文字的算术运算是在编译时还是运行时计算的？ [英] Are arithmetic operations on literals calculated at compile time or run time?

问题描述

我有以下内容：

double timeInMinutes = (double) timeInMilliseconds / (1000 * 60);

操作是否为（1000 * 60）在编译时或运行时完成？换句话说，在上面的代码片段和运行时间之间是否存在性能差异：

Is the operation (1000 * 60) done at compile time or at run time? In other words, are there performance differences during run time between the code snippet above and:

double timeInMinutes = (double) timeInMilliseconds / 60000;

编辑：我的问题与 Java编译器是否会预先计算文字的总和？，因为我在算术运算中混合了变量和文字的使用。这是一个小小的差异，但正如@TagirValeev在评论中指出的那样（对文字的算术运算是在编译时还是运行时计算的？），有些文字虽然可以预先编译，但有些文字不会被预编译。

my question is different from Will the Java compiler precalculate sums of literals?, as I'm mixing the use of variables and literals in the arithmetic operations. It's a small difference, but as @TagirValeev noted in the comments (Are arithmetic operations on literals calculated at compile time or run time?), there are instances where some literals aren't pre-compiled even though they could be.

推荐答案

根据JLS§15.2 - 表达形式

有些表达式的值可以确定在编译时。
这些是常量表达式（§15.28）。

Some expressions have a value that can be determined at compile time. These are constant expressions (§15.28).

多重运算符，如 *，/和％属于常量表达式，因此它将在编译时确定。

Multiplicative operators like *, /, and % falls under constant expressions, so it would be determined at compile time.

@SergeyMorozov比我更快写入并获得字节码证明（＃2 =整数60000 ）但这里是实际证据，上面是理论/官方声明：

@SergeyMorozov was faster than me to write and get byte code proof (#2 = Integer 60000) but here is the practical proof and above is theoretical/official statement:

尝试使用 1000 * 60 和 60000 生成字节代码，您将看到相同的字节代码指令，因此会有相同的运行时性能。

Try generating byte code at your end as well using 1000 * 60 and 60000, and you will see same byte code instructions, and hence there would be same runtime performance.

Java类：

public class Test {
    public static void main(String[] args) {
        int compileTest = 1000 * 60;
    }
}

字节代码：

Classfile /E:/Test.class
  Last modified Oct 9, 2015; size 265 bytes
  MD5 checksum fd115be769ec6ef7995e4c84f7597d67
  Compiled from "Test.java"
public class Test
  SourceFile: "Test.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#13         //  java/lang/Object."<init>":()V
   #2 = Integer            60000
   #3 = Class              #14            //  Test
   #4 = Class              #15            //  java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Utf8               Code
   #8 = Utf8               LineNumberTable
   #9 = Utf8               main
  #10 = Utf8               ([Ljava/lang/String;)V
  #11 = Utf8               SourceFile
  #12 = Utf8               Test.java
  #13 = NameAndType        #5:#6          //  "<init>":()V
  #14 = Utf8               Test
  #15 = Utf8               java/lang/Object
{
  public Test();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=2, args_size=1
         0: ldc           #2                  // int 60000
         2: istore_1
         3: return
      LineNumberTable:
        line 3: 0
        line 4: 3
}

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