Hello.
I've got a question about LinkedHashMap created with accessOrder=true flag.
Usually this constructor is used when we want to build LRU cache.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/LinkedHashMap.html#<init>(int,float,boolean)
LinkedHashMap<MyKey, MyValue> cache = new LinkedHashMap<>(8, 1f, true) {
@Override
protected boolean removeEldestEntry(Map.Entry<MyKey,
MyValue> eldest) {
return size() > 32;
}
};
LinkedHashMap is not a thread-safe data structure, so to be able to
use it in a concurrent environment, access to it should be guarded by
some kind of synchronization.
I often see in many projects that such access is guarded by
ReentrantReadWriteLock.
Example in OpenJDK:
https://github.com/openjdk/jdk/blob/0e1c1b793d43064aabe9571057284899c9580f30/src/java.desktop/share/classes/sun/awt/image/ImageCache.java#L46
private final ReentrantReadWriteLock readWriteLock = new
ReentrantReadWriteLock();
public MyValue get(MyKey key) {
readWriteLock.readLock().lock();
try {
return cache.get(key);
} finally {
readWriteLock.readLock().unlock();
}
}
public void put(MyKey key, MyValue value) {
readWriteLock.writeLock().lock();
try {
cache.put(key, value);
} finally {
readWriteLock.writeLock().unlock();
}
}
I can see in LHM javadoc that LinkedHashMap.get in such a case can
cause structural modification:
"In access-ordered linked hash maps, merely querying the map with get
is a structural modification."
As I understand, it means that LinkedHashMap.get can't be guarded
_just_ by ReadLock.
Is my assumption correct?
Andrey Turbanov
