JVM20——弱引用

与软引用十分类似。

/**
 * 演示弱引用
 * -Xmx20m -XX:+PrintGCDetails 
 */
public class Demo2_5 {
    private static final int _4MB = 4 * 1024 * 1024;

    public static void main(String[] args) {
        //  list --> WeakReference --> byte[]
        List<WeakReference<byte[]>> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            WeakReference<byte[]> ref = new WeakReference<>(new byte[_4MB]);
            list.add(ref);
            for (WeakReference<byte[]> w : list) {
                System.out.print(w.get()+" ");
            }
            System.out.println();

        }
        System.out.println("循环结束：" + list.size());
    }
}

打印的结果如下。其中第10 次循环时，由于弱引用本身也占有一定的内存，触发Full GC。

[B@7f31245a 
[B@7f31245a [B@6d6f6e28 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 
[GC (Allocation Failure) [PSYoungGen: 2209K->504K(6144K)] 14497K->13139K(19968K), 0.0023913 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 [B@45ee12a7 
[GC (Allocation Failure) [PSYoungGen: 4712K->496K(6144K)] 17347K->13326K(19968K), 0.0025155 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 null [B@330bedb4 
[GC (Allocation Failure) [PSYoungGen: 4704K->504K(6144K)] 17534K->13350K(19968K), 0.0020861 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 null null [B@2503dbd3 
[GC (Allocation Failure) [PSYoungGen: 4711K->504K(6144K)] 17557K->13382K(19968K), 0.0017767 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 null null null [B@4b67cf4d 
[GC (Allocation Failure) [PSYoungGen: 4710K->456K(6144K)] 17588K->13334K(19968K), 0.0017985 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 null null null null [B@7ea987ac 
[GC (Allocation Failure) [PSYoungGen: 4775K->504K(5120K)] 17653K->13398K(18944K), 0.0011480 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@7f31245a [B@6d6f6e28 [B@135fbaa4 null null null null null [B@12a3a380 
[GC (Allocation Failure) [PSYoungGen: 4735K->256K(5632K)] 17629K->13531K(19456K), 0.0016537 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Ergonomics) [PSYoungGen: 256K->0K(5632K)] [ParOldGen: 13275K->782K(8192K)] 13531K->782K(13824K), [Metaspace: 3345K->3345K(1056768K)], 0.0108212 secs] [Times: user=0.17 sys=0.00, real=0.01 secs] 
null null null null null null null null null [B@29453f44 
循环结束：10

当然，弱引用与软引用的区别是，只要触发垃圾回收，无论内存是否充足都会回收其引用对象。

public class WeakReferenceDemo {
    public  static  void  main(String[] args) {

        WeakReference<String> sr =  new  WeakReference<String>( new  String( "hello" ));

        System.out.println(sr.get());
        System.gc();                 //通知JVM的gc进行垃圾回收
        System.out.println(sr.get());
    }
}

输出结果。

hello
null