core/src/com/google/inject/Scope.java

/*
 * Copyright (C) 2006 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;

/**
 * A scope is a level of visibility that instances provided by Guice may have. By default, an
 * instance created by the {@link Injector} has <i>no scope</i>, meaning it has no state from the
 * framework's perspective -- the {@code Injector} creates it, injects it once into the class that
 * required it, and then immediately forgets it. Associating a scope with a particular binding
 * allows the created instance to be "remembered" and possibly used again for other injections.
 *
 * <p>An example of a scope is {@link Scopes#SINGLETON}.
 *
 * @author crazybob@google.com (Bob Lee)
 */
public interface Scope {

  /**
   * Scopes a provider. The returned provider returns objects from this scope. If an object does not
   * exist in this scope, the provider can use the given unscoped provider to retrieve one.
   *
   * <p>Scope implementations are strongly encouraged to override {@link Object#toString} in the
   * returned provider and include the backing provider's {@code toString()} output.
   *
   * @param key binding key
   * @param unscoped locates an instance when one doesn't already exist in this scope.
   * @return a new provider which only delegates to the given unscoped provider when an instance of
   *     the requested object doesn't already exist in this scope
   */
  public <T> Provider<T> scope(Key<T> key, Provider<T> unscoped);

  /**
   * A short but useful description of this scope. For comparison, the standard scopes that ship
   * with guice use the descriptions {@code "Scopes.SINGLETON"}, {@code "ServletScopes.SESSION"} and
   * {@code "ServletScopes.REQUEST"}.
   */
  @Override
  String toString();
}