先看下面的例子
public class FinalDemo {
public static void main(String[] args) {
System.out.println(A.s);
}
}
class A{
public static final String s = "jetty";
public static String ss = "docker";
public final String sss = "zookeper";
static {
System.out.println("A class init。。。");
}
}
会输出什么，简单啊~~~
jetty
A class init。。。
然而不是这样的，正确的输出应该是
jetty
tell me why? 怎么回事呢
这就要说到class文件中的ConstantValue属性
我们先来看看class文件吧
C:\demo\target\classes\com\example\demo\test>javap -v A.class
Classfile /C:/demo/target/classes/com/example/demo/test/A.class
Last modified 2021-8-19; size 652 bytes
MD5 checksum 5edd6c444c043e853ed1cd7f678e85f2
Compiled from "FinalDemo.java"
class com.example.demo.test.A
minor version: 0
major version: 52
flags: ACC_SUPER
Constant pool: //常量池
#1 = Methodref #9.#26 // java/lang/Object."<init>":()V
#2 = String #27 // docker
#3 = Fieldref #8.#28 // com/example/demo/test/A.sss:Ljava/lang/String;
#4 = Fieldref #8.#29 // com/example/demo/test/A.ss:Ljava/lang/String;
#5 = Fieldref #30.#31 // java/lang/System.out:Ljava/io/PrintStream;
#6 = String #32 // class init。。。
#7 = Methodref #33.#34 // java/io/PrintStream.println:(Ljava/lang/String;)V
#8 = Class #35 // com/example/demo/test/A
#9 = Class #36 // java/lang/Object
#10 = Utf8 s
#11 = Utf8 Ljava/lang/String;
#12 = Utf8 ConstantValue
#13 = String #37 // jetty
#14 = Utf8 ss
#15 = Utf8 sss
#16 = Utf8 <init>
#17 = Utf8 ()V
#18 = Utf8 Code
#19 = Utf8 LineNumberTable
#20 = Utf8 LocalVariableTable
#21 = Utf8 this
#22 = Utf8 Lcom/example/demo/test/A;
#23 = Utf8 <clinit>
#24 = Utf8 SourceFile
#25 = Utf8 FinalDemo.java
#26 = NameAndType #16:#17 // "<init>":()V
#27 = Utf8 docker
#28 = NameAndType #15:#11 // sss:Ljava/lang/String;
#29 = NameAndType #14:#11 // ss:Ljava/lang/String;
#30 = Class #38 // java/lang/System
#31 = NameAndType #39:#40 // out:Ljava/io/PrintStream;
#32 = Utf8 class init。。。
#33 = Class #41 // java/io/PrintStream
#34 = NameAndType #42:#43 // println:(Ljava/lang/String;)V
#35 = Utf8 com/example/demo/test/A
#36 = Utf8 java/lang/Object
#37 = Utf8 jetty
#38 = Utf8 java/lang/System
#39 = Utf8 out
#40 = Utf8 Ljava/io/PrintStream;
#41 = Utf8 java/io/PrintStream
#42 = Utf8 println
#43 = Utf8 (Ljava/lang/String;)V
{
public static final java.lang.String s;
descriptor: Ljava/lang/String;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: String jetty //这里ConstantValue 的类型是string，值为jetty
public static java.lang.String ss;
descriptor: Ljava/lang/String;
flags: ACC_PUBLIC, ACC_STATIC
public final java.lang.String sss;
descriptor: Ljava/lang/String;
flags: ACC_PUBLIC, ACC_FINAL
ConstantValue: String docker //这里ConstantValue 的类型是string，值为docker
com.example.demo.test.A();
descriptor: ()V
flags:
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String docker
7: putfield #3 // Field sss:Ljava/lang/String;
10: return
LineNumberTable:
line 10: 0
line 14: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/example/demo/test/A;
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=2, locals=0, args_size=0
0: ldc #2 // String docker
2: putstatic #4 // Field ss:Ljava/lang/String;
5: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
8: ldc #6 // String class init。。。
10: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
13: return
LineNumberTable:
line 13: 0
line 16: 5
line 17: 13
}
SourceFile: "FinalDemo.java"
分析class文件，发现 jetty 并没有在静态代码块或者构造块中赋值，可是能够直接输出 jetty ，原因就只有一个，类初始化之前就已经有值了，所以当你获取直接获取一个被 final static 修饰的常量时，可以不用加载类而直接进行获取。
在实际的程序中，只有同时被final和static修饰的字段才有ConstantValue属性，且仅限于基本类型和String。编译时Javac将会为该常量生成ConstantValue属性，在类加载的准备阶段，会用ConstantValue中的值来初始化，而不是用默认的值来进行初始化。
为什么仅限于基本类型和字符串
因为常量池中只能引用到基本类型和string类型的字面量
