在Java中;从强到弱依次为:强、软、弱、幻

强引用是一种普通引用，用于保护被引用的对象不被GC收集。即从不收集垃圾。

软引用可以被垃圾回收器收集，但可能直到需要它的内存时才会被收集。即在OutOfMemoryError之前进行垃圾收集。

弱引用是指不保护被引用对象不被GC收集的引用。即，当没有强或软引用时，垃圾收集。

幻影引用是在对象完成后，但在已分配的内存被回收之前，对对象的幻影引用。

源

类比:假设JVM是一个王国，对象是王国的国王，GC是王国的攻击者，试图杀死国王(对象)。

当国王强大时，GC不能杀死他。 当国王是软的，GC攻击他，但国王统治王国的保护，直到资源可用。 当国王虚弱时，GC攻击他，但在没有保护的情况下统治王国。 当国王是幻影时，GC已经杀死了他，但国王可以通过他的灵魂获得。

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.

另一方面，softreference对于缓存外部的、可重新创建的资源很有好处 因为GC通常会延迟清除它们。尽管如此，这是有保证的 在抛出OutOfMemoryError之前，sofreferferences将被清除 理论上不会引起OOME[*]。

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.

现在是[*]。保持一个SoftReference本身不会导致OOME。如果 另一方面，你错误地把软参考用在了一个弱参考的任务上 (也就是说，你以某种方式保存与对象相关的信息 ，并在Reference对象获得时丢弃它 清除)，您可以运行OOME作为您的代码轮询referencqueuue 而丢弃关联对象可能会发生不及时运行的情况 时尚。

所以，决定取决于使用情况 -如果你缓存的信息构造起来很昂贵，但是 尽管如此，可从其他数据重构，使用软引用 -如果您保留对某些数据的规范实例的引用，或者 您希望拥有对对象的引用，但不“拥有”它(因此 防止它被GC)，使用弱引用。

软引用和弱引用之间唯一真正的区别是

垃圾收集器使用算法来决定是否 回收软可达对象，但总是回收弱可达对象 可访问的对象。

这篇文章对于理解强引用、软引用、弱引用和幻影引用非常有帮助。

总结一下，

如果对一个对象只有弱引用(没有强引用)，那么该对象将在下一个GC循环中被GC回收。

如果对对象只有软引用(没有强引用)，那么只有当JVM内存耗尽时，GC才会回收该对象。

所以你可以说，强引用具有强大的功能(GC永远不会收集)。

软引用比弱引用更强大(因为它们可以逃避GC循环，直到JVM耗尽内存)

弱引用甚至不如软引用强大(因为它们不能逃避任何GC循环，如果对象没有其他强引用，就会被回收)。

餐厅的类比

服务员- GC 堆中的对象 餐厅区域/空间-堆空间 新客户-在餐厅想要桌子的新对象

现在，如果您是一个强客户(类似于强引用)，那么即使餐厅里来了一个新客户或发生了其他事情，您也不会离开您的表(堆上的内存区域)。服务员没有权利告诉你(甚至要求你)离开餐厅。

如果你是一个软顾客(类似于软参考)，那么如果餐厅来了一个新顾客，服务员不会让你离开桌子，除非没有其他空桌子来容纳新顾客。(换句话说，只有当一个新顾客进来，并且没有其他的桌子给这个新顾客时，服务员才会叫你离开桌子)

如果你是一个弱顾客(类似于弱参考)，那么服务员可以根据他的意愿(在任何时候)要求你离开餐厅:P

SoftReference is designed for caches. When it is found that a WeakReference references an otherwise unreachable object, then it will get cleared immediately. SoftReference may be left as is. Typically there is some algorithm relating to the amount of free memory and the time last used to determine whether it should be cleared. The current Sun algorithm is to clear the reference if it has not been used in as many seconds as there are megabytes of memory free on the Java heap (configurable, server HotSpot checks against maximum possible heap as set by -Xmx). SoftReferences will be cleared before OutOfMemoryError is thrown, unless otherwise reachable.

摘自Ethan Nicholas的《理解弱引用》:

Weak references A weak reference, simply put, is a reference that isn't strong enough to force an object to remain in memory. Weak references allow you to leverage the garbage collector's ability to determine reachability for you, so you don't have to do it yourself. You create a weak reference like this: WeakReference weakWidget = new WeakReference(widget); and then elsewhere in the code you can use weakWidget.get() to get the actual Widget object. Of course the weak reference isn't strong enough to prevent garbage collection, so you may find (if there are no strong references to the widget) that weakWidget.get() suddenly starts returning null. ... Soft references A soft reference is exactly like a weak reference, except that it is less eager to throw away the object to which it refers. An object which is only weakly reachable (the strongest references to it are WeakReferences) will be discarded at the next garbage collection cycle, but an object which is softly reachable will generally stick around for a while. SoftReferences aren't required to behave any differently than WeakReferences, but in practice softly reachable objects are generally retained as long as memory is in plentiful supply. This makes them an excellent foundation for a cache, such as the image cache described above, since you can let the garbage collector worry about both how reachable the objects are (a strongly reachable object will never be removed from the cache) and how badly it needs the memory they are consuming.

Peter Kessler补充道:

The Sun JRE does treat SoftReferences differently from WeakReferences. We attempt to hold on to object referenced by a SoftReference if there isn't pressure on the available memory. One detail: the policy for the "-client" and "-server" JRE's are different: the -client JRE tries to keep your footprint small by preferring to clear SoftReferences rather than expand the heap, whereas the -server JRE tries to keep your performance high by preferring to expand the heap (if possible) rather than clear SoftReferences. One size does not fit all.