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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>This class and its references are not thread-safe and should not be used s
imultaneously by |
| 24 * multiple threads. |
| 25 */ |
| 26 public class DiscardableReferencePool { |
| 27 /** |
| 28 * The underlying data storage. The wildcard type parameter allows using a s
ingle pool for |
| 29 * references of any type. |
| 30 */ |
| 31 private final Set<DiscardableReference<?>> mPool; |
| 32 |
| 33 public DiscardableReferencePool() { |
| 34 WeakHashMap<DiscardableReference<?>, Boolean> map = new WeakHashMap<>(); |
| 35 mPool = Collections.newSetFromMap(map); |
| 36 } |
| 37 |
| 38 /** |
| 39 * A reference to an object in the pool. Will be nulled out when the pool is
drained. |
| 40 * @param <T> The type of the object. |
| 41 */ |
| 42 public static class DiscardableReference<T> { |
| 43 @Nullable |
| 44 private T mPayload; |
| 45 |
| 46 private DiscardableReference(T payload) { |
| 47 assert payload != null; |
| 48 mPayload = payload; |
| 49 } |
| 50 |
| 51 /** |
| 52 * @return The referent, or null if the pool has been drained. |
| 53 */ |
| 54 @Nullable |
| 55 public T get() { |
| 56 return mPayload; |
| 57 } |
| 58 |
| 59 /** |
| 60 * Clear the referent. |
| 61 */ |
| 62 private void discard() { |
| 63 assert mPayload != null; |
| 64 mPayload = null; |
| 65 } |
| 66 } |
| 67 |
| 68 /** |
| 69 * @param <T> The type of the object. |
| 70 * @param payload The payload to add to the pool. |
| 71 * @return A new reference to the {@code payload}. |
| 72 */ |
| 73 public <T> DiscardableReference<T> put(T payload) { |
| 74 assert payload != null; |
| 75 DiscardableReference<T> reference = new DiscardableReference<>(payload); |
| 76 mPool.add(reference); |
| 77 return reference; |
| 78 } |
| 79 |
| 80 /** |
| 81 * Drains the pool, removing all references to objects in the pool and there
fore allowing them |
| 82 * to be garbage collected. |
| 83 */ |
| 84 public void drain() { |
| 85 for (DiscardableReference<?> ref : mPool) { |
| 86 ref.discard(); |
| 87 } |
| 88 mPool.clear(); |
| 89 } |
| 90 } |
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