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
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.
# 1 楼答案
关于这个主题的另一篇好文章：
Java Reference Objects or How I Learned to Stop Worrying and Love OutOfMemoryError，有漂亮的图表
摘录：
记住并非所有对象都附加到引用对象上，这一点也很重要-事实上，很少有对象应该附加到引用对象上
引用对象是一个间接层：您通过引用对象到达引用对象，显然您不希望在整个代码中使用该间接层
事实上，大多数程序将使用引用对象访问程序创建的相对较少的对象
引用和引用
引用对象在程序代码和其他对象（称为referent）之间提供了一个间接层
每个引用对象都围绕其引用对象构造，并提供一个get（）方法来访问引用对象。创建引用后，不能更改其referent。收集引用对象后，get（）方法返回null。应用程序代码、软/弱引用和引用之间的关系
甚至更多的例子：Java Programming: References' Package
alt text http://www.pabrantes.net/blog/space/start/2007-09-16/1/referenceTypes.png
# 2 楼答案
解释这些引用类型的文章（包括示例）：Understanding Weak References - weblogs.java.net (From the Web Archive)
# 3 楼答案
有一个非常简单的规则：
Object a = new Object()这样的标准代码位。只要引用（a，上面）是“可访问的”，引用对象就不是垃圾。因此，任何没有可到达的强引用的东西都可以被视为垃圾然后我们来看看非强引用类型：
WeakReference被清除）。对a的弱引用看起来像new WeakReference<Object>(a)。弱引用在需要缓存的情况下非常有用，因为只有在其他地方（例如HttpSessions）可以强访问密钥时，才需要缓存数据我从来都不太确定幻影