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1 /**
2  * Copyright (C) 2006 Google Inc.
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16
17 package com.google.inject;
18
19 /**
20  * Simply, any object capable of providing instances of type {@code T}.
21  * Providers are used in numerous ways by the Guice framework:
22  *
23  * <ul>
24  * <li>When the default means for obtaining instances (an injectable or
25  * parameterless constructor) is insufficient for a particular binding, the
26  * module can specify a custom {@code Provider} instead, to control exactly how
27  * Guice creates or obtains instances for the binding.
28  *
29  * <li>An implementation class may always choose to have a {@code Provider<T>}
30  * instance injected, rather than having a {@code T} injected directly. This
31  * may give you access to multiple instances, instances you wish to safely
32  * mutate and discard, instances which are out of scope (e.g. using a
33  * {@code @RequestScoped} object from within a {@code @SessionScoped} object),
34  * or instances you don't want to initialize until they are absolutely needed.
35  *
36  * <li>A custom {@link Scope} is implemented as a decorator of
37  * {@code Provider<T>}, which decides when to delegate to the backing provider
38  * and when to provide the instance some other way.
39  *
40  * <li>The {@link Injector} offers access to the {@code Provider<T>} it uses
41  * to fulfill requests for a given key, via the {@link Injector#getProvider}
42  * methods.
43  * </ul>
44  *
45  * @param <T> the type of object this provider provides
46  *
47  * @author crazybob@google.com (Bob Lee)
48  */
49 public interface Provider<T> {
50
51   /**
52    * Provides an instance of {@code T}. Must never return {@code null}.
53    */
54   T get();
55 }
56
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