Java基础之引用相关知识总结

package org.pdool.ref;
​
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.LinkedList;
​
public class ReferenceTest {
​
    private static ReferenceQueue<VeryBig> rq = new ReferenceQueue<VeryBig>();
​
    public static void checkQueue() {
        Reference<? extends VeryBig> ref = null;
        while ((ref = rq.poll()) != null) {
            //   只留有引用，没有对象
            System.out.println("In queue: "    + ((VeryBigWeakReference) (ref)).id);
        }
    }
​
    public static void main(String args[]) {
        int size = 3;
        LinkedList<WeakReference<VeryBig>> weakList = new LinkedList<WeakReference<VeryBig>>();
        for (int i = 0; i < size; i++) {
            weakList.add(new VeryBigWeakReference(new VeryBig("Weak " + i), rq));
            System.out.println("Just created weak: " + weakList.getLast());
        }
​
        System.gc();
        try {
            // 暂停6s，让上面的垃圾回收线程运行完成
            Thread.currentThread().sleep(6000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        checkQueue();
    }
}
​
class VeryBig {
    public String id;
    // 占用空间,让线程进行回收
    byte[] b = new byte[2 * 1024];
​
    public VeryBig(String id) {
        this.id = id;
    }
​
    protected void finalize() {
        System.out.println("Finalizing VeryBig " + id);
    }
}
​
class VeryBigWeakReference extends WeakReference<VeryBig> {
    public String id;
​
    public VeryBigWeakReference(VeryBig big, ReferenceQueue<VeryBig> rq) {
        super(big, rq);
        this.id = big.id;
    }
​
    protected void finalize() {
        System.out.println("Finalizing VeryBigWeakReference " + id);
    }
}