@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);
} @Test
public void weakReference() {
Object referent = new Object();
WeakReference<Object> weakRerference = new WeakReference<Object>(referent); assertSame(referent, weakRerference.get()); referent = null;
System.gc(); /**
* 参考
* http://blog.csdn.net/matrix_xu/article/details/8424038
* 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());
} @Test
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);
/*
* Returns this reference object's referent. If this reference object has been cleared,
either by the program or by the garbage collector, then this method returns null.
*/
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());
} /*
将一个 ReferenceQueue 传给一个 Reference的构造函数， 当对象被回收时，
虚拟机会自动将这个对象插入到 ReferenceQueue中。 WeakHashMap 就是利用
ReferenceQueue 来清除 key 已经没有强引用的 entries。
*/
@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();
/*
* 一旦没有指向 referent 的强引用, weak reference 在 GC 后会被回收
当对象被回收时， 虚拟机会自动将这个对象插入到 ReferenceQueue
*/
assertTrue(weakReference.isEnqueued());
Reference<? extends Object> removed = referenceQueue.remove();
assertNotNull(removed);
}