Add new binding to a Guice Module?

Question

I've my own Guice module and in the configure method, I've provided my own bindings as under -

public void configure() {
    MapBinder<String, ExternalDatabaseConnection> m = MapBinder.newMapBinder(binder(), String.class, ExternalDatabaseConnection.class);
    m.addBinding("DBServer1").to(ExternalDBServer1Connection.class);
    m.addBinding("DBServer2").to(ExternalDBServer2Connection.class);
}  

The above is deployed as a web application. I want to let third party providers be able to provide there own implementations and give a jar file for connection class. How do I do that? That is instead of modifying code above to add new binding like below -

m.addBinding("DBServer3").to(ExternalDBServer3Connection.class);

Answer 1

You may combine modules. Here is an example. Let's say you have extenral and internal modules that live separately.

public class InternalModule extends AbstractModule {

    @Override
    protected void configure() {
        MapBinder<String, String> m = MapBinder.newMapBinder(binder(), String.class, String.class);
        m.addBinding("DBServer1").toInstance("Value1");
        m.addBinding("DBServer2").toInstance("Value2");
    }

}

and:

public class ExternalModule extends AbstractModule {

    @Override
    protected void configure() {
        MapBinder<String, String> m = MapBinder.newMapBinder(binder(), String.class, String.class);
        m.addBinding("DBServer3").toInstance("Value3");
    }

}

Here is an injector based on the combination of the two modules (e.g. this may live in your app and you may implement some simple registration mechanism):

InternalModule moduleInt = new InternalModule();
ExternalModule moduleExt = new ExternalModule();

Module combined = Modules.combine(moduleInt, moduleExt);

Injector injector = Guice.createInjector(combined);

When this injector injects a map, e.g.:

@Inject
private Map<String, String> stringMap;

this map will contain the following values:

{DBServer1=Value1, DBServer2=Value2, DBServer3=Value3}

Javadoc of Modules.

It is also possible to override modules instead of combining them. In this case external libraries will replace your own implementations.

Hope this helps.

Answer 2

Contributing mapbindings from different modules is supported.

Check this for more details.