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System.currentTimeMillis() 和 System.nanoTime() 哪个更快?别用错了!
2022-07-04 14:33:00 【Java技术栈】
作者:捏造的信仰
来源:https://segmentfault.com/a/1190000041276485
Java有两个取时间戳的方法:System.currentTimeMillis()
和 System.nanoTime()
,它们的使用场景是有区别的,当前网上一些文章对于这两个方法的性能讨论存在一些片面的描述,本文希望能给出一个简单的最终答案。
System.currentTimeMillis() 存在性能问题?
答案是否定的。这两个方法性能差异取决于操作系统。
Windows:
在 Windows 下,System.currentTimeMillis()
比 System.nanoTime()
要快很多,这是因为 Windows 系统为前者提供的只是一个缓存变量,而后者则是实时的去硬件底层获取计数。
所以如果你的生产环境是 Windows,请尽可能避免使用
System.nanoTime()
。
Linux:
在 Linux 下,两者的执行耗时相差不大,不论是单线程还是多线程。
不同的虚拟机实现会带来性能差异
如今的云主机主要有 Xen 和 KVM 两种实现方式,网上有文章发现它们在取系统时间方面存在性能差异。
当你的虚拟机用的是 Xen 时,取时间的耗时会是 KVM 的十倍以上。不过上文也提供了遇到此类问题该如何解决的方案。
需要写一个专门的类来提升 System.currentTimeMillis() 性能吗?
不需要。那属于画蛇添足。
我的测试代码
我的测试代码如下,没有任何依赖,可以直接用 javac 编译然后运行。读者有兴趣可以试试:
import java.util.ArrayList;import java.util.List;import java.util.function.Consumer;public class TimePerformance { public static final int LOOP_COUNT = 9999999; public static final int THREAD_COUNT = 30; public static void main(String[] args) { Runnable millisTest = () -> { long start = System.currentTimeMillis(); for (int i = 0; i < LOOP_COUNT; i++) { System.currentTimeMillis(); } long end = System.currentTimeMillis(); System.out.printf("%s : %f ns per call\n", Thread.currentThread().getName(), ((double)end - start) * 1000000 / LOOP_COUNT); }; Runnable nanoTest = () -> { long start = System.currentTimeMillis(); for (int i = 0; i < LOOP_COUNT; i++) { System.nanoTime(); } long end = System.currentTimeMillis(); System.out.printf("%s : %f ns per call\n", Thread.currentThread().getName(), ((double)end - start) * 1000000 / LOOP_COUNT); }; Consumer<Runnable> testing = test -> { System.out.println("Single thread test:"); test.run(); System.out.println(THREAD_COUNT + " threads test:"); List<Thread> threads = new ArrayList<>(); for (int i = 0; i < THREAD_COUNT; i++) { Thread t = new Thread(test); t.start(); threads.add(t); } // Wait for all threads to finish threads.forEach(thread -> { try { thread.join(); } catch (InterruptedException e) { e.printStackTrace(); } }); }; System.out.println("//// Test System.nanoTime()"); testing.accept(nanoTest); System.out.println("//// Test System.currentTimeMillis()"); testing.accept(millisTest); }}
因为我用的是 Windows,所以执行输出当中 System.nanoTime()
明显非常慢。
具体输出内容我就不放出来了,因为不具有参考价值,大多数生产环境用的是 Linux。
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