无法从 ReferenceQueue 中取出刚放进去的 Reference 元素，一直为 null

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最近在读 TIJ4，在 P518 页`持有引用`一节，书中讲述了有关 java.lang.ref 包下的 Reference 类

Java 新手，查了些资料还是没有头绪，实在不行的话我换个 JDK 试下...
以下是原书部分样例代码

package sust.p600.p519;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.LinkedList;

/**
 * Created by SunnyGrocery on 2019/11/17 18:05
 */
public class References {
    private static ReferenceQueue<VeryBig> referenceQueue = new ReferenceQueue<>();

    public static void main(String[] args) {
        int size = 10;

        LinkedList<SoftReference<VeryBig>> referenceLinkedList = new LinkedList<>();
        for (int i = 0; i < size; i++) {
            referenceLinkedList.add(new SoftReference<>(
                    new VeryBig("Soft:" + i),
                    referenceQueue));
            System.out.println("Just created:" + referenceLinkedList.getLast());
            checkQueue();
        }

        LinkedList<WeakReference<VeryBig>> referenceLinkedList1 = new LinkedList<>();
        for (int i = 0; i < size; i++) {
            referenceLinkedList1.add(new WeakReference<>(
                    new VeryBig("Weak:" + i),
                    referenceQueue));
            System.out.println("Just created:" + referenceLinkedList1.getLast());
            checkQueue();
        }

    }

    private static void checkQueue() {
        Reference<? extends VeryBig> reference = referenceQueue.poll();
        if (reference != null) {
            System.out.println("In queue: " + reference.get());
        }
    }

}

class VeryBig {
    private static final int SiZE = 10000;
    private long[] la = new long[SiZE];
    private String ident;

    public VeryBig(String id) {
        ident = id;
    }

    @Override
    public String toString() {
        return "id = " + ident;
    }

    protected void finalize() {
        System.out.println("Finalizing:" + ident);
    }
}

输出

Just created:java.lang.ref.SoftReference@677327b6
Just created:java.lang.ref.SoftReference@14ae5a5
Just created:java.lang.ref.SoftReference@7f31245a
Just created:java.lang.ref.SoftReference@6d6f6e28
Just created:java.lang.ref.SoftReference@135fbaa4
Just created:java.lang.ref.SoftReference@45ee12a7
Just created:java.lang.ref.SoftReference@330bedb4
Just created:java.lang.ref.SoftReference@2503dbd3
Just created:java.lang.ref.SoftReference@4b67cf4d
Just created:java.lang.ref.SoftReference@7ea987ac
Just created:java.lang.ref.WeakReference@12a3a380
Just created:java.lang.ref.WeakReference@29453f44
Just created:java.lang.ref.WeakReference@5cad8086
Just created:java.lang.ref.WeakReference@6e0be858
Just created:java.lang.ref.WeakReference@61bbe9ba
Just created:java.lang.ref.WeakReference@610455d6
Just created:java.lang.ref.WeakReference@511d50c0
Just created:java.lang.ref.WeakReference@60e53b93
Just created:java.lang.ref.WeakReference@5e2de80c
Just created:java.lang.ref.WeakReference@1d44bcfa

结果中没有 checkQueue 中当 reference != null 时的输出也就是说, referenceQueue.poll() 一直为空

2 条回复 • 2019-11-18 18:10:55 +08:00