Java中GC与四种引用的关系详解

@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();  
    /** 
     * 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);  
    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());  
}  

@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);  
}