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Java example source code file (Providers.java)

This example Java source code file (Providers.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

constantprovider, guicifiedprovider, guicifiedproviderwithdependencies, inject, override, provider, providers, set, string, suppresswarnings, util

The Providers.java Java example source code

 * 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,
 * 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 of(final T instance) {
    return new ConstantProvider<T>(instance);

  private static final class ConstantProvider<T> implements Provider {
    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 guicify(javax.inject.Provider 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 =
    if(injectionPoints.isEmpty()) {
      return new GuicifiedProvider<T>(delegate);
    } else {
      Set<Dependency mutableDeps = Sets.newHashSet();
      for(InjectionPoint ip : injectionPoints) {
      final Set<Dependency dependencies = ImmutableSet.copyOf(mutableDeps);
      return new GuicifiedProviderWithDependencies<T>(dependencies, delegate);

  private static class GuicifiedProvider<T> implements Provider {
    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 {
    private final Set<Dependency dependencies;

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

    void initialize(Injector injector) {

    public Set<Dependency getDependencies() {
      return dependencies;

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