References.java

package learnrefclasses;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashSet;
import java.util.Set;

/**
 * 
 * 
 * @author Moon Zang
 * 
 */

class Grocery {
    private static final int SIZE = 10000;
    // 属性d使得每个Grocery对象占用较多内存，有80K左右
    public double[] d = new double[SIZE];
    private String id;

    public Grocery(String id) {
        this.id = id;
    }

    public String toString() {
        return id;
    }

    public void finalize() {
//        System.out.println("!!!!!!!!!!!!!!!!Finalizing " + id);
    }
}

public class References {
    private static ReferenceQueue<Grocery> rq = new ReferenceQueue<Grocery>();

    public static void checkQueue() {
        Reference<? extends Grocery> inq = null; // 从队列中取出一个引用
        while ((inq = rq.poll()) != null) {
            System.out.println("TTTTTTTTTTTTTTTTTTIn queue: " + inq + " : "
                    + inq.get());
        }
    }

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

        // 创建10个Grocery对象以及10个软引用
        Set<SoftReference<Grocery>> sa = new HashSet<SoftReference<Grocery>>();
        for (int i = 0; i < size; i++) {
            SoftReference<Grocery> ref = new SoftReference<Grocery>(
                    new Grocery("Soft " + i), rq);
            System.out.println("Just created: " + ref.get());
            sa.add(ref);
        }
        checkQueue();
        System.gc();
        checkQueue();

        // 创建10个Grocery对象以及10个弱引用
        Set<WeakReference<Grocery>> wa = new HashSet<WeakReference<Grocery>>();
        for (int i = 0; i < size; i++) {
            WeakReference<Grocery> ref = new WeakReference<Grocery>(
                    new Grocery("Weak " + i), rq);
            System.out.println("Just created: " + ref.get());
            wa.add(ref);
        }
        checkQueue();
        System.gc();
        checkQueue();

        // 创建10个Grocery对象以及10个虚引用
        Set<PhantomReference<Grocery>> pa = new HashSet<PhantomReference<Grocery>>();
        for (int i = 0; i < size; i++) {
            PhantomReference<Grocery> ref = new PhantomReference<Grocery>(
                    new Grocery("Phantom " + i), rq);
            System.out.println("Just created: " + ref.get());
            pa.add(ref);
        }
        checkQueue();
        System.gc();
        checkQueue();
        try {
            Thread.sleep(1000);
            checkQueue();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}