1 /**
2 * Copyright 2008 Atlassian Pty Ltd
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.atlassian.util.concurrent;
18
19 import static com.atlassian.util.concurrent.Assertions.notNull;
20
21 import java.lang.ref.ReferenceQueue;
22 import java.lang.ref.WeakReference;
23 import java.util.concurrent.ConcurrentHashMap;
24 import java.util.concurrent.ConcurrentMap;
25 import java.util.concurrent.locks.Lock;
26 import java.util.concurrent.locks.ReentrantLock;
27
28 /**
29 * {@link WeakLockMap} holds a {@link Lock} per Descriptor.
30 * <p>
31 * Each {@link Lock} is {@link WeakReference weakly referenced} internally.
32 *
33 * @param <D> comparable descriptor, should have a good hash function.
34 */
35 class WeakLockMap<D> implements Function<D, Lock> {
36 private final ConcurrentMap<D, WeakReference<Lock>> locks;
37 private final ReferenceQueue<Lock> queue;
38
39 /**
40 * Construct a new {@link WeakLockMap} instance.
41 *
42 * @param initialCapacity how large the internal map should be initially.
43 * @throws IllegalArgumentException if the initial capacity of elements is negative.
44 */
45 WeakLockMap(final int initialCapacity) {
46 locks = new ConcurrentHashMap<D, WeakReference<Lock>>(initialCapacity);
47 queue = new ReferenceQueue<Lock>();
48 }
49
50 /**
51 * Get a Lock for the supplied Descriptor.
52 *
53 * @param descriptors
54 * @return descriptor lock
55 */
56 public Lock get(final D descriptor) {
57 expungeStaleEntries();
58
59 while (true) {
60 final WeakReference<Lock> reference = locks.get(descriptor);
61 if (reference != null) {
62 final Lock lock = reference.get();
63 if (lock != null) {
64 return lock;
65 }
66 locks.remove(descriptor, reference);
67 }
68 locks.putIfAbsent(descriptor, new LockReference<D>(descriptor, queue));
69 }
70 }
71
72 // expunge descriptors whose lock reference has been collected
73 @SuppressWarnings("unchecked") private void expungeStaleEntries() {
74 LockReference<D> ref;
75 while ((ref = (LockReference<D>) queue.poll()) != null) {
76 locks.remove(ref.getDescriptor(), ref);
77 }
78 }
79
80 /**
81 * A weak reference that maintains a reference to the descriptor so that it can be removed from
82 * the map when garbage collected
83 */
84 private static class LockReference<D> extends WeakReference<Lock> {
85 private final D descriptor;
86
87 public LockReference(final D descriptor, final ReferenceQueue<? super Lock> q) {
88 super(new ReentrantLock(), q);
89 this.descriptor = notNull("descriptor", descriptor);
90 }
91
92 public D getDescriptor() {
93 return descriptor;
94 }
95 }
96 }