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Googler edited this page Nov 22, 2021 · 17 revisions

Multibindings

Overview of Multibinder, MapBinder and OptionalBinder

NOTE: Since Guice 4.2, multibindings support has moved to Guice core. Before that, you need to depend on the guice-multibindings extension.

Multibinder and MapBinder are intended for plugin-type architectures, where you've got several modules contributing Servlets, Actions, Filters, Components or even just names.

OptionalBinder is intended to be used by frameworks to:

  • define an injection point that may or may not be bound by users.
  • supply a default value that can be changed by users.

Multibinding

Using Multibinder or MapBinder to host plugins.

Multibinder

Multibindings make it easy to support plugins in your application. Made popular by IDEs and browsers, this pattern exposes APIs for extending the behaviour of an application.

Neither the plugin consumer nor the plugin author need write much setup code for extensible applications with Guice. Simply define an interface, bind implementations, and inject sets of implementations! Any module can create a new Multibinder to contribute bindings to a set of implementations. To illustrate, we'll use plugins to summarize ugly URIs like http://bit.ly/1mzgW1 into something readable on Twitter.

First, we define an interface that plugin authors can implement. This is usually an interface that lends itself to several implementations. For this example, we would write a different implementation for each website that we could summarize.

interface UriSummarizer {
  /**
   * Returns a short summary of the URI, or null if this summarizer doesn't
   * know how to summarize the URI.
   */
  String summarize(URI uri);
}

Next, we'll get our plugin authors to implement the interface. Here's an implementation that shortens Flickr photo URLs:

class FlickrPhotoSummarizer implements UriSummarizer {
  private static final Pattern PHOTO_PATTERN
      = Pattern.compile("http://www\\.flickr\\.com/photos/[^/]+/(\\d+)/");

  public String summarize(URI uri) {
    Matcher matcher = PHOTO_PATTERN.matcher(uri.toString());
    if (!matcher.matches()) {
      return null;
    } else {
      String id = matcher.group(1);
      Photo photo = lookupPhoto(id);
      return photo.getTitle();
    }
  }
}

The plugin author registers their implementation using a multibinder. Some plugins may bind multiple implementations, or implementations of several extension-point interfaces.

public class FlickrPluginModule extends AbstractModule {
  public void configure() {
    Multibinder<UriSummarizer> uriBinder = Multibinder.newSetBinder(binder(), UriSummarizer.class);
    uriBinder.addBinding().to(FlickrPhotoSummarizer.class);

    ... // bind plugin dependencies, such as our Flickr API key
  }
}

Now we can consume the services exposed by our plugins. In this case, we're summarizing tweets:

public class TweetPrettifier {

  private final Set<UriSummarizer> summarizers;
  private final EmoticonImagifier emoticonImagifier;

  @Inject TweetPrettifier(Set<UriSummarizer> summarizers,
      EmoticonImagifier emoticonImagifier) {
    this.summarizers = summarizers;
    this.emoticonImagifier = emoticonImagifier;
  }

  public Html prettifyTweet(String tweetMessage) {
    ... // split out the URIs and call prettifyUri() for each
  }

  public String prettifyUri(URI uri) {
    // loop through the implementations, looking for one that supports this URI
    for (UrlSummarizer summarizer : summarizers) {
      String summary = summarizer.summarize(uri);
      if (summary != null) {
        return summary;
      }
    }

    // no summarizer found, just return the URI itself
    return uri.toString();
  }
}

Note: The method Multibinder.newSetBinder(binder, type) can be confusing. This operation creates a new binder, but doesn't override any existing bindings. A binder created this way contributes to the existing Set of implementations for that type. It would create a new set only if one is not already bound.

Finally we must register the plugins themselves. The simplest mechanism to do so is to list them programatically:

public class PrettyTweets {
  public static void main(String[] args) {
    Injector injector = Guice.createInjector(
        new GoogleMapsPluginModule(),
        new BitlyPluginModule(),
        new FlickrPluginModule()
        ...
    );

    injector.getInstance(Frontend.class).start();
  }
}

If it is infeasible to recompile each time the plugin set changes, the list of plugin modules can be loaded from a configuration file.

Note that this mechanism cannot load or unload plugins while the system is running. If you need to hot-swap application components, investigate Guice's OSGi.

