extensions/multibindings/src/com/google/inject/multibindings/MapBinder.java

/**
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject.multibindings;

import com.google.inject.Binder;
import com.google.inject.Inject;
import com.google.inject.Key;
import com.google.inject.Module;
import com.google.inject.Provider;
import com.google.inject.TypeLiteral;
import com.google.inject.binder.LinkedBindingBuilder;
import com.google.inject.multibindings.Multibinder.RealMultibinder;
import static com.google.inject.multibindings.Multibinder.checkConfiguration;
import static com.google.inject.multibindings.Multibinder.checkNotNull;
import com.google.inject.util.Types;
import static com.google.inject.util.Types.newParameterizedType;
import static com.google.inject.util.Types.newParameterizedTypeWithOwner;
import java.lang.annotation.Annotation;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

/**
 * An API to bind multiple map entries separately, only to later inject them as
 * a complete map. MapBinder is intended for use in your application's module:
 * <pre><code>
 * public class SnacksModule extends AbstractModule {
 *   protected void configure() {
 *     MapBinder&lt;String, Snack&gt; mapbinder
 *         = MapBinder.newMapBinder(binder(), String.class, Snack.class);
 *     mapbinder.addBinding("twix").toInstance(new Twix());
 *     mapbinder.addBinding("snickers").toProvider(SnickersProvider.class);
 *     mapbinder.addBinding("skittles").to(Skittles.class);
 *   }
 * }</code></pre>
 *
 * <p>With this binding, a {@link Map}{@code <String, Snack>} can now be 
 * injected:
 * <pre><code>
 * class SnackMachine {
 *   {@literal @}Inject
 *   public SnackMachine(Map&lt;String, Snack&gt; snacks) { ... }
 * }</code></pre>
 * 
 * <p>In addition to binding {@code Map<K, V>}, a mapbinder will also bind
 * {@code Map<K, Provider<V>>} for lazy value provision:
 * <pre><code>
 * class SnackMachine {
 *   {@literal @}Inject
 *   public SnackMachine(Map&lt;String, Provider&lt;Snack&gt;&gt; snackProviders) { ... }
 * }</code></pre>
 *
 * <p>Creating mapbindings from different modules is supported. For example, it
 * is okay to have both {@code CandyModule} and {@code ChipsModule} both
 * create their own {@code MapBinder<String, Snack>}, and to each contribute 
 * bindings to the snacks map. When that map is injected, it will contain 
 * entries from both modules.
 *
 * <p>Values are resolved at map injection time. If a value is bound to a
 * provider, that provider's get method will be called each time the map is
 * injected (unless the binding is also scoped).
 *
 * <p>Annotations are be used to create different maps of the same key/value
 * type. Each distinct annotation gets its own independent map.
 *
 * <p><strong>Keys must be distinct.</strong> If the same key is bound more than
 * once, map injection will fail.
 *
 * <p><strong>Keys must be non-null.</strong> {@code addBinding(null)} will 
 * throw an unchecked exception.
 *
 * <p><strong>Values must be non-null to use map injection.</strong> If any
 * value is null, map injection will fail (although injecting a map of providers
 * will not).
 *
 * @author dpb@google.com (David P. Baker)
 */
public abstract class MapBinder<K, V> {
  private MapBinder() {}

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with no binding annotation.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder, 
      TypeLiteral<K> keyType, TypeLiteral<V> valueType) {
    binder = binder.skipSources(MapBinder.class, RealMapBinder.class);
    return newMapBinder(binder, valueType,
        Key.get(mapOf(keyType, valueType)),
        Key.get(mapOfProviderOf(keyType, valueType)),
        Multibinder.newSetBinder(binder, entryOfProviderOf(keyType, valueType)));
  }

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with no binding annotation.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder,
      Class<K> keyType, Class<V> valueType) {
    return newMapBinder(binder, TypeLiteral.get(keyType), TypeLiteral.get(valueType));
  }

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with {@code annotation}.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder, 
      TypeLiteral<K> keyType, TypeLiteral<V> valueType, Annotation annotation) {
    binder = binder.skipSources(MapBinder.class, RealMapBinder.class);
    return newMapBinder(binder, valueType,
        Key.get(mapOf(keyType, valueType), annotation),
        Key.get(mapOfProviderOf(keyType, valueType), annotation),
        Multibinder.newSetBinder(binder, entryOfProviderOf(keyType, valueType), annotation));
  }

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with {@code annotation}.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder,
      Class<K> keyType, Class<V> valueType, Annotation annotation) {
    return newMapBinder(binder, TypeLiteral.get(keyType), TypeLiteral.get(valueType), annotation);
  }

