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Class文件的存储格式

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一、Class文件结构概览

1.1 概述
  • 每一个class文件包含一个类或者接口的定义
  • 以大端方式存储
  • u1、u2、u4分别表示1、2、4个字节
1.2 数据结构
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ClassFile {
    u4             magic;
    u2             minor_version;
    u2             major_version;
    u2             constant_pool_count;
    cp_info        constant_pool[constant_pool_count-1];
    u2             access_flags;
    u2             this_class;
    u2             super_class;
    u2             interfaces_count;
    u2             interfaces[interfaces_count];
    u2             fields_count;
    field_info     fields[fields_count];
    u2             methods_count;
    method_info    methods[methods_count];
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}
1.3 图形化表示
1.4 示例
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public interface Car {
    void drive();
}

public class BMWCar implements Car{

    private String name;
    
    public BMWCar() {
        name = "宝马";
    }
    
    @Override
    public void drive() {
        System.out.println("BMW car drive." + name);
    }
}
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Last modified 2017-11-10; size 644 bytes
  MD5 checksum ac6d7477d45479490e4ea3f660b1dcdd
  Compiled from "BMWCar.java"
public class BMWCar implements Car
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #12.#23        // java/lang/Object."<init>":()V
   #2 = String             #24            // 宝马
   #3 = Fieldref           #11.#25        // BMWCar.name:Ljava/lang/String;
   #4 = Fieldref           #26.#27        // java/lang/System.out:Ljava/io/PrintStream;
   #5 = Class              #28            // java/lang/StringBuilder
   #6 = Methodref          #5.#23         // java/lang/StringBuilder."<init>":()V
   #7 = String             #29            // BMW car drive.
   #8 = Methodref          #5.#30         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #9 = Methodref          #5.#31         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #10 = Methodref          #32.#33        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #11 = Class              #34            // BMWCar
  #12 = Class              #35            // java/lang/Object
  #13 = Class              #36            // Car
  #14 = Utf8               name
  #15 = Utf8               Ljava/lang/String;
  #16 = Utf8               <init>
  #17 = Utf8               ()V
  #18 = Utf8               Code
  #19 = Utf8               LineNumberTable
  #20 = Utf8               drive
  #21 = Utf8               SourceFile
  #22 = Utf8               BMWCar.java
  #23 = NameAndType        #16:#17        // "<init>":()V
  #24 = Utf8               宝马
  #25 = NameAndType        #14:#15        // name:Ljava/lang/String;
  #26 = Class              #37            // java/lang/System
  #27 = NameAndType        #38:#39        // out:Ljava/io/PrintStream;
  #28 = Utf8               java/lang/StringBuilder
  #29 = Utf8               BMW car drive.
  #30 = NameAndType        #40:#41        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #31 = NameAndType        #42:#43        // toString:()Ljava/lang/String;
  #32 = Class              #44            // java/io/PrintStream
  #33 = NameAndType        #45:#46        // println:(Ljava/lang/String;)V
  #34 = Utf8               BMWCar
  #35 = Utf8               java/lang/Object
  #36 = Utf8               Car
  #37 = Utf8               java/lang/System
  #38 = Utf8               out
  #39 = Utf8               Ljava/io/PrintStream;
  #40 = Utf8               append
  #41 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #42 = Utf8               toString
  #43 = Utf8               ()Ljava/lang/String;
  #44 = Utf8               java/io/PrintStream
  #45 = Utf8               println
  #46 = Utf8               (Ljava/lang/String;)V
{
  public BMWCar();
    descriptor: ()V
    flags: ACC_PUBLIC
    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 宝马
         7: putfield      #3                  // Field name:Ljava/lang/String;
        10: return
      LineNumberTable:
        line 6: 0
        line 7: 4
        line 8: 10

  public void drive();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=1, args_size=1
         0: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: new           #5                  // class java/lang/StringBuilder
         6: dup
         7: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
        10: ldc           #7                  // String BMW car drive.
        12: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        15: aload_0
        16: getfield      #3                  // Field name:Ljava/lang/String;
        19: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        22: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        25: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        28: return
      LineNumberTable:
        line 12: 0
        line 13: 28
}
SourceFile: "BMWCar.java"

二、常量池的结构

常量池是一个cp_info的数组：通过tag定位到具体的数据结构，再通过数据结构的表示，判断下面几个字节怎么解析。

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cp_info {
    u1 tag; //cp-info的类型
    u1 info[];//类型对应的数组
}
tag info 数据结构 描述
1 CONSTANT_Utf8_info CONSTANT_NameAndType_info {
 u1 tag;
 u2 name_index;
 u2 descriptor_index;
}		 UTF-8编码的字符串
3 CONSTANT_Integer_info CONSTANT_Integer_info {
 u1 tag;
 u4 bytes;
}		 整形字面量，boolean、byte、char、short等类型都用int存放
4 CONSTANT_Float_info CONSTANT_Float_info {
 u1 tag;
 u4 bytes;
}		 浮点型字面量
5 CONSTANT_Long_info CONSTANT_Long_info {
 u1 tag;
 u4 high_bytes;
 u4 low_bytes;
}		 浮点型字面量
6 CONSTANT_Double_info CONSTANT_Long_info {
 u1 tag;
 u4 high_bytes;
 u4 low_bytes;
}		 双精度浮点型字面量
7 CONSTANT_Class_info CONSTANT_Class_info {
 u1 tag;
 u2 name_index;
}		 类或接口的符号引用
8 CONSTANT_String_info CONSTANT_String_info {
 u1 tag;
 u2 string_index;
}		 字符串类型字面量
9 CONSTANT_Fieldref_info CONSTANT_Fieldref_info {
 u1 tag;
 u2 class_index;//索引值
 u2 name_and_type_index;
}		 字段的符号引用
10 CONSTANT_Methodref_info CONSTANT_Methodref_info {
 u1 tag;
 u2 class_index;;//索引值
 u2 name_and_type_index;
}		 类中方法的符号引用
11 CONSTANT_InterfaceMethodref CONSTANT_InterfaceMethodref_info {
 u1 tag;
 u2 class_index;;//索引值
 u2 name_and_type_index;
}		 接口中方法的符号引用
12 CONSTANT_NameAndType_info CONSTANT_NameAndType_info {
 u1 tag;
  u2 name_index;
  u2 descriptor_index;
}		 接口中方法的符号引用

三、字段的结构

每一个字段用一个 field_info的结构体表示

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field_info {
    u2             access_flags;
    u2             name_index;//指向常量池的index，表示字段名
    u2             descriptor_index;//指向常量池的inde，字段的描述符
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

四、方法的结构

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method_info {
    u2             access_flags;
    u2             name_index;
    u2             descriptor_index;
    u2             attributes_count;
    attribute_info attributes[attributes_count];//Code字节码信息
}

五、属性的结构

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attribute_info {
    u2 attribute_name_index;
    u4 attribute_length;
    u1 info[attribute_length];
}

六、参考链接

Chapter 4. The class File Format

Java Class文件结构解析

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