【JVM】Class结构之常量池

时间：2023-03-09 08:03:06
【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）

【JVM】Class结构之常量池

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

【JVM】Class结构之常量池

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;

}

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