【JVM】Class结构之常量池

常量池

主要包括下面2类：

1. 字面量（Literal）：如int,double,String等；
2. 符号引用（Symbolic Reference）；

符号引用

1. 类和接口的全限定名；
2. 字段的名称和描述符；
3. 方法的名称和描述符；

当虚拟机运行时，需要从常量池中获取对应的符号引用，再在类创建时或运行时解析、翻译到具体的内存地址之中；

常量池的项目类型（常量类型）

常量池中常量项的结构总表

说明： 下图中的tag值即上图中的标志值

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解析Class文件指令

javap -verbose xxxx 

其中xxxx表示的是xxxx.class

如程序TestClass.java

public class TestClass {
    private int m;

    public int inc() {
        return m + 1;
    }
}

编译后生成的二进制Class文件内容如下：

 Offset: 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F    
00000000: CA FE BA BE 00 00 00 32 00 16 07 00 02 01 00 09    J~:>...2........
00000010: 54 65 73 74 43 6C 61 73 73 07 00 04 01 00 10 6A    TestClass......j
00000020: 61 76 61 2F 6C 61 6E 67 2F 4F 62 6A 65 63 74 01    ava/lang/Object.
00000030: 00 01 6D 01 00 01 49 01 00 06 3C 69 6E 69 74 3E    ..m...I...<init>
00000040: 01 00 03 28 29 56 01 00 04 43 6F 64 65 0A 00 03    ...()V...Code...
00000050: 00 0B 0C 00 07 00 08 01 00 0F 4C 69 6E 65 4E 75    ..........LineNu
00000060: 6D 62 65 72 54 61 62 6C 65 01 00 12 4C 6F 63 61    mberTable...Loca
00000070: 6C 56 61 72 69 61 62 6C 65 54 61 62 6C 65 01 00    lVariableTable..
00000080: 04 74 68 69 73 01 00 0B 4C 54 65 73 74 43 6C 61    .this...LTestCla
00000090: 73 73 3B 01 00 03 69 6E 63 01 00 03 28 29 49 09    ss;...inc...()I.
000000a0: 00 01 00 13 0C 00 05 00 06 01 00 0A 53 6F 75 72    ............Sour
000000b0: 63 65 46 69 6C 65 01 00 0E 54 65 73 74 43 6C 61    ceFile...TestCla
000000c0: 73 73 2E 6A 61 76 61 00 21 00 01 00 03 00 00 00    ss.java.!.......
000000d0: 01 00 02 00 05 00 06 00 00 00 02 00 01 00 07 00    ................
000000e0: 08 00 01 00 09 00 00 00 2F 00 01 00 01 00 00 00    ......../.......
000000f0: 05 2A B7 00 0A B1 00 00 00 02 00 0C 00 00 00 06    .*7..1..........
00000100: 00 01 00 00 00 02 00 0D 00 00 00 0C 00 01 00 00    ................
00000110: 00 05 00 0E 00 0F 00 00 00 01 00 10 00 11 00 01    ................
00000120: 00 09 00 00 00 31 00 02 00 01 00 00 00 07 2A B4    .....1........*4
00000130: 00 12 04 60 AC 00 00 00 02 00 0C 00 00 00 06 00    ...`,...........
00000140: 01 00 00 00 06 00 0D 00 00 00 0C 00 01 00 00 00    ................
00000150: 07 00 0E 00 0F 00 00 00 01 00 14 00 00 00 02 00    ................
00000160: 15                                                 .

解析示例：
Offset:00000008~00000009： 0x0016-->22 表示有22-1=21个常量；

使用

javap -verbose TestClass

进行解析，后生成如下：

Compiled from "TestClass.java"
public class TestClass extends java.lang.Object
  SourceFile: "TestClass.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class    #2; //  TestClass
const #2 = Asciz    TestClass;
const #3 = class    #4; //  java/lang/Object
const #4 = Asciz    java/lang/Object;
const #5 = Asciz    m;
const #6 = Asciz    I;
const #7 = Asciz    <init>;
const #8 = Asciz    ()V;
const #9 = Asciz    Code;
const #10 = Method  #3.#11; //  java/lang/Object."<init>":()V
const #11 = NameAndType #7:#8;//  "<init>":()V
const #12 = Asciz   LineNumberTable;
const #13 = Asciz   LocalVariableTable;
const #14 = Asciz   this;
const #15 = Asciz   LTestClass;;
const #16 = Asciz   inc;
const #17 = Asciz   ()I;
const #18 = Field   #1.#19; //  TestClass.m:I
const #19 = NameAndType #5:#6;//  m:I
const #20 = Asciz   SourceFile;
const #21 = Asciz   TestClass.java;

{
public TestClass();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #10; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable: 
   line 2: 0

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      5      0    this       LTestClass;


public int inc();
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   aload_0
   1:   getfield    #18; //Field m:I
   4:   iconst_1
   5:   iadd
   6:   ireturn
  LineNumberTable: 
   line 6: 0

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      7      0    this       LTestClass;


}