如何确保没有jvm和编译器优化发生 [英] how to make sure no jvm and compiler optimization occurs

问题描述

我有这个代码测试 Calendar.getInstance（）。getTimeInMillis（） vs System.currentTimeMilli（）：

I have this code that is testing Calendar.getInstance().getTimeInMillis() vs System.currentTimeMilli() :

long before = getTimeInMilli();
for (int i = 0; i < TIMES_TO_ITERATE; i++)
{
  long before1 = getTimeInMilli();
  doSomeReallyHardWork();
  long after1 = getTimeInMilli();
}
long after = getTimeInMilli();
System.out.println(getClass().getSimpleName() + " total is " + (after - before));

我想确保没有JVM或编译器优化发生，所以测试将是有效的，显示差异。

I want to make sure no JVM or compiler optimization happens, so the test will be valid and will actually show difference.

如何确定？

EDIT 代码示例所以会更清楚。我在这里检查是在不同的实现中调用getTimeInMilli（）需要多少时间 - 日历vs系统。

EDIT: I changed the code example so it will be more clear. What I am checking here is how much time it takes to call getTimeInMilli() in different implementations - Calendar vs System.

推荐答案

认为你需要禁用JIT。添加到您的运行命令下一个选项：

I think you need to disable JIT. Add to your run command next option:

-Djava.compiler=NONE

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