JOL 26 Defragmentation

2016-12-06 Java jol 评论

本篇文章基于V0.16 JOLSample_26_Defragmentation

本例是用来演示VM如何对堆进行碎片整理的。

在本例中我们有一个对象数组，这些对象很紧密地分配在了一起，而且在多次GC中都存活了下来。然后我们随机清理一半的元素。现在对象布局就很稀疏了。
然后继续GC。

运行这个例子最好使用参数

1	-Xmx1g -XX:+UseParallelGC -XX:ParallelGCThreads=1

public class JOLSample_26_Defragmentation {

    public static volatile Object sink;

    public static void main(String[] args) throws Exception {
        out.println(VM.current().details());

        // allocate some objects to beef up generations
        for (int t = 0; t < 1000000; t++) {
            sink = new Object();
        }
        System.gc();

        final int COUNT = 10000;

        Object[] array = new Object[COUNT];
        for (int c = 0; c < COUNT; c++) {
            array[c] = new Object();
        }

        Object obj = array;

        GraphLayout.parseInstance(obj).toImage("array-1-new.png");

        for (int c = 2; c <= 5; c++) {
            for (int t = 0; t < 1000000; t++) {
                sink = new Object();
            }
            System.gc();
            GraphLayout.parseInstance(obj).toImage("array-" + c + "-before.png");
        }

        for (int c = 0; c < COUNT; c++) {
            if (Math.random() < 0.5) {
                array[c] = null;
            }
        }

        GraphLayout.parseInstance(obj).toImage("array-6-after.png");

        for (int c = 7; c <= 10; c++) {
            for (int t = 0; t < 1000000; t++) {
                sink = new Object();
            }
            System.gc();
            GraphLayout.parseInstance(obj).toImage("array-" + c + "-after-gc.png");
        }
    }

}