Java reference types. weak, strong phantom and soft

Java Weak Reference

Releasing unused objects for garbage collection can be done efficiently using java weak reference. Yes. weak reference is related to garbage collection. In java we need to not anything explicitly for garbage collection (GC), this memory management overhead is taken care by java run-time itself.

Then what is the use of java weak reference? Let us see an example scenario, which will help us understand the problem in detail. Before that, we should be aware there are four types of references in java and they are strong reference, weak reference, soft reference and phantom reference.

Weak Reference

When there are one or more reference to an object it will not be garbage collected in Java. But this rule depends on what type of reference it is. If an object has only weak reference associated with other objects, then it is a valid candidate for garbage collection.

Let us take a sample scenario to understand it better. Let TextView be an object (recently programming in Android and so using its class for example :-)) and we will have program generated ids used for its identification. These ids are used in some other object for referencing the TextViews.

...
Map textViewIdMap = new HashMap();
textViewIdMap.put(textView1, iD1);
textViewIdMap.put(textView2, iD2)
...

Key is TextView object and value is the Id. Now, during the execution of the program we have removed a TextView object say textView1. We do not require that view object so we have made it null. Now, what will happen to the key-value pair(textView1, iD1) stored in HashMap. This pair as of now makes no sense and it is not required as that textview itself is null.

So, programmatic we need to ensure that, when a textView is removed then its corresponding entry in the map should be removed. Only then, that object becomes a candidate for garbage collection. Otherwise, even though it is not used at run-time, this stale object will not be garbage collected.

WeakHashMap

There is a predefined Map which uses weak reference. This is an implementation of Map interface and exactly same as HashMap but with only difference of weakrefernce. Key-value pairs extext WeakReference class.

...
private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V>
...

In WeakHashMap there is a private method name expungeStaleEntires(), which is used to remove stale entries as given in above scenario. This method is internally used before get/getEntry/put/resize/size /…. all methods. Only after expunge done the regular operation is done. It just makes a stale entry ‘null’ and releases it for garbage collection. You may read its source code for more detail.

Strong Reference vs Weak Reference

Strong reference is not something new, it is nothing but what we use in our daily programming. The default reference for objects. Strong reference is strongest of all references, if there there is a strong reference garbage collecter will not even come to this object :-)

StringBuilder iD1 = new StringBuilder();

WeakReference weakTextView1 = new WeakReference(textView1);

Types of References in Java

There are four types of references in Java,

1. Strong Reference
2. Soft Reference
3. Weak Reference
4. Phantom Reference

above is listed in the same order as their strength.

Soft Reference

Soft Reference is slightly stronger that weak reference. Soft reference allows for garbage collection, but begs the garbage collector to clear it only if there is no other option. That is, it is eligible for garbage collection, but garbage collector can remove it based on the memory crunch it has. If it is left with little memory and it is in a position to reclaim memory, then it will collect the soft references.

Phantom Reference

Phantom reference is different from soft/weak reference. This can never be reached before it is cleared in the program. Calling the get() on it return null always. When a phantom referenced object is garbage collected, it is notified via the ReferenceQueue. We can use this to find when it is garbage collected and no use other than that.