Question

我在以下代码中体验了（对我来说）奇怪的运行时行为：

public class Main{

    private final static long ROUNDS = 1000000;
    private final static double INITIAL_NUMBER = 0.45781929d;
    private final static double DIFFERENCE = 0.1250120303d;


    public static void main(String[] args){

        doSomething();
        doSomething();
        doSomething();
    }


    private static void doSomething(){

        long begin, end;
        double numberToConvert, difference;


        numberToConvert = INITIAL_NUMBER;
        difference = DIFFERENCE;

        begin = System.currentTimeMillis();

        for(long i=0; i<ROUNDS; i++){

            String s = "" + numberToConvert;

            if(i % 2 == 0){
                numberToConvert += difference;
            }
            else{
                numberToConvert -= difference;
            }
        }

        end = System.currentTimeMillis();

        System.out.println("String appending conversion took " + (end - begin) + "ms.");
    }
}

我希望程序每次打印出类似的运行时。但是，我得到的输出总是这样：

String appending conversion took 473ms.
String appending conversion took 362ms.
String appending conversion took 341ms.

第一次通话比之后的通话慢约30％。大多数情况下，第二次通话也比第三次通话稍慢。

java / javac版本：

javac 1.7.0_09 java version "1.7.0_09" OpenJDK Runtime Environment (IcedTea7 2.3.3) (7u9-2.3.3-0ubuntu1~12.04.1) OpenJDK 64-Bit Server VM (build 23.2-b09, mixed mode)

所以，我的问题：为什么会发生这种情况？

Answer 1

即时（JIT）编译器正在动态分析您的代码并优化执行。执行代码的次数越多，优化效果就越好。

例如，请参阅此问题以获取更多信息：(How) does the Java JIT compiler optimize my code?

Answer 2

您运行的其他应用程序可能会影响您在计算机上分配给JVM的内存量。尝试在运行java命令时为JVM设置相同的最小和最大内存：

java -Xms512M -Xmx512M ...

尝试运行时，我得到了相当一致的间隔：

String appending conversion took 1153ms.
String appending conversion took 1095ms.
String appending conversion took 1081ms.

Java：main中的奇怪运行时行为

2 个答案: