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    <p>Weak references are collected eagerly. If GC finds that an object is weakly reachable (reachable only through weak references), it'll clear the weak references to that object immediately. As such, they're good for keeping a reference to an object for which your program also keeps (strongly referenced) "associated information" somewere, like cached reflection information about a class, or a wrapper for an object, etc. Anything that makes no sense to keep after the object it is associated with is GC-ed. When the weak reference gets cleared, it gets enqueued in a reference queue that your code polls somewhere, and it discards the associated objects as well. That is, you keep extra information about an object, but that information is not needed once the object it refers to goes away. Actually, in certain situations you can even subclass WeakReference and keep the associated extra information about the object in the fields of the WeakReference subclass. Another typical use of WeakReference is in conjunction with Maps for keeping canonical instances.</p> <p>SoftReferences on the other hand are good for caching external, recreatable resources as the GC typically delays clearing them. It is guaranteed though that all SoftReferences will get cleared before OutOfMemoryError is thrown, so they theoretically can't cause an OOME[*].</p> <p>Typical use case example is keeping a parsed form of a contents from a file. You'd implement a system where you'd load a file, parse it, and keep a SoftReference to the root object of the parsed representation. Next time you need the file, you'll try to retrieve it through the SoftReference. If you can retrieve it, you spared yourself another load/parse, and if the GC cleared it in the meantime, you reload it. That way, you utilize free memory for performance optimization, but don't risk an OOME.</p> <p>Now for the [*]. Keeping a SoftReference can't cause an OOME in itself. If on the other hand you mistakenly use SoftReference for a task a WeakReference is meant to be used (namely, you keep information associated with an Object somehow strongly referenced, and discard it when the Reference object gets cleared), you can run into OOME as your code that polls the ReferenceQueue and discards the associated objects might happen to not run in a timely fashion.</p> <p>So, the decision depends on usage - if you're caching information that is expensive to construct, but nonetheless reconstructible from other data, use soft references - if you're keeping a reference to a canonical instance of some data, or you want to have a reference to an object without "owning" it (thus preventing it from being GC'd), use a weak reference.</p>
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    1. POWhat's the difference between SoftReference and WeakReference in Java?
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    1. COParticularly useful for the explanation of when weak objects would be used.
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      1. USJack BeNimble
 

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