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with {@code annotationType}.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder, TypeLiteral<K> keyType,
      TypeLiteral<V> valueType, Class<? extends Annotation> annotationType) {
    binder = binder.skipSources(MapBinder.class, RealMapBinder.class);
    return newMapBinder(binder, valueType,
        Key.get(mapOf(keyType, valueType), annotationType),
        Key.get(mapOfProviderOf(keyType, valueType), annotationType),
        Multibinder.newSetBinder(binder, entryOfProviderOf(keyType, valueType), annotationType));
  }

  /**
   * Returns a new mapbinder that collects entries of {@code keyType}/{@code valueType} in a
   * {@link Map} that is itself bound with {@code annotationType}.
   */
  public static <K, V> MapBinder<K, V> newMapBinder(Binder binder, Class<K> keyType,
      Class<V> valueType, Class<? extends Annotation> annotationType) {
    return newMapBinder(
        binder, TypeLiteral.get(keyType), TypeLiteral.get(valueType), annotationType);
  }

  @SuppressWarnings("unchecked") // a map of <K, V> is safely a Map<K, V>
  private static <K, V> TypeLiteral<Map<K, V>> mapOf(
      TypeLiteral<K> keyType, TypeLiteral<V> valueType) {
    return (TypeLiteral<Map<K, V>>) TypeLiteral.get(
        Types.mapOf(keyType.getType(), valueType.getType()));
  }

  @SuppressWarnings("unchecked") // a provider map <K, V> is safely a Map<K, Provider<V>>
  private static <K, V> TypeLiteral<Map<K, Provider<V>>> mapOfProviderOf(
      TypeLiteral<K> keyType, TypeLiteral<V> valueType) {
    return (TypeLiteral<Map<K, Provider<V>>>) TypeLiteral.get(
        Types.mapOf(keyType.getType(), newParameterizedType(Provider.class, valueType.getType())));
  }
  
  @SuppressWarnings("unchecked") // a provider entry <K, V> is safely a Map.Entry<K, Provider<V>>
  private static <K, V> TypeLiteral<Map.Entry<K, Provider<V>>> entryOfProviderOf(
      TypeLiteral<K> keyType, TypeLiteral<V> valueType) {
    return (TypeLiteral<Entry<K, Provider<V>>>) TypeLiteral.get(newParameterizedTypeWithOwner(
        Map.class, Entry.class, keyType.getType(), Types.providerOf(valueType.getType())));
  }

  private static <K, V> MapBinder<K, V> newMapBinder(Binder binder, TypeLiteral<V> valueType,
      Key<Map<K, V>> mapKey, Key<Map<K, Provider<V>>> providerMapKey,
      Multibinder<Entry<K, Provider<V>>> entrySetBinder) {
    RealMapBinder<K, V> mapBinder = new RealMapBinder<K, V>(
        binder, valueType, mapKey, providerMapKey, entrySetBinder);
    binder.install(mapBinder);
    return mapBinder;
  }

  /**
   * Returns a binding builder used to add a new entry in the map. Each
   * key must be distinct (and non-null). Bound providers will be evaluated each
   * time the map is injected.
   *
   * <p>It is an error to call this method without also calling one of the
   * {@code to} methods on the returned binding builder.
   *
   * <p>Scoping elements independently is supported. Use the {@code in} method
   * to specify a binding scope.
   */
  public abstract LinkedBindingBuilder<V> addBinding(K key);

  /**
   * The actual mapbinder plays several roles:
   *
   * <p>As a MapBinder, it acts as a factory for LinkedBindingBuilders for
   * each of the map's values. It delegates to a {@link Multibinder} of
   * entries (keys to value providers).
   *
   * <p>As a Module, it installs the binding to the map itself, as well as to
   * a corresponding map whose values are providers. It uses the entry set 
   * multibinder to construct the map and the provider map.
   * 
   * <p>As a module, this implements equals() and hashcode() in order to trick 
   * Guice into executing its configure() method only once. That makes it so 
   * that multiple mapbinders can be created for the same target map, but
   * only one is bound. Since the list of bindings is retrieved from the
   * injector itself (and not the mapbinder), each mapbinder has access to
   * all contributions from all equivalent mapbinders.
   *
   * <p>Rather than binding a single Map.Entry&lt;K, V&gt;, the map binder
   * binds keys and values independently. This allows the values to be properly
   * scoped.
   *
   * <p>We use a subclass to hide 'implements Module' from the public API.
   */
  private static final class RealMapBinder<K, V> extends MapBinder<K, V> implements Module {
    private final TypeLiteral<V> valueType;
    private final Key<Map<K, V>> mapKey;
    private final Key<Map<K, Provider<V>>> providerMapKey;
    private final RealMultibinder<Map.Entry<K, Provider<V>>> entrySetBinder;

