1 /*
2 * Grapht, an open source dependency injector.
3 * Copyright 2014-2015 various contributors (see CONTRIBUTORS.txt)
4 * Copyright 2010-2014 Regents of the University of Minnesota
5 *
6 * This program is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU Lesser General Public License as
8 * published by the Free Software Foundation; either version 2.1 of the
9 * License, or (at your option) any later version.
10 *
11 * This program is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
13 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
14 * details.
15 *
16 * You should have received a copy of the GNU General Public License along with
17 * this program; if not, write to the Free Software Foundation, Inc., 51
18 * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
19 */
20 package org.grouplens.grapht.util;
21
22 import javax.annotation.Nonnull;
23 import javax.annotation.concurrent.ThreadSafe;
24 import javax.inject.Provider;
25
26 /**
27 * MemoizingProvider is a Provider that enforces memoization or caching on
28 * another Provider that it wraps.
29 *
30 * @param <T>
31 * @author <a href="http://grouplens.org">GroupLens Research</a>
32 */
33 @ThreadSafe
34 public class MemoizingProvider<T> implements TypedProvider<T> {
35 private final Provider<T> wrapped;
36
37 // We track a boolean because this supports providing null instances, in
38 // which case we can't just check against null to see if we've already
39 // queried the base provider
40 private volatile T cached;
41 private volatile boolean invoked;
42
43 public MemoizingProvider(@Nonnull Provider<T> provider) {
44 Preconditions.notNull("provider", provider);
45 wrapped = provider;
46 cached = null;
47 invoked = false;
48 }
49
50 @Override
51 public Class<?> getProvidedType() {
52 return Types.getProvidedType(wrapped);
53 }
54
55 @Override
56 public T get() {
57 if (!invoked) {
58 synchronized (this) {
59 if (!invoked) {
60 cached = wrapped.get();
61 invoked = true;
62 }
63 }
64 }
65 return cached;
66 }
67 }