Java中的4种类型的引用

Java中一共有4种类型的引用:StrongReference、SoftReference、WeakReference以及 PhantomReference (幽灵引用)。

 

1. Strong Reference
   StrongReference 是 Java 的默认引用实现,它会尽可能长时间的存活于 JVM 内， 当没有任何对象指向它时Java GC 执行后将会被回收
   
   
2. SoftReference
   SoftReference 所引用的对象在JVM内部不再有强引用时会被回收(虚拟机保证), 但会尽可能长的保留引用直到 JVM 内存不足时才会回收,这一特性使得 SoftReference 非常适合缓存应用
   
   
3. WeakReference
   WeakReference 当所引用的对象在 JVM 内不再有强引用时, weak reference 将会被自动回收
   
   
4. PhantomReference
   Phantom Reference(幽灵引用) 与 WeakReference 和 SoftReference 有很大的不同,因为它的 get() 方法永远返回 null, 这也正是它名字的由来
   
   诸位可能要问, 一个永远返回 null 的 reference 要来何用,请注意构造 PhantomReference 时的第二个参数 ReferenceQueue(事实上 WeakReference & SoftReference 也可以有这个参数)，PhantomReference 唯一的用处就是跟踪 referent何时被 enqueue 到 ReferenceQueue 中.

对比

Soft vs Weak vs Phantom References Type Purpose Use When GCed Implementing Class

Strong Reference	An ordinary reference. Keeps objects alive as long as they are referenced.	normal reference.	Any object not pointed to can be reclaimed.	default
Soft Reference	Keeps objects alive provided there’s enough memory.	to keep objects alive even after clients have removed their references (memory-sensitive caches), in case clients start asking for them again by key.	After a first gc pass, the JVM decides it still needs to reclaim more space.	java.lang.ref. SoftReference
Weak Reference	Keeps objects alive only while they’re in use (reachable) by clients.	Containers that automatically delete objects no longer in use.	After gc determines the object is only weakly reachable	java.lang.ref. WeakReference   java.util. WeakHashMap
Phantom Reference	Lets you clean up after finalization but before the space is reclaimed (replaces or augments the use offinalize ())	Special clean up processing	After finalization.	java.lang.ref. PhantomReference