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。
