apolinkevych/spring-guice

Tools for using Spring in Guice and Guice in Spring

This project provides bridges between Spring and Guice so that you can use one from the other (and vice versa)

Using a Spring ApplicationContext as a Module in Guice

The main bridge in this case is a Guice Module that wraps an existing Spring ApplicationContext. Example:

AnnotationConfigApplicationContext context = 
    new AnnotationConfigApplicationContext(ApplicationConfiguration.class);
Injector injector = Guice.createInjector(new SpringModule(context), new MyModule());
Service service = injector.getInstance(Service.class);

Note that the ApplicationContext in this example might contain the Service definition or it might be in the Guice Module (MyModule), or if Service is a concrete class it could be neither, but Guice creates an instance and wires it for us.

If the ApplicationConfiguration is annotated @GuiceModule then it can filter the types of bean that are registered with the Guice binder. Example:

@Configuration
@GuiceModule(includeFilters=@Filter(pattern=.*\\.Service))
public class ApplicationConfiguration {
    @Bean
    public MyService service() {
        ...
    }
}

In this case, only bean types (or interfaces) called "Service" will match the include filter, and only those beans will be bound.

If there are multiple @Beans of the same type in the ApplicationContext then the SpringModule will register them all, and there will be a runtime exception if an Injector needs one. As with normal Spring dependency resolution, you can add the @Primary marker to a single bean to differentiate and hint to the Injector which instance to use.

Using existing Guice Modules in a Spring ApplicationContext

The main feature here is a Spring @Configuration annotation: @EnableGuiceModules. If you have Guice Modules that you want to re-use (e.g. if they come from a third party) you can declare them in a Spring ApplicationContext as @Beans, and expose all their bindings. Example:

@EnableGuiceModules
@Configuration
public static class TestConfig extends AbstractModule {

    @Bean
    public MyModule myModule() {
        return new MyModule();
    }

    @Bean
    public Spam spam(Service service) {
        return new Spam(service);
    }

}

The Service was defined in the Guice module MyModule, and then it was be bound to the autowired spam() method when Spring started.

Using Guice as an API for accessing a Spring ApplicationContext

In this case the main feature is an Injector implementation that wraps a Spring ApplicationContext. Example:

AnnotationConfigApplicationContext context = 
    new AnnotationConfigApplicationContext(ApplicationConfiguration.class);
Injector injector = new SpringInjector(context);
Service service = injector.getInstance(Service.class);

If there is a @Bean of type Service it will be returned from the Injector. But there may actually not be a @Bean definition of type Service, and if it is a concrete type then the Injector will create it and autowire its dependencies for you. A side effect of this is that a BeanDefinition will be created.

In the example above, if ApplicationConfiguration was annotated @EnableGuiceModules then there is an Injector bean already waiting to be used, including wiring it into the application itself if you need it. So this works as well:

@Configuration
@EnableGuiceModules
public class ApplicationConfiguration {

    @Autowired
    private Injector injector;
    
    @Bean
    public Foo foo() {
        // Guice creates and does the wiring of Foo instead of Spring
        return injector.getInstance(Foo.class);
    }
}

In this example if the Injector has a binding for a Provider of Foo.class then the foo() method is redundant - it is already resolvable as a Spring dependency. But if Guice is being used as a factory for new objects that it doesn't have bindings for, then it makes sense.

Note: if you also have @GuiceModule in your context, then using the injector to create a @Bean directly is a bad idea (there's a dependency cycle). You can do it, and break the cycle, if you exclude the @Bean type from the Injector bindings using the @GuiceModule exclude filters.

Limitations

• So far there is no support for the Guice SPI methods in SpringInjector so tooling may not work. It wouldn't be hard to do.

• SpringInjector only knows about raw types, so it ignores additional meta-information in factory requests (like annotations). Should be easy enough to fix, but some compromises might hav eto be made.

• SpringInjector has no support for creating child or parent Injectors. Probably not difficult.

• SpringModule treats all beans as singletons.

• SpringModule binds all interfaces of a bean it can find. This should work out OK, as long as those interfaces are not needed for injection (and if there is no @Primary bean).