Duplicate elements in Multibinder

By default Multibinder does not allow duplicates and a DUPLICATE_ELEMENT error will be thrown when duplicate elements are added to the Multibinder. Note that Guice itself deduplicates if you bind the same constant value twice, this error is only thrown if a duplicate is encountered during provisioning, e.g. when two providers return the same value. To allow duplicates, you can use permitDuplicates API on Multibinder:

public class FlickrPluginModule extends AbstractModule {
  @Override
  protected void configure() {
    Multibinder<UriSummarizer> uriBinder =
        Multibinder.newSetBinder(binder(), UriSummarizer.class);
    uriBinder.permitDuplicates();
  }
}

MapBinder

The previous example shows how to bind a Set<UrlSummarizer> with multiple modules using Multibinder. Guice also supports binding Map<K, V> using MapBinder (e.g a Map<String, UrlSummarizer>):

public class FlickrPluginModule extends AbstractModule {
  public void configure() {
    MapBinder<String, UriSummarizer> uriBinder =
        MapBinder.newMapBinder(binder(), String.class, UriSummarizer.class);
    uriBinder.addBinding("Flickr").to(FlickrPhotoSummarizer.class);

    ... // bind plugin dependencies, such as our Flickr API key
  }
}

Applications then can inject Map<String, UriSummarizer> like:

public class TweetPrettifier {

  private final Map<String, UriSummarizer> summarizers;

  @Inject TweetPrettifier(Map<String, UriSummarizer> summarizers) {
    this.summarizers = summarizers;
    ...
  }
}

Duplicate keys in MapBinder

Like Multibinder, MapBinder by default does not allow duplicates and a DUPLICATE_ELEMENT error will be thrown when duplicate elements are added to the MapBinder. However, unlike Multibinder, the uniqueness requirement is on the key of the entry and not the value.

To allow duplicates, you can use permitDuplicates API on MapBinder. When there are duplicate keys, the actual value that ends up in the final Map is unspecified:

public final class FooModule extends AbstractModule {
  @Override
  protected void configure() {
    MapBinder.newMapBinder(binder(), String.class, String.class)
        .permitDuplicates();
  }
}

public final class BarModule extends AbstractModule {
  @ProvidesIntoMap
  @StringMapKey("letter")
  String provideKeyValue() {
    return "a";
  }
}

public final class BazModule extends AbstractModule {
  @ProvidesIntoMap
  @StringMapKey("letter")
  String provideKeyValue() {
    return "b";
  }
}

In the above example, the key letter in the final Map<String, String> may have value a or b.

OptionalBinder

OptionalBinder can be used to provide optional bindings.

Frameworks often expose configuration APIs for application developers to customize the framework's behavior. OptionalBinder can make requiring optional binding easy when Guice bindings are used to customize this type of configurations.

For example, a web framework might have an API for application to supply an optional RequestLogger to log request and response:

 public class FrameworkLoggingModule extends AbstractModule {
   protected void configure() {
     OptionalBinder.newOptionalBinder(binder(), RequestLogger.class);
   }
 }

With this module, an Optional<RequestLogger> can be injected by the framework code to log the request and response after processing a request:

public class RequestHandler {
  private final Optional<RequestLogger> requestLogger;

  @Inject
  RequestHandler(Optional<RequestLogger> requestLogger) {
    this.requestLogger = requestLogger;
  }

  void handleRequest(Request request) {
    Response response = ...;
    if (requestLogger.isPresent()) {
      requestLogger.get().logRequest(request, response);
    }
  }
}

When the application doesn't provide a RequestLogger, no logging is done. If the application installs a module like:

public class ConsoleLoggingModule extends AbstractModule {
  @Provides
  RequestLogger provideRequestLogger() {
    return new ConsoleLogger(System.out);
  }
}

The framework code will get a present value that contains an instance of ConsoleLogger to log the request and response.

OptionalBinder with a default

In the above example, a RequestLogger is optional to the framework but that is not always the case. When a binding is required, the framework can use OptionalBinder to set a default binding that can be overidden by the application:

public class FrameworkLoggingModule extends AbstractModule {
 protected void configure() {
   OptionalBinder.newOptionalBinder(binder(), RequestLogger.class)
       .setDefault()
       .to(DefaultRequestLoggerImpl.class);
 }
}

With the above module, the framework's default DefaultRequestLoggerImpl is used when no application binding for RequestLogger is supplied.

Overriding the default

If one module uses setDefault the only way to override the default is to use setBinding. It is an error for a user to specify the binding without using OptionalBinder if setDefault or setBinding are called.

So to override the framework's default logger binding, application can install a module like:

public class ConsoleLoggingModule extends AbstractModule {
  @Override
  protected void configure() {
    OptionalBinder.newOptionalBinder(binder(), RequestLogger.class)
        .setBinding()
        .to(ConsoleLogger.class);
  }
}

Using @Provides-like methods

Besides using the Multibinder and MapBinder to create multibindings in a module's configure method, you can also use @Provides-like methods to add bindings to a Multibinder, MapBinder or OptionalBinder.

