Using Google Guice to inject java properties

The library Guice configuration can inject for you values from Properties or JSON files to your services.

You can inject from the file application.properties to your service as :

@BindConfig(value = "application", syntax = PROPERTIES)
public class Service {

    @InjectConfig
    private int port;

    @InjectConfig
    private String url;

    @InjectConfig
    private Optional<Integer> timeout;
}

You must simply install the modules ConfigurationModule

public class GuiceModule extends AbstractModule {
    @Override
    protected void configure() {
        install(ConfigurationModule.create());
        requestInjection(Service.class);
    }
}

Pass the injector to all subclasses and then use injector.getInstance(...) to create the subclasses?

no, by doing this you are defeating the purpose of the dependency injection pattern and also coupling all your implementation to Guice. Your implementations should not interact at all with guice, except through the (now standardized) annotations.

Instanciate a new injector like

TestConfiguration config = new TestConfiguration(); 
Injector injector = Guice.createInjector(config); 
TestImpl test = injector.getInstance(TestImpl.class); 

in all nested classes?

no, and this is even worse cause you will end up with multiple injectors, hence multiple contexts which will prevent a proper usage of the scopes.

Ideally, you should only use the injector during the bootstrapping of your application. Of course the way to bootstrap it will largely depend of the application.

Is there an other approach to make the properties available in all classes?

The properties could be injected the same way you did for TestImpl. If you want TestImpl to use let say a service which also needs some properties (or other services), just let Guice inject it to TestImpl. Guice is taking care of all the instantiation/wiring. You should only tell Guice "how to proceed", by using the binder, when Guice cannot figure this out itself :

public class TestImpl {
    private final String property1;
    private final Integer property2;
    private final IService service;


        @Inject
        public TestImpl(@Named("Property1") String property1, @Named("Property2") Integer property2, IService service) {
           this.property1 = property1;
           this.property2 = property2;
           this.service= service;
        }
    }
}

Library "Governator" provide a configuration mapping feature for guice injection. The approach is different, but load from properties files is available.

https://github.com/Netflix/governator/wiki/Configuration-Mapping

guice-config

---

Usage example: At first we have a properties file 'config.properties':

test.key=test value

And we want to inject this value like this:

@Inject
@Config( Property.TEST_KEY )
private String injectedValue;

We need to load contents of file 'config.properties' into java.util.Properties and pass it to Config module:

Properties props = new Properties();
props.load(...);

Module configModule = new ConfigModule( props, Property.values() ); ... and injecting:

Injector injector = Guice.createInjector( configModule );
TestClass testClass = injector.getInstance( TestClass.class );
String injectedValue = testClass.getInjectedValue();

injected value will be 'test value'...