a Class Salutationís constant pool 分析

首先：1、建立一张像书一样的目录索引表1,2,3........

          2、具体的值再根据 Value 去查。

一、jvm找类常量字符串（CONSTANT_String_info）：static final 类常量 3个。

  1--30 CONSTANT_Utf8_info "Greetings, planet!"
  2--31 CONSTANT_Utf8_info "Hello, world!"
  3--39 CONSTANT_Utf8_info "Salutations, orb!"

二、jvm找类相关信息（CONSTANT_Class_info ）：5个

 1、类名 Salutation :4--37 CONSTANT_Utf8_info  "Salutation"

 2、System.out用到两个类：1>PrintStream 5--44  CONSTANT_Utf8_info "java/io/PrintStream"

                  2>System 8--47 CONSTANT_Utf8_info "java/lang/System" 

   3、choice 变量用到的类 java/lang/Math：6--45 CONSTANT_Utf8_info "java/lang/Math"

  4、默认的父类 java/lang/Object ：7--46 CONSTANT_Utf8_info   "java/lang/Object"

三、jvm 查找类变量：成员变量有两个:本类choice和System类的out.

   choice,默认执行方法 <init：执行后得结果。

      必须有一个对象（默认Object）执行方法.<init
    1、查找类变量需要的类和方法：CONSTANT_Methodref_info方法连接信息（类，方法）,7开始查找相应类，16开始查找相应的方法。

       

 2、存放类成员的名字(choice）,类型(int),

  1》属于哪个类:CONSTANT_Fieldref_info 3个

   

 2》成员变量out本类没有：是System类里面的 public final static PrintStream out = null; 

  

四、jvm 查找类用的方法：out.print(s)，Math.random() 两个。CONSTANT_Methodref_info

  1、输出时用的方法

   如print(s)：类，方法名，参数类型。 

   

  2、给类变量赋值用的方法。

   

五、编译时常量值保存：14 CONSTANT_Double_info 2.99
  

package Linking.ex5;

public class Salutation {
//** 这些都是类成员
    private static final String hello="Hello,world!";
    private static final String greeting="Greetings,planet!";
    private static final String saulutation="Saulutations,orb!";
    private static int choice=(int)(Math.random()*2.99);
    
public static void main(String[] args) {
// TODO Auto-generated method stub

String s=hello;//建立新的字符串对象。
s.intern();
System.out.print("choice:"+choice);
if (choice==1)
 s=greeting;
else if (choice==2)
s=saulutation;
System.out.print(s);   
}

}

Table 8-1. Class Salutationís constant pool
Index Type Value
1 CONSTANT_String_info 30
2 CONSTANT_String_info 31
3 CONSTANT_String_info 39
4 CONSTANT_Class_info 37
5 CONSTANT_Class_info 44
6 CONSTANT_Class_info 45
7 CONSTANT_Class_info 46
8 CONSTANT_Class_info 47
9 CONSTANT_Methodref_info 7, 16
10 CONSTANT_Fieldref_info 4, 17
11 CONSTANT_Fieldref_info 8, 18
12 CONSTANT_Methodref_info 5, 19
13 CONSTANT_Methodref_info 6, 20
14 CONSTANT_Double_info 2.99
16 CONSTANT_NameAndType_info 26, 22
17 CONSTANT_NameAndType_info 41, 32
18 CONSTANT_NameAndType_info 49, 34
19 CONSTANT_NameAndType_info 50, 23
20 CONSTANT_NameAndType_info 51, 21
21 CONSTANT_Utf8_info "()D"
22 CONSTANT_Utf8_info "()V"
23 CONSTANT_Utf8_info "(Ljava/lang/String;)V"
24 CONSTANT_Utf8_info "([Ljava/lang/String;)V"
25 CONSTANT_Utf8_info "<clinit"
26 CONSTANT_Utf8_info "<init"
27 CONSTANT_Utf8_info "Code"
28 CONSTANT_Utf8_info "ConstantValue"
29 CONSTANT_Utf8_info "Exceptions"
30 CONSTANT_Utf8_info "Greetings, planet!"
31 CONSTANT_Utf8_info "Hello, world!"
32 CONSTANT_Utf8_info "I"
33 CONSTANT_Utf8_info "LineNumberTable"
34 CONSTANT_Utf8_info "Ljava/io/PrintStream;"
35 CONSTANT_Utf8_info "Ljava/lang/String;"
36 CONSTANT_Utf8_info "LocalVariables"
37 CONSTANT_Utf8_info "Salutation"
38 CONSTANT_Utf8_info "Salutation.java"
39 CONSTANT_Utf8_info "Salutations, orb!"
40 CONSTANT_Utf8_info "SourceFile"
41 CONSTANT_Utf8_info "choice"
42 CONSTANT_Utf8_info "greeting"
43 CONSTANT_Utf8_info "hello"
44 CONSTANT_Utf8_info "java/io/PrintStream"
45 CONSTANT_Utf8_info "java/lang/Math"
46 CONSTANT_Utf8_info "java/lang/Object"
47 CONSTANT_Utf8_info "java/lang/System"
48 CONSTANT_Utf8_info "main"
49 CONSTANT_Utf8_info "out"
50 CONSTANT_Utf8_info "println"
51 CONSTANT_Utf8_info "random"
52 CONSTANT_Utf8_info "salutation"

使用 javap -c -verbose  Salutation.class  可以查看常量池

文章来源：Inside the Java Virtual Machine.pdf  p353

参考：http://www.cnblogs.com/lrh-xl/p/5334755.html