深入理解Java虚拟机－(1)引用计数算法

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引用计数算法是常用的内存管理算法，该算法的缺陷是会产生循环引用，导致对象永远不会被回收。下面证明JVM不是通过引用计数算法来判断对象是否存活。

//Debug Configuration://-verbose:gc: -Xms20M -Xmx20M -Xmn10M -verbose:gc -XX:+PrintGCDetails -XX:SurvivorRatio=8public class ReferenceCountingGC {    public Object instance = null;    private static final int _1MB = 1024 * 1024;    private byte[] bigSize = new byte[2 * _1MB];    public static void testGC()    {        ReferenceCountingGC objA = new ReferenceCountingGC();        ReferenceCountingGC objB = new ReferenceCountingGC();        objA.instance = objB;        objB.instance = objA;        objA = null;        objB = null;        System.gc();    }    public static void main(String[] args)    {        testGC();    }}

结果：

[Full GC[Tenured: 0K->376K(10240K), 0.0080588 secs] 4684K->376K(19456K), [Perm : 149K->149K(12288K)], 0.0334432 secs] [Times: user=0.02 sys=0.00, real=0.03 secs] Heap def new generation   total 9216K, used 327K [0x32f00000, 0x33900000, 0x33900000)  eden space 8192K,   4% used [0x32f00000, 0x32f51f90, 0x33700000)  from space 1024K,   0% used [0x33700000, 0x33700000, 0x33800000)  to   space 1024K,   0% used [0x33800000, 0x33800000, 0x33900000) tenured generation   total 10240K, used 376K [0x33900000, 0x34300000, 0x34300000)   the space 10240K,   3% used [0x33900000, 0x3395e090, 0x3395e200, 0x34300000) compacting perm gen  total 12288K, used 149K [0x34300000, 0x34f00000, 0x38300000)   the space 12288K,   1% used [0x34300000, 0x34325638, 0x34325800, 0x34f00000)    ro space 10240K,  44% used [0x38300000, 0x38778300, 0x38778400, 0x38d00000)    rw space 12288K,  52% used [0x38d00000, 0x3934f100, 0x3934f200, 0x39900000)

4684K->376K(19456K)表明，GC发生前，该内存已使用4684K，GC发生后，该内存使用了376K，表明对象已被回收，从而表明JVM没有使用引用计数算法。Java中使用了可达性分析算法来来判定对象是否存活，

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