an injection point
which requests a type not bound.
Guice will look for an Constructor with @Inject annotation or a default
constructor in that type. When it encounters a matching constructor it
will add "just in time" a binding to the injector.
I mostly try to avoid using just in time bindings because I like the
explicity of the bindings in my modules.
On 06/20/2013 04:31 AM, Puneet Lakhina wrote:
HI,
I am trying to migrate some existing code to guice and following is a
self contained example of the setup I have:
public class GuiceMailingListQuestion {
public static interface CountrySpecificArtifact {
public abstract ISOCountry getCountry();
}
public interface CountrySpecificArtifactFactory<T extends
CountrySpecificArtifact> {
public T create(ISOCountry country);
}
public static abstract class Base<T> {
}
public static abstract class CountrySpecificBase<T> extends
Base<T> implements CountrySpecificArtifact {
private ISOCountry country;
@Inject
public CountrySpecificBase(@Assisted ISOCountry country) {
this.country = country;
}
@Override
public ISOCountry getCountry() {
return country;
}
public String toString() {
return getClass().getSimpleName() + " country=" +
country.getId();
}
}
public static class A extends CountrySpecificBase<String> {
@Inject
public A(@Assisted ISOCountry country) {
super(country);
}
}
public static class B implements CountrySpecificArtifact {
private ISOCountry country;
private A baseObject;
@Inject
public B(A baseObject, @Assisted ISOCountry country) {
this.country = country;
this.baseObject = baseObject;
}
@Override
public ISOCountry getCountry() {
return country;
}
public String toString() {
return "B country=" + country.getId() + " baseObject=" +
baseObject.toString();
}
}
public static void main(String[] args) {
Injector i = Guice.createInjector(new AbstractModule() {
@Override
protected void configure() {
install(new FactoryModuleBuilder().build(
new
TypeLiteral<CountrySpecificArtifactFactory<A>>() {}));
install(new FactoryModuleBuilder().build(
new
TypeLiteral<CountrySpecificArtifactFactory<B>>() {}));
}
});
System.out.println(i.getInstance(Key.get(new
TypeLiteral<CountrySpecificArtifactFactory<B>>(){})).create(NFCountry.US));
System.out.println(i.getInstance(Key.get(new
TypeLiteral<CountrySpecificArtifactFactory<B>>(){})).create(NFCountry.MX));
}
}
The part that surprises me about the above example is how does it work
at all? When I try to create a B using the
CountrySpecificArtiFactory<B> , how does guice figure out how to
create an A to pass to B's constructor. All I have told guice is how
to create CountrySpecificArtifactFactory<A> not how to create A.
Can you explain why this works at all?
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