@Test public void strongReference() { Object referent = new Object(); /** * 通过赋值创建 StrongReference */ Object strongReference = referent; assertSame(referent, strongReference); referent = null; System.gc(); /** * StrongReference 在 GC 后不会被回收 */ assertNotNull(strongReference); }
WeakReference, 顾名思义, 是一个弱引用, 当所引用的对象在 JVM 内不再有强引用时, GC 后 weak reference 将会被自动回收
public void weakReference() { Object referent = new Object(); WeakReference<Object> weakRerference = new WeakReference<Object>(referent); assertSame(referent, weakRerference.get()); referent = null; System.gc(); /** * 一旦没有指向 referent 的强引用, weak reference 在 GC 后会被自动回收 */ assertNull(weakRerference.get()); }
public void weakHashMap() throws InterruptedException { Map<Object, Object> weakHashMap = new WeakHashMap<Object, Object>(); Object key = new Object(); Object value = new Object(); weakHashMap.put(key, value); assertTrue(weakHashMap.containsValue(value)); key = null; System.gc(); /** * 等待无效 entries 进入 ReferenceQueue 以便下一次调用 getTable 时被清理 */ Thread.sleep(1000); /** * 一旦没有指向 key 的强引用, WeakHashMap 在 GC 后将自动删除相关的 entry */ assertFalse(weakHashMap.containsValue(value)); }
@Test public void softReference() { Object referent = new Object(); SoftReference<Object> softRerference = new SoftReference<Object>(referent); assertNotNull(softRerference.get()); referent = null; System.gc(); /** * soft references 只有在 jvm OutOfMemory 之前才会被回收, 所以它非常适合缓存应用 */ assertNotNull(softRerference.get()); }
@Test public void phantomReferenceAlwaysNull() { Object referent = new Object(); PhantomReference<Object> phantomReference = new PhantomReference<Object>(referent, new ReferenceQueue<Object>()); /** * phantom reference 的 get 方法永远返回 null */ assertNull(phantomReference.get()); }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),
@Test public void referenceQueue() throws InterruptedException { Object referent = new Object(); ReferenceQueue<Object> referenceQueue = new ReferenceQueue<Object>(); WeakReference<Object> weakReference = new WeakReference<Object>(referent, referenceQueue); assertFalse(weakReference.isEnqueued()); Reference<? extends Object> polled = referenceQueue.poll(); assertNull(polled); referent = null; System.gc(); assertTrue(weakReference.isEnqueued()); Reference<? extends Object> removed = referenceQueue.remove(); assertNotNull(removed); }
6. PhantomReference vs WeakReference
PhantomReference 有两个好处, 其一, 它可以让我们准确地知道对象何时被从内存中删除, 这个特性可以被用于一些特殊的需求中(例如 Distributed GC, XWork 和 google-guice 中也使用 PhantomReference 做了一些清理性工作).
其二, 它可以避免 finalization 带来的一些根本性问题, 上文提到 PhantomReference 的唯一作用就是跟踪 referent 何时被 enqueue 到 ReferenceQueue 中, 但是 WeakReference 也有对应的功能, 两者的区别到底在哪呢 ?
这就要说到 Object 的 finalize 方法, 此方法将在 gc 执行前被调用, 如果某个对象重载了 finalize 方法并故意在方法内创建本身的强引用, 这将导致这一轮的 GC 无法回收这个对象并有可能
引起任意次 GC, 最后的结果就是明明 JVM 内有很多 Garbage 却 OutOfMemory, 使用 PhantomReference 就可以避免这个问题, 因为 PhantomReference 是在 finalize 方法执行后回收的,也就意味着此时已经不可能拿到原来的引用, 也就不会出现上述问题, 当然这是一个很极端的例子, 一般不会出现.
7. 对比
taken from http://mindprod.com/jgloss/phantom.html
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 |