README.md

This project provides bridges between Spring and Guice so that you can use one from the other (and vice versa). It works with Spring 6 (or at least the tests are green), but Guice does not support the Jakarta annotations so it may break in unexpected ways.

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);

SpringModule (org.springframework.guice.module.SpringModule) will wrap the existing spring configurations for you.

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.

Registering Spring Configuration Classes as a Guice Module

If your Spring @Configuration has dependencies that can only come from a Guice Module and you prefer to use the Guice APIs to build up the configuration (so you can't use @EnableGuiceModules below), then you can create a SpringModule from a Provider<ConfigurableListableBeanFactory> instead of from an existing ApplicationContext. There are some additional features that may also apply:

• If the bean factory created by the provider is a DefaultListableBeanFactory (mostly it would be if it came from an ApplicationContext), then it will pick up a special Guice-aware AutowireCandidateResolver, meaning that it will be able to inject dependencies from Guice modules that are not registered as beans.

• If the bean factory contains any beans of type ProvisionListener (a Guice lifecysle listener), then those will be instantiated and registered with Guice.

To take advantage of the autowiring the bean factory must come from an ApplicationContext that is not fully refreshed (refreshing would resolve all the dependencies and fail because the Guice resolver is not yet registered). To help you build bean factories that have this quality there is a convenience class called BeanFactoryProvider with static methods which you can use to create a provider to inject into a SpringModule. Example:

Injector injector = Guice.createInjector(new SimpleGuiceModule(), 
    new SpringModule(BeanFactoryProvider.from(SpringConfiguration.class)));

The SimpleGuiceModule contains a component that the SpringConfiguration depends on.

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 {

    @Bean
    public static 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.

Filtering out modules from startup of ApplicationContext

In certain cases you might need to ensure that some modules are not configured at all, even though they will be absent from the final ApplicationContext. This might be due to external code that may be hard to change that causes side effects at binding time, for example. To ensure that these modules are not called at all you can define one or moreModuleFilter beans that will be applied to filter out modules from the context before they are touched by the Spring-Guice bridge.

import org.springframework.context.annotation.Bean;
import org.springframework.guice.annotation.ModuleFilter;

public static class TestConfig {


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

  @Bean
  public ModuleFilter myFilter() {
    return module -> !(module instanceof MyModule);
  }
}

This will ensure that no configure() methods are called on the filtered modules.

Configuration Class Enhancements

Note that the Module bean definition in the example above is declared in a static method. This is intentional and can be used to avoid accidentally preventing Spring from being able to enhance the parent @Configuration class. The default behaviour for Spring is to create a proxy for @Configuration classes so the @Bean methods can call each other and Spring will preserve the singleton nature of the bean factory - one bean of each type per bean id.

If you see logs like this when the context starts:

Mar 21, 2022 8:30:35 AM org.springframework.context.annotation.ConfigurationClassPostProcessor enhanceConfigurationClasses
INFO: Cannot enhance @Configuration bean definition 'TestConfig' since its singleton instance has been created too early. The typical cause is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor return type: Consider declaring such methods as 'static'.

that is a sign that you might want to use static methods to define any beans of type Module. It is logged at INFO because Spring doesn't know if it was intentional or not. Most likely it was unintentional, and it is better to avoid nasty surprises later if we can.

Another way to avoid the warning is to declare the parent class as @Configuration(proxyBeanMethods = false) so that Spring knows that you don't even want to enhance the class. It's not a bad idea to use that flag wherever you can because it saves some time on start up if the proxy doesn't need to be created.

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.

Configurable Options

For a full list of configuration options, see the configuration metadata file.

Binding Deduplication - When using @EnableGuiceModules, if a Spring Bean and a Guice Binding both exist for the same type and Qualifier, creation of the Injector will fail. You may instead prefer to keep Spring's instance of the type instead of receiving this error. To accomplish this, you may set the property spring.guice.dedup=true.

Disable Guice just-in-time bindings - When enabled (default enabled), beans without explicit definitions will be created using Guice just-in-time bindings. Otherwise, it will fail with UnsatisfiedDependencyException. To disable, set the property spring.guice.autowireJIT=false.

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 and bean names, so it ignores additional meta-information in factory requests (like annotations other than @Named). Should be easy enough to fix, but some compromises might have to 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 and all names 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).