Null / Object和Null / Null比较效率

Question

This question让我做一些测试：

public class Stack
{
    public static void main(String[] args)
    {
        Object obj0 = null;
        Object obj1 = new Object();
        long start;
        long end;
        double difference;  
        double differenceAvg = 0;

        for (int j = 0; j < 100; j++)
        {
            start = System.nanoTime();

            for (int i = 0; i < 1000000000; i++)
                if (obj0 == null);

            end = System.nanoTime();
            difference = end - start;
            differenceAvg +=difference;
        }

        System.out.println(differenceAvg/100);
        differenceAvg = 0;

        for (int j = 0; j < 100; j++)
        {
            start = System.nanoTime();

            for (int i = 0; i < 1000000000; i++)
                if (null == obj0);

            end = System.nanoTime();
            difference = end - start;
            differenceAvg +=difference;
        }

        System.out.println(differenceAvg/100);
        differenceAvg = 0;

        for (int j = 0; j < 100; j++)
        {
            start = System.nanoTime();

            for (int i = 0; i < 1000000000; i++)
                if (obj1 == null);

            end = System.nanoTime();
            difference = end - start;
            differenceAvg +=difference;
        }

        System.out.println(differenceAvg/100);
        differenceAvg = 0;

        for (int j = 0; j < 100; j++)
        {
            start = System.nanoTime();

            for (int i = 0; i < 1000000000; i++)
                if (null == obj1);

            end = System.nanoTime();
            difference = end - start;
            differenceAvg +=difference;
        }

        System.out.println(differenceAvg/100);      
    }
}

与other post相切，有趣的是，当我们比较的Object被初始化时，比较的速度有多快。每个输出中的前两个数字是Object为null时，后两个数字是Object初始化时的数字。在所有30次执行中，我运行了21次额外的程序执行，Object初始化时的比较速度要快得多。这里发生了什么？

Answer 1

如果将最后两个循环移到开头，您将得到相同的结果，因此比较无关紧要。

这是关于JIT编译器预热的全部内容。在前2个循环中，java以解释字节码开始。在一些迭代之后，它确定代码路径是“热”的，因此它将其编译为机器代码并删除无效的循环，因此您基本上测量System.nanotime和double算术。

我不确定为什么两个循环很慢。我认为在找到两条热门路径后，它决定优化整个方法。