測試WeakReference和PhantomReference

介紹強引用(strong reference)、軟引用(soft reference)、弱引用(weak reference)和虛引用(phantom reference)

下面測試WeakReference和PhantomReference

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;

public class ReferenceTest {
	@SuppressWarnings("unchecked")
	public static void main(String[] args) {
		Reference<Object> ref;
		
		Object weakObj = new String("weak reference");
		Object phantomObj = new String("phantom reference");
		ReferenceQueue<Object> weakQueue = new ReferenceQueue<Object>();
		ReferenceQueue<Object> phantomQueue = new ReferenceQueue<Object>();
		WeakReference<Object> weakReference = new WeakReference<Object>(weakObj, weakQueue);
		PhantomReference<Object> phantomReference = new PhantomReference<Object>(phantomObj, phantomQueue);
		
		weakObj = null;
		phantomObj = null;
		System.out.println("Weak Reference: "+weakReference.get());
		System.out.println("Phantom Reference: "+phantomReference.get());//null
		System.gc();//弱引用、虛引用被回收，同時加入到相關ReferenceQueue
		System.out.println("Weak Reference: "+weakReference.get());//null
		System.out.println("Phantom Reference: "+phantomReference.get());//null
		
		if(!phantomReference.isEnqueued()) {
			System.out.println("Request finalization.");
			System.runFinalization();
		}
		System.out.println("weak is queued: "+weakReference.isEnqueued());//true
		System.out.println("Phantom Queued: " + phantomReference.isEnqueued());//true
		
		try {  
			ref = (Reference<Object>) weakQueue.remove();  
			System.out.println("Weak Reference: " + ref.get());//null
			ref = (Reference<Object>) phantomQueue.remove();  
			System.out.println("Phantom Reference: " + ref.get());//null
			ref.clear();  
		} catch(InterruptedException e) {  
			e.printStackTrace();  
		} 
	}
}