java中的编译时常量与运行时常量

常量是程序运行期间恒定不变的量，许多程序设计语言都有某种方式，向编译器告知一块数据是恒定不变的，例如C++中的const和Java中的final。

根据编译器的不同行为，常量又分为编译时常量和运行时常量，其实编译时常量肯定就是运行时常量，只是编译时常量在编译的时候就被执行计算，并带入到程序中一切可能用到它的计算式中。

以Java为例，static final int a = 1将是一个编译时常量，编译后的符号表中将找不到a，所有对a的引用都被替换成了1。
而static final int b = “”.length()将是一个运行时常量。测试代码如下：

class Test {
    public static final int a = 10;
    public static final int b = "test".length();
    static {
        System.out.println("Class Test Was Loaded !");
    }
}

public class CompilConstant {
    public static void main(String[] args) {
        System.out.println(Test.a);
        System.out.println(Test.b);
    }
}

输出：

10
Class Test Was Loaded !
4

即：
1. a被作为编译期全局常量，并不依赖于类，而b作为运行期的全局常量，其值还是依赖于类的。
2. 编译时常量在编译时就可以确定值，上例中的a可以确定值，但是b在编译期是不可能确定值的。
3. 由于编译时常量不依赖于类，所以对编译时常量的访问不会引发类的初始化。

测试：

//System.out.println(Test.b);//注释掉该条代码

执行

javac CompilConstant.java
del Test.class

也就是把编译生成的Test.class 删除，并执行java CompilConstant，输出：
10

执行反汇编：
javap -v CompilConstant

D:\N3verL4nd\Desktop>javap -v CompilConstant
Classfile /D:/N3verL4nd/Desktop/CompilConstant.class
  Last modified 2017-10-21; size 440 bytes
  MD5 checksum faf8cf7e41ff58ae190d78fbb7e7635c
  Compiled from "CompilConstant.java"
public class CompilConstant
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #7.#16         // java/lang/Object."<init>":()V
   #2 = Fieldref           #17.#18        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Class              #19            // Test
   #4 = Methodref          #20.#21        // java/io/PrintStream.println:(I)V
   #5 = Fieldref           #3.#22         // Test.c:I
   #6 = Class              #23            // CompilConstant
   #7 = Class              #24            // java/lang/Object
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               main
  #13 = Utf8               ([Ljava/lang/String;)V
  #14 = Utf8               SourceFile
  #15 = Utf8               CompilConstant.java
  #16 = NameAndType        #8:#9          // "<init>":()V
  #17 = Class              #25            // java/lang/System
  #18 = NameAndType        #26:#27        // out:Ljava/io/PrintStream;
  #19 = Utf8               Test
  #20 = Class              #28            // java/io/PrintStream
  #21 = NameAndType        #29:#30        // println:(I)V
  #22 = NameAndType        #31:#32        // b:I
  #23 = Utf8               CompilConstant
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/System
  #26 = Utf8               out
  #27 = Utf8               Ljava/io/PrintStream;
  #28 = Utf8               java/io/PrintStream
  #29 = Utf8               println
  #30 = Utf8               (I)V
  #31 = Utf8               b
  #32 = Utf8               I
{
  public CompilConstant();
    descriptor: ()V
    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 9: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: bipush        10
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
         8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        11: getstatic     #5                  // Field Test.b:I
        14: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
        17: return
      LineNumberTable:
        line 11: 0
        line 12: 8
        line 13: 17
}
SourceFile: "CompilConstant.java"

D:\N3verL4nd\Desktop>

可见，对于Test.a的输出直接把bipush 10 10压入栈中。

转载自 ：作者：N3verL4nd 出处： https://blog.csdn.net/lgh1992314/article/details/78303623