src/com/google/inject/Binding.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;

import com.google.inject.spi.Element;
import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;

/**
 * A mapping from a key (type and optional annotation) to the strategy for getting instances of the
 * type. This interface is part of the introspection API and is intended primarily for use by 
 * tools. Bindings exist on both {@link Module}s and on {@link Injector}s, and their behaviour is
 * different for each.
 *
 * <p><strong>Module bindings</storng> are incomplete and cannot be used to provide instances. This
 * is because the applicable scopes and interceptors may not be known until an injector is created.
 * From a tool's perspective, module bindings are like the injector's source code. They can be
 * inspected or rewritten, but this analysis must be done statically.
 *
 * <p><strong>Injector bindings</strong> are complete and valid and can be used to provide
 * instances. From a tools' perspective, injector bindings are like reflection for an injector.
 * They have full runtime information, including the complete graph of injections necessary to
 * satisfy a binding.
 *
 * @author crazybob@google.com (Bob Lee)
 * @author jessewilson@google.com (Jesse Wilson)
 */
public interface Binding<T> extends Element {

  /**
   * Returns the key for this binding.
   */
  Key<T> getKey();

  /**
   * Returns an arbitrary object containing information about the "place"
   * where this binding was configured. Used by Guice in the production of
   * descriptive error messages.
   *
   * <p>Tools might specially handle types they know about;
   * {@code StackTraceElement} is a good example. Tools should simply call
   * {@code toString()} on the source object if the type is unfamiliar.
   */
  Object getSource();

  /**
   * Returns the scoped provider guice uses to fulfill requests for this
   * binding.
   *
   * @throws UnsupportedOperationException when invoked on a {@link Binding}
   *      created via {@link com.google.inject.spi.Elements#getElements}. This
   *      method is only supported on {@link Binding}s returned from an injector.
   */
  Provider<T> getProvider();

  <V> V acceptVisitor(Visitor<V> visitor);

  <V> V acceptTargetVisitor(TargetVisitor<? super T, V> visitor);

  <V> V acceptScopingVisitor(ScopingVisitor<V> visitor);

  /**
   * A strategy to find an instance that satisfies an injection.
   */
  interface TargetVisitor<T, V> {

    /**
     * Visit a instance binding. The same instance is returned for every injection. This target is
     * used in both module and injector bindings.
     */
    V visitInstance(T instance);

    V visitProvider(Provider<? extends T> provider);
    V visitProviderKey(Key<? extends Provider<? extends T>> providerKey);
    V visitKey(Key<? extends T> key);

    // module-only bindings
    V visitUntargetted();

    // injector-only bindings
    V visitConstructor(Constructor<? extends T> constructor);
    V visitConstant(T value);
    V visitConvertedConstant(T value);
    V visitProviderBinding(Key<?> provided);
  }

  interface ScopingVisitor<V> {
    V visitEagerSingleton();
    V visitScope(Scope scope);
    V visitScopeAnnotation(Class<? extends Annotation> scopeAnnotation);
    V visitNoScoping();
  }
}