Aleksey Shipilev cebaad1c94 8280041: Retry loop issues in java.io.ClassCache
Co-authored-by: Peter Levart <plevart@openjdk.org>
Reviewed-by: rkennke, rriggs, plevart
2022-01-25 19:22:07 +00:00

117 lines
3.9 KiB
Java

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