Can someone explain me what is the difference between Strong, Soft, Weak and Phantom references and the usage of it?

Question

I have been trying to understand the difference between different references but the theory does not provoke any ideas for me to visualize the same.

Could anyone please explain in brief the different references?

An example for each would do better.

Answer 1

An article explaining these types of references (including examples): http://weblogs.java.net/blog/enicholas/archive/2006/05/understanding_w.html

Answer 2

Another good article on the topic:
Java Reference Objects _{or How I Learned to Stop Worrying and Love OutOfMemoryError}, with nice diagrams

Extract:

As you might guess, adding three new optional states to the object life-cycle diagram makes for a mess.
Although the documentation indicates a logical progression from strongly reachable through soft, weak, and phantom, to reclaimed, the actual progression depends on what reference objects your program creates.
If you create a WeakReference but don't create a SoftReference, then an object progresses directly from strongly-reachable to weakly-reachable to finalized to collected. object life-cycle, with reference objects

It's also important to remember that not all objects are attached to reference objects — in fact, very few of them should be.
A reference object is a layer of indirection: you go through the reference object to reach the referred object, and clearly you don't want that layer of indirection throughout your code.
Most programs, in fact, will use reference objects to access a relatively small number of the objects that the program creates.

References and Referents

A reference object provides a layer of indirection between your program code and some other object, called the referent.
Each reference object is constructed around its referent, and provides a get() method to access the referent. Once you create a reference, you cannot change its referent. Once the referent has been collected, the get() method returns null. relationships between application code, soft/weak reference, and referent

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Even more examples: Java Programming: References' Package

• Case 1: This is the regular case where Object is said to be strongly reachable.

• Case 2: There are two paths to Object, so the strongest one is chosen, which is the one with the strong reference hence the object is strongly reachable.

• Case 3: Once again there are two paths to the Object, the strongest one is the Weak Reference (since the other one is a Phantom Reference), so the object is said to be weakly reachable.

• Case 4: There is only one path and the weakest link is a weak reference, so the object is weakly reachable.

• Case 5: Only one path and the weakest link is the phantom reference hence the object is phantomly reachable.

• Case 6: There are now two paths and the strongest path is the one with a soft reference, so the object is now said to be softly reachable.

Answer 3

There's a really simple rule:

• strongly-referenced objects are standard bits of code like Object a = new Object(). The referenced Objects are not garbage as long as the reference (a, above) is "reachable". Hence anything which has no reachable strong reference can be deemed garbage.

So then we look at the non-strong reference types:

• weakly-referenced objects will probably get collected by the JVM as soon as they become eligible for GC (and the WeakReference cleared). A weak reference to a would look like new WeakReference<Object>(a). Weak references are useful in the case that you want a cache whereby the data is only needed if the keys exist as strongly-reachable elsewhere (e.g. HttpSessions)
• softly-referenced objects will probably hang around in the JVM until it absolutely needs to recover memory. Soft references are useful for caches where the values are long-lived but can collected if necessary

I'm never too sure about phantom ones!