Sometimes, you need to bind one instance to two interfaces in Guice, for example:
interface Foo { ... }
interface Bar extends Foo { ... }
class FooBarImpl implements Bar { ... }
Let’s imaging FooBarImpl is a stateful bean which is saved in a session scope:
bind(Bar.class)
.to(FooBarImpl.class)
.in(Session.class);
This works when someone injects Bar but injecting Foo fails. There are two solutions for this: Bind Foo to Bar or use a provider method to transform the type.
Solution 1: Binding Interfaces
bind(Foo.class)
.to(Bar.class);
This will use the mapping for Bar to satisfy requests for Foo.
Solution 2: Provider Method
Provider methods are a nice way to build complex objects from bound instances. In our case, we can map the types like so:
@Provides
@Session
public Foo fooProvider(Bar impl) {
return impl;
}
Note: These approaches will put two keys into your scope but they will point to the same instance.
Posted by digulla 
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