----- Original Message ----- > From: "Andrey Turbanov" <turban...@gmail.com> > To: "core-libs-dev" <core-libs-...@openjdk.java.net> > Sent: Thursday, November 7, 2024 9:17:00 AM > Subject: Can LinkedHashMap(accessOrder=true) be guarded by ReadWriteLock ?
> 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 Hello Andrey, yes ! This flavor of LinkedHashMap has always been a little dubious because having a Map.get() that does a structural modification can be qualified at best as surprising, at worst as not compliant with the contract of Map. regards, Rémi
