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8251397: NPE on ClassValue.ClassValueMap.cacheArray

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Add release fence to ClassValueMap constructor.

* Release fence guarantees that cacheArray field will published
with a non-null value.
* Without this fix, CacheValueMap.cacheArray can sometimes be
seen as null.

Reviewed-by: shade, psandoz
This commit is contained in:
Galder Zamarreño 2020-09-17 13:42:15 +00:00 committed by Severin Gehwolf
parent cca3a26e43
commit 81e2cf82d9
1 changed files with 14 additions and 2 deletions
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16
src/java.base/share/classes/java/lang/ClassValue.java
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@ -29,6 +29,8 @@ import java.util.WeakHashMap;
import java.lang.ref.WeakReference;
import java.util.concurrent.atomic.AtomicInteger;
import jdk.internal.misc.Unsafe;
import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;
@ -369,12 +371,22 @@ public abstract class ClassValue<T> {
}
private static final Object CRITICAL_SECTION = new Object();
private static final Unsafe UNSAFE = Unsafe.getUnsafe();
private static ClassValueMap initializeMap(Class<?> type) {
ClassValueMap map;
synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks
// happens about once per type
if ((map = type.classValueMap) == null)
type.classValueMap = map = new ClassValueMap();
if ((map = type.classValueMap) == null) {
map = new ClassValueMap();
// Place a Store fence after construction and before publishing to emulate
// ClassValueMap containing final fields. This ensures it can be
// published safely in the non-volatile field Class.classValueMap,
// since stores to the fields of ClassValueMap will not be reordered
// to occur after the store to the field type.classValueMap
UNSAFE.storeFence();
type.classValueMap = map;
}
}
return map;
}