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1 // Copyright 2017 The Chromium Authors. All rights reserved. | |
2 // Use of this source code is governed by a BSD-style license that can be | |
3 // found in the LICENSE file. | |
4 | |
5 package org.chromium.base; | |
6 | |
7 import android.support.annotation.Nullable; | |
8 | |
9 import java.util.Collections; | |
10 import java.util.Set; | |
11 import java.util.WeakHashMap; | |
12 | |
13 /** | |
14 * A DiscardableReferencePool allows handing out typed references to objects ("p ayloads") that can | |
15 * be dropped in one batch ("drained"), e.g. under memory pressure. In contrast to {@link | |
16 * java.lang.ref.WeakReference}s, which drop their referents when they get garba ge collected, a | |
17 * reference pool gives more precise control over when exactly it is drained. | |
18 * | |
19 * <p>Internally it uses a {@link WeakHashMap} with the reference itself as a ke y to allow the | |
20 * payloads to be garbage collected regularly when the last reference goes away before the pool is | |
21 * drained. | |
22 * | |
23 * <p>Note that certain kinds of reference cycles that would be garbage-collecte d when using weak | |
DmitrySkiba
2017/05/10 22:04:19
This is not accurate anymore, right? Since now the
Bernhard Bauer
2017/05/11 08:43:02
True. Removed (and added a comment about thread sa
| |
24 * (or even strong) references could lead to leaks with a DiscardableReferencePo ol: For example, if | |
25 * a {@link DiscardableReference} is held by its own payload (including indirect ly) but is otherwise | |
26 * not referenced, it will not get garbage-collected, because the DiscardableRef erencePool will | |
27 * still hold a strong reference to the payload. | |
28 */ | |
29 public class DiscardableReferencePool { | |
30 /** | |
31 * The underlying data storage. The wildcard type parameter allows using a s ingle pool for | |
32 * references of any type. | |
33 */ | |
34 private final Set<DiscardableReference<?>> mPool; | |
35 | |
36 public DiscardableReferencePool() { | |
37 WeakHashMap<DiscardableReference<?>, Boolean> map = new WeakHashMap<>(); | |
38 mPool = Collections.newSetFromMap(map); | |
39 } | |
40 | |
41 /** | |
42 * A reference to an object in the pool. Will be nulled out when the pool is drained. | |
43 * @param <T> The type of the object. | |
44 */ | |
45 public static class DiscardableReference<T> { | |
46 @Nullable | |
47 private T mPayload; | |
48 | |
49 private DiscardableReference(T payload) { | |
50 assert payload != null; | |
51 mPayload = payload; | |
52 } | |
53 | |
54 /** | |
55 * @return The referent, or null if the pool has been drained. | |
56 */ | |
57 @Nullable | |
58 public T get() { | |
59 return mPayload; | |
60 } | |
61 | |
62 /** | |
63 * Clear the referent. | |
64 */ | |
65 private void discard() { | |
66 assert mPayload != null; | |
67 mPayload = null; | |
68 } | |
69 } | |
70 | |
71 /** | |
72 * @param <T> The type of the object. | |
73 * @param payload The payload to add to the pool. | |
74 * @return A new reference to the {@code payload}. | |
75 */ | |
76 public <T> DiscardableReference<T> put(T payload) { | |
77 assert payload != null; | |
78 DiscardableReference<T> reference = new DiscardableReference<>(payload); | |
79 mPool.add(reference); | |
80 return reference; | |
81 } | |
82 | |
83 /** | |
84 * Drains the pool, removing all references to objects in the pool and there fore allowing them | |
85 * to be garbage collected. | |
86 */ | |
87 public void drain() { | |
88 for (DiscardableReference<?> ref : mPool) { | |
89 ref.discard(); | |
90 } | |
91 mPool.clear(); | |
92 } | |
93 } | |
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