ProvidesIntoSet

public class FlickrPluginModule extends AbstractModule {
  @ProvidesIntoSet
  UriSummarizer provideFlickerUriSummarizer() {
    return new FlickrPhotoSummarizer(...);
  }
}

ProvidesIntoMap

public class FlickrPluginModule extends AbstractModule {
  @StringMapKey("Flickr")
  @ProvidesIntoMap
  UriSummarizer provideFlickrUriSummarizer() {
    return new FlickrPhotoSummarizer(...);
  }
}

@ProvidesIntoMap requires an extra annotation to specify the key associated with the binding. The above example uses @StringMapKey annotation, which is one of the built-in annotations that can be used with @ProvidesIntoMap, to associate the binding provided by provideFlickerUriSummarizer with the key "Flickr".

You can create custom annotation that can be used with @ProvidesIntoMap by annotating an annotation with MapKey annotation like:

@MapKey(unwrapValue=true)
@Retention(RUNTIME)
public @interface MyCustomEnumKey {
  MyCustomEnum value();
}

If unwrapValue = true, then the value of the custom annotation is used as the key of the map, otherwise the whole annotation is used as the key. The above example of MyCustomEnumKey has unwrapValue = true, so the corresponding MapBinder uses MyCustomEnum as the key instead of MyCustomEnumKey itself.

ProvidesIntoOptional

Framework code can use @ProvidesIntoOptional(ProvidesIntoOptional.Type.DEFAULT) to provide a default binding and application code can use @ProvidesIntoOptional(ProvidesIntoOptional.Type.ACTUAL) to override the default binding.

public class FrameworkModule extends AbstractModule {
  @ProvidesIntoOptional(ProvidesIntoOptional.Type.DEFAULT)
  @Singleton
  RequestLogger provideConsoleLogger() {
    return new DefaultRequestLoggerImpl();
  }
}
public class RequestLoggingModule extends AbstractModule {
  @ProvidesIntoOptional(ProvidesIntoOptional.Type.ACTUAL)
  @Singleton
  RequestLogger provideConsoleLogger() {
    return new ConsoleLogger(System.out);
  }
}

NOTE: Currently @ProvidesIntoOptional can't be used to create an absent/empty optional binding and OptionalBinder.newOptionalBinder must be used instead.

Limitations

When you use PrivateModules with multibindings, all of the elements must be bound in the same environment. You cannot create collections whose elements span private modules. Otherwise injector creation will fail.

Inspecting Multibindings

(new in Guice 3.0)

Sometimes you need to inspect the elements that make up a Multibinder or MapBinder. For example, you may need a test that strips all elements of a MapBinder out of a series of modules. You can visit a binding with a MultibindingsTargetVisitor to get details about Multibindings or MapBindings. After you have an instance of a MapBinderBinding or a MultibinderBinding you can learn more.

   // Find the MapBinderBinding and use it to remove elements within it.
   Module stripMapBindings(Key<?> mapKey, Module... modules) {
     MapBinderBinding<?> mapBinder = findMapBinder(mapKey, modules);
     List<Element> allElements = Lists.newArrayList(Elements.getElements(modules));
     if (mapBinder != null) {
       List<Element> mapElements = getMapElements(mapBinder, modules);
       allElements.removeAll(mapElements);
     }
     return Elements.getModule(allElements);
  }

  // Look through all Elements in the module and, if the key matches,
  // then use our custom MultibindingsTargetVisitor to get the MapBinderBinding
  // for the matching binding.
  MapBinderBinding<?> findMapBinder(Key<?> mapKey, Module... modules) {
    for(Element element : Elements.getElements(modules)) {
      MapBinderBinding<?> binding =
          element.acceptVisitor(new DefaultElementVisitor<MapBinderBinding<?>>() {
            MapBinderBinding<?> visit(Binding<?> binding) {
              if(binding.getKey().equals(mapKey)) {
                return binding.acceptTargetVisitor(new Visitor());
              }
              return null;
            }
          });
      if (binding != null) {
        return binding;
      }
    }
    return null;
  }

  // Get all elements in the module that are within the MapBinderBinding.
  List<Element> getMapElements(MapBinderBinding<?> binding, Module... modules) {
    List<Element> elements = Lists.newArrayList();
    for(Element element : Elements.getElements(modules)) {
      if(binding.containsElement(element)) {
        elements.add(element);
      }
    }
    return elements;
  }

  // A visitor that just returns the MapBinderBinding for the binding.
  class Visitor
      extends DefaultBindingTargetVisitor<Object, MapBinderBinding<?>>
      implements MultibindingsTargetVisitor<Object, MapBinderBinding<?>> {
    MapBinderBinding<?> visit(MapBinderBinding<?> mapBinder) {
      return mapBinder;
    }

    MapBinderBinding<?> visit(MultibinderBinding<?> multibinder) {
      return null;
    }
  }
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