WeakReferences (java.lang.ref and more)
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The document discusses different types of references in Java, including strong, soft, weak, and phantom references, each with varying levels of reachability and garbage collection behavior. It explains how these references function, their use cases, and mentions implementations in other programming languages like Ruby, Python, Objective-C, and C++. Additionally, it emphasizes the importance of cautious handling of weak references to avoid issues like premature optimization.
![Python - weakref
ā weakref.ref(obj [,callback])
ā Also WeakKeyDictionary and
WeakValueDictionary
ā Not all objects can be weakly referenced.
ā Some built-in types donāt support weak
references unless subclassed.
ā str can never be
ā Good to avoid cyclic references (before 2.0)](https://image.slidesharecdn.com/java-140402094656-phpapp02/85/WeakReferences-java-lang-ref-and-more-16-320.jpg)
![Python - weakref
ā weakref.getweakrefcount(obj)
ā weakref.proxy(object [,callback]) //
Alternative to subclassing
ā Extension types can be made to support
weak references](https://image.slidesharecdn.com/java-140402094656-phpapp02/85/WeakReferences-java-lang-ref-and-more-17-320.jpg)
![Be Careful
ā Weak maps usually check references for
equality.
String s1 = āHello!ā;
String s2 = new String([āHā, āeā, ālā, ālā, āoā, ā!ā]);
strongMap.put(s1, value); weakMap.put(s1, value);
strongMap.contains(s2) //true;
weakMap.contains(s2) // false;](https://image.slidesharecdn.com/java-140402094656-phpapp02/85/WeakReferences-java-lang-ref-and-more-24-320.jpg)
WeakReferences (java.lang.ref and more)
- 2. ... references? ā A reference is, as its name suggests, what you use to point to (reference) an object somewhere else in the memory. ā You use it all the time! String s = āstringā; // s is a reference
- 3. There are more? ā Probably, all our life, weāve been using only one type of references; strong references. ā Object o = new Object(); As long as you keep o to your side, the object it references stays in memory.
- 4. There are more? ā There are more levels of āstrength or reachabilityā for references. ā Soft ā Weak ā Phantom ā You can get these by using package java. lang.ref.
- 5. How to? ā You use them as you use the usual strong reference, but use them for special purposes. ā The GC handles them as it handles the strong references. So itās done for you and you canāt add to it or extend it.
- 6. Reference<T> ā Constructor: Reference(T referent) ā T get() ā void clear () ā Thatās it. Once you initialize it, you canāt change the āreferentā. Clearing is your only option.
- 7. Softly reachable.. ā The next level after strong references is soft references. ā A softly reachable object is an object that has no strong references, but has soft ones. ā Object o = new Object(); SoftReference<Object> softRef = new SoftReference<Object>(o); o = null; // o is now softly reachable
- 8. Softly reachableā¦ so? ā Important point about references is when the GC decides to reclaim referents. ā Itās guaranteed to live till an OutOfMemoryError is thrown, then thereās no guarantee. ā Sounds good for simple caches. ā Some JRE implementations start collection with the oldest objects.
- 9. Weak references ā Itās weakly reachable when there are no strong or soft references to it, and it has weak references to it. ā Once the object is weakly reachable, itās eligible for finalization at once.
- 10. Weak references ā java.util.WeakHashMap ā References the keys weakly and the values strongly. Once the key goes weak, its record is removed from the map.
- 11. Phantom references ā No strong. No soft. No weak. And some phantom references to the object. ā If the object is phantomly reachable, itās dead! So get() returns null. ā In fact, it always returns null.
- 12. Phantom references ā Sounds unuseful! ā The only value is to get notified of the object getting in that level. i.e. get notified that itās really dead, to do post-mortem stuff. ā P.S. finalize() isnāt immune to fooling around. You can resurrect the object from death there.
- 13. ReferenceQueue<? super T> ā Nothing really! It just tells you when the objects reach registered reachability levels. ā If the object moves from strongly reachable to softly reachable, youāll get the call
- 14. ReferenceQueue ā Reference(T referent, ReferenceQueue<? super T> queue) ā Reference<? extends T> poll() // doesnāt block ā remove(), remove(long timeOut) // blocks ā As mentioned before, phantom references are only good with queues. However, weak and soft references are good with it too.
- 15. Ruby - WeakRef ā Inherits from Delegator -> You can deal with it the same way you deal with the object. ā Uses object ID to keep track ā Raises an exception when referenced after being collected
- 16. Python - weakref ā weakref.ref(obj [,callback]) ā Also WeakKeyDictionary and WeakValueDictionary ā Not all objects can be weakly referenced. ā Some built-in types donāt support weak references unless subclassed. ā str can never be ā Good to avoid cyclic references (before 2.0)
- 17. Python - weakref ā weakref.getweakrefcount(obj) ā weakref.proxy(object [,callback]) // Alternative to subclassing ā Extension types can be made to support weak references
- 18. Objective-C ā Reference counting and ARC -> NOT GC ā strong vs weak @property becomes nil automatically ā Avoid cyclic strong references
- 19. Objective-C ā NSMapTable: Dictionary (weak keys, weak values, both) ā NSHashTable: Set ā NSPointerArray:Array ā NSPointerFunctions
- 20. C++11 ā Use std::shared_ptr instead of a regular pointer. ā Use std::weak_ptr with the shared pointer ā There are other libraries for pre C++11
- 21. Uses ā Simple Caching: Storing the big object in memory until everybody stops using it. ā Javaās SoftReference. Weak references wonāt be much useful for that ā Listeners: Keeps notifying objects until theyā re out of reach. ā Problem with anonymous objects in Java.
- 22. Uses ā Associating data with various objects āmeta- data storingā: Doesnāt make sense to override ā Store data specific to a thread ā Store data specific to a view object; like a tag or a serial ā Store data specific to a resource object; like a file
- 23. Uses ā Canonicalized mapping ā Store one reference for the same objects and remove them when nobody needs them anymore. Map employeesByID = new HashMap<String, WeakReference<Employee>(); // Maps employee IDs to weak references of Employee objects retrieved from DB
- 24. Be Careful ā Weak maps usually check references for equality. String s1 = āHello!ā; String s2 = new String([āHā, āeā, ālā, ālā, āoā, ā!ā]); strongMap.put(s1, value); weakMap.put(s1, value); strongMap.contains(s2) //true; weakMap.contains(s2) // false;
- 25. Be Careful ā āPremature optimization is the root of all evilā ā Weak references and their dependencies are great for special uses, not everything.
- 26. References ā Javadoc ā Understanding Weak References ā Getting to know the Ruby Standard Library - WeakRef ā docs.python.org ā Canonicalized Mapping ā Weak Object Pools with WeakHashMap ā Some others I canāt remember.