117 lines
3.9 KiB
Java
117 lines
3.9 KiB
Java
/*
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* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.io;
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import java.lang.ref.Reference;
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import java.lang.ref.ReferenceQueue;
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import java.lang.ref.SoftReference;
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import java.util.Objects;
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// Maps Class instances to values of type T. Under memory pressure, the
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// mapping is released (under soft references GC policy) and would be
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// recomputed the next time it is queried. The mapping is bound to the
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// lifetime of the class: when the class is unloaded, the mapping is
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// removed too.
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abstract class ClassCache<T> {
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private static class CacheRef<T> extends SoftReference<T> {
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private final Class<?> type;
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private T strongReferent;
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CacheRef(T referent, ReferenceQueue<T> queue, Class<?> type) {
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super(referent, queue);
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this.type = type;
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this.strongReferent = referent;
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}
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Class<?> getType() {
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return type;
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}
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T getStrong() {
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return strongReferent;
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}
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void clearStrong() {
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strongReferent = null;
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}
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}
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private final ReferenceQueue<T> queue;
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private final ClassValue<CacheRef<T>> map;
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protected abstract T computeValue(Class<?> cl);
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protected ClassCache() {
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queue = new ReferenceQueue<>();
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map = new ClassValue<>() {
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@Override
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protected CacheRef<T> computeValue(Class<?> type) {
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T v = ClassCache.this.computeValue(type);
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Objects.requireNonNull(v);
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return new CacheRef<>(v, queue, type);
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}
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};
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}
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T get(Class<?> cl) {
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while (true) {
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processQueue();
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CacheRef<T> ref = map.get(cl);
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// Case 1: A recently created CacheRef.
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// We might still have strong referent, and can return it.
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// This guarantees progress for at least one thread on every CacheRef.
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// Clear the strong referent before returning to make the cache soft.
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T strongVal = ref.getStrong();
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if (strongVal != null) {
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ref.clearStrong();
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return strongVal;
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}
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// Case 2: Older or recently cleared CacheRef.
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// Check if its soft referent is still available, and return it.
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T val = ref.get();
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if (val != null) {
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return val;
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}
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// Case 3: The reference was cleared.
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// Clear the mapping and retry.
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map.remove(cl);
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}
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}
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private void processQueue() {
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Reference<? extends T> ref;
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while((ref = queue.poll()) != null) {
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CacheRef<? extends T> cacheRef = (CacheRef<? extends T>)ref;
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map.remove(cacheRef.getType());
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}
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}
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}
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