    /* the target injector's binder. non-null until initialization, null afterwards */
    private Binder binder;

    private RealMapBinder(Binder binder, TypeLiteral<V> valueType,
        Key<Map<K, V>> mapKey, Key<Map<K, Provider<V>>> providerMapKey,
        Multibinder<Map.Entry<K, Provider<V>>> entrySetBinder) {
      this.valueType = valueType;
      this.mapKey = mapKey;
      this.providerMapKey = providerMapKey;
      this.entrySetBinder = (RealMultibinder<Entry<K, Provider<V>>>) entrySetBinder;
      this.binder = binder;
    }

    /**
     * This creates two bindings. One for the {@code Map.Entry<K, Provider<V>>}
     * and another for {@code V}.
     */
    @Override public LinkedBindingBuilder<V> addBinding(K key) {
      checkNotNull(key, "key");
      checkConfiguration(!isInitialized(), "MapBinder was already initialized");

      Key<V> valueKey = Key.get(valueType, new RealElement(entrySetBinder.getSetName()));
      entrySetBinder.addBinding().toInstance(new MapEntry<K, Provider<V>>(key,
          binder.getProvider(valueKey)));
      return binder.bind(valueKey);
    }

    public void configure(Binder binder) {
      checkConfiguration(!isInitialized(), "MapBinder was already initialized");

      // binds a Map<K, Provider<V>> from a collection of Map<Entry<K, Provider<V>>
      final Provider<Set<Entry<K, Provider<V>>>> entrySetProvider = binder
          .getProvider(entrySetBinder.getSetKey());
      binder.bind(providerMapKey).toProvider(new Provider<Map<K, Provider<V>>>() {
        private Map<K, Provider<V>> providerMap;

        @SuppressWarnings("unused")
        @Inject void initialize() {
          RealMapBinder.this.binder = null;

          Map<K, Provider<V>> providerMapMutable = new LinkedHashMap<K, Provider<V>>();
          for (Entry<K, Provider<V>> entry : entrySetProvider.get()) {
            checkConfiguration(providerMapMutable.put(entry.getKey(), entry.getValue()) == null,
                "Map injection failed due to duplicated key \"%s\"", entry.getKey());
          }

          providerMap = Collections.unmodifiableMap(providerMapMutable);
        }

        public Map<K, Provider<V>> get() {
          return providerMap;
        }
      });

      final Provider<Map<K, Provider<V>>> mapProvider = binder.getProvider(providerMapKey);
      binder.bind(mapKey).toProvider(new Provider<Map<K, V>>() {
        public Map<K, V> get() {
          Map<K, V> map = new LinkedHashMap<K, V>();
          for (Entry<K, Provider<V>> entry : mapProvider.get().entrySet()) {
            V value = entry.getValue().get();
            K key = entry.getKey();
            checkConfiguration(value != null,
                "Map injection failed due to null value for key \"%s\"", key);
            map.put(key, value);
          }
          return Collections.unmodifiableMap(map);
        }
      });
    }

    private boolean isInitialized() {
      return binder == null;
    }

    @Override public boolean equals(Object o) {
      return o instanceof RealMapBinder
          && ((RealMapBinder<?, ?>) o).mapKey.equals(mapKey);
    }

    @Override public int hashCode() {
      return mapKey.hashCode();
    }

    private static final class MapEntry<K, V> implements Map.Entry<K, V> {
      private final K key;
      private final V value;

      private MapEntry(K key, V value) {
        this.key = key;
        this.value = value;
      }

      public K getKey() {
        return key;
      }

      public V getValue() {
        return value;
      }

      public V setValue(V value) {
        throw new UnsupportedOperationException();
      }

      @Override public boolean equals(Object obj) {
        return obj instanceof Map.Entry 
            && key.equals(((Map.Entry<?, ?>) obj).getKey()) 
            && value.equals(((Map.Entry<?, ?>) obj).getValue());
      }

      @Override public int hashCode() {
        return 127 * ("key".hashCode() ^ key.hashCode())
            + 127 * ("value".hashCode() ^ value.hashCode());
      }

      @Override public String toString() {
        return "MapEntry(" + key + ", " + value + ")";
      }
    }
  }
}