core/src/com/google/inject/util/Providers.java

/*
 * Copyright (C) 2007 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.util;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.base.Objects;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Provider;
import com.google.inject.spi.Dependency;
import com.google.inject.spi.InjectionPoint;
import com.google.inject.spi.ProviderWithDependencies;

import java.util.Set;

/**
 * Static utility methods for creating and working with instances of
 * {@link Provider}.
 *
 * @author Kevin Bourrillion (kevinb9n@gmail.com)
 * @since 2.0
 */
public final class Providers {

  private Providers() {}

  /**
   * Returns a provider which always provides {@code instance}.  This should not
   * be necessary to use in your application, but is helpful for several types
   * of unit tests.
   *
   * @param instance the instance that should always be provided.  This is also
   *     permitted to be null, to enable aggressive testing, although in real
   *     life a Guice-supplied Provider will never return null.
   */
  public static <T> Provider<T> of(final T instance) {
    return new ConstantProvider<T>(instance);
  }

  private static final class ConstantProvider<T> implements Provider<T> {
    private final T instance;

    private ConstantProvider(T instance) {
      this.instance = instance;
    }

    public T get() {
      return instance;
    }

    @Override public String toString() {
      return "of(" + instance + ")";
    }

    @Override public boolean equals(Object obj) {
      return (obj instanceof ConstantProvider)
          && Objects.equal(instance, ((ConstantProvider<?>) obj).instance);
    }

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

  /**
   * Returns a Guice-friendly {@code com.google.inject.Provider} for the given
   * JSR-330 {@code javax.inject.Provider}. The converse method is unnecessary,
   * since Guice providers directly implement the JSR-330 interface.
   * 
   * @since 3.0
   */
  public static <T> Provider<T> guicify(javax.inject.Provider<T> provider) {
    if (provider instanceof Provider) {
      return (Provider<T>) provider;
    }
  
    final javax.inject.Provider<T> delegate = checkNotNull(provider, "provider");
    
    // Ensure that we inject all injection points from the delegate provider.
    Set<InjectionPoint> injectionPoints =
        InjectionPoint.forInstanceMethodsAndFields(provider.getClass());
    if(injectionPoints.isEmpty()) {
      return new GuicifiedProvider<T>(delegate);
    } else {
      Set<Dependency<?>> mutableDeps = Sets.newHashSet();
      for(InjectionPoint ip : injectionPoints) {
        mutableDeps.addAll(ip.getDependencies());
      }
      final Set<Dependency<?>> dependencies = ImmutableSet.copyOf(mutableDeps);
      return new GuicifiedProviderWithDependencies<T>(dependencies, delegate);
    }
  }

  private static class GuicifiedProvider<T> implements Provider<T> {
    protected final javax.inject.Provider<T> delegate;

    private GuicifiedProvider(javax.inject.Provider<T> delegate) {
      this.delegate = delegate;
    }

    public T get() {
      return delegate.get();
    }

    @Override public String toString() {
      return "guicified(" + delegate + ")";
    }

    @Override public boolean equals(Object obj) {
      return (obj instanceof GuicifiedProvider)
          && Objects.equal(delegate, ((GuicifiedProvider<?>) obj).delegate);
    }

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

  private static final class GuicifiedProviderWithDependencies<T>
      extends GuicifiedProvider<T> implements ProviderWithDependencies<T> {
    private final Set<Dependency<?>> dependencies;

    private GuicifiedProviderWithDependencies(Set<Dependency<?>> dependencies,
        javax.inject.Provider<T> delegate) {
      super(delegate);
      this.dependencies = dependencies;
    }

    @SuppressWarnings("unused")
    @Inject
    void initialize(Injector injector) {
      injector.injectMembers(delegate);
    }

    public Set<Dependency<?>> getDependencies() {
      return dependencies;
    }
  }
}