JAVA class文件格式

 

 

class文件结构

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ClassFile { u4 magic;//固定值0xCAFEBABE u2 minor_version; u2 major_version; u2 constant_pool_count;//常量池计数 cp_info constant_pool[constant_pool_count-1];//常量池 u2 access_flags;//访问标记 u2 this_class;//本类名在常量池中的位置 u2 super_class;//超类 u2 interfaces_count;//接口数 u2 interfaces[interfaces_count];//接口 u2 fields_count;//字段数 field_info fields[fields_count];//字段 u2 methods_count;//方法数 method_info methods[methods_count];//方法 u2 attributes_count;//属性数 attribute_info attributes[attributes_count];//属性}

访问标记

public final static int ACC_PUBLIC = 0x0001; public final static int ACC_FINAL = 0x0010; public final static int ACC_SUPER = 0x0020; public final static int ACC_INTERFACE = 0x0200; public final static int ACC_ABSTRACT = 0x0400;

 

类型标记 

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 /*B bytesigned byte C charcharacter D doubledouble-precision IEEE 754 float F floatsingle-precision IEEE 754 float I intinteger J longlong integer L<classname>;...an instance of the class S shortsigned short Z booleantrue or false [ ...one array dimension*/ 

 

 

 

 常量池

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    cp_info {

    u1 tag;    u1 info[];    }

 常量类型表

public final static int CONSTANT_Class=7; public final static int CONSTANT_Fieldref=9;public final static int CONSTANT_Methodref=10;public final static int CONSTANT_InterfaceMethodref=11;public final static int CONSTANT_String=8;public final static int CONSTANT_Integer=3;public final static int CONSTANT_Float=4;public final static int CONSTANT_Long=5;public final static int CONSTANT_Double=6;public final static int CONSTANT_NameAndType=12;public final static int CONSTANT_Utf8=1;

 

 

 

1) CONSTANT_Class

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 The CONSTANT_Class_info structure is used to represent a class or an interface:

 
CONSTANT_Class_info { u1 tag; u2 name_index; }

name_index 指向 CONSTANT_Utf8_info

 

2) CONSTANT_Fieldref, CONSTANT_Methodref, and CONSTANT_InterfaceMethodref

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 Fields, methods, and interface methods are represented by similar structures:

 
CONSTANT_Fieldref_info { u1 tag; u2 class_index; u2 name_and_type_index; } CONSTANT_Methodref_info { u1 tag; u2 class_index; u2 name_and_type_index; } CONSTANT_InterfaceMethodref_info { u1 tag; u2 class_index; u2 name_and_type_index; }

class_index 指向 CONSTANT_Class_info

name_and_type_index 指向 CONSTANT_Methodref_info CONSTANT_InterfaceMethodref_info

 

3) CONSTANT_String 

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The CONSTANT_String_info structure is used to represent constant objects of the type java.lang.String:

CONSTANT_String_info { u1 tag; u2 string_index; }

   string_index 指向 CONSTANT_Utf8_info

 

  

4) CONSTANT_Integer and CONSTANT_Float 

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The CONSTANT_Integer_info and CONSTANT_Float_info structures represent four-byte numeric (int and float) constants: 
CONSTANT_Integer_info { u1 tag; u4 bytes; } CONSTANT_Float_info { u1 tag; u4 bytes; }

 

 

5) CONSTANT_Long and CONSTANT_Double

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The CONSTANT_Long_info and CONSTANT_Double_info represent eight-byte numeric (long and double) constants:

 
CONSTANT_Long_info { u1 tag; u4 high_bytes; u4 low_bytes; } CONSTANT_Double_info { u1 tag; u4 high_bytes; u4 low_bytes; } 

6) CONSTANT_NameAndType

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The CONSTANT_NameAndType_info structure is used to represent a field or method, without indicating which class or interface type it belongs to:

 
CONSTANT_NameAndType_info { u1 tag; u2 name_index; u2 descriptor_index; }

        name_index 指向 CONSTANT_Utf8_info

        descriptor_index 指向 CONSTANT_Utf8_info

 

  

 

 

7) CONSTANT_Utf8

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The CONSTANT_Utf8_info structure is used to represent constant string values.

CONSTANT_Utf8_info { u1 tag; u2 length; u1 bytes[length]; }

 

 

 

字段 

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field_info { u2 access_flags; u2 name_index; u2 descriptor_index; u2 attributes_count; attribute_info attributes[attributes_count]; }

The items of the field_info structure are as follows:

 

access_flags

public final static int ACC_PUBLIC = 0x0001;public final static int ACC_PRIVATE = 0x0002;public final static int ACC_PROTECTED = 0x0004;public final static int ACC_STATIC = 0x0008;public final static int ACC_FINAL = 0x0010;public final static int ACC_VOLATILE = 0x0040;public final static int ACC_TRANSIENT = 0x0080;

name_index 指向CONSTANT_Utf8_info

 

descriptor_index 指向 CONSTANT_Utf8_info

attributes_count 属性计数

attributes[] 所有属性

 

 

 

方法 

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Each method, and each instance initialization method <init>, is described by a variable-length method_info structure. The structure has the following format:

 

method_info { u2 access_flags; u2 name_index; u2 descriptor_index; u2 attributes_count; attribute_info attributes[attributes_count]; }
access_flags

public final static int ACC_PUBLIC = 0x0001;public final static int ACC_PRIVATE = 0x0002;public final static int ACC_PROTECTED = 0x0004;public final static int ACC_STATIC = 0x0008;public final static int ACC_FINAL = 0x0010;public final static int ACC_SYNCHRONIZED= 0x0020;public final static int ACC_NATIVE = 0x0100;public final static int ACC_ABSTRACT = 0x0400;

name_index 指向CONSTANT_Utf8_info

descriptor_index 指向 CONSTANT_Utf8_info

attributes_count 属性计数

attributes[] 所有属性

 属性

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

1) SourceFile Attribute

 

  
SourceFile_attribute { u2 attribute_name_index; u4 attribute_length; u2 sourcefile_index; }

attribute_name_index 指向 CONSTANT_Utf8_info "SourceFile"

attribute_length 属性长度 = 2

sourcefile_index 指向CONSTANT_Utf8_info的源文件名 如:foo.java ,没有路径

 

 

 

 

2) ConstantValue Attribute

 
ConstantValue_attribute { u2 attribute_name_index; u4 attribute_length; u2 constantvalue_index; }

attribute_name_index 指向 CONSTANT_Utf8_info "ConstantValue"

attribute_length 定长2

constantvalue_index 指向常量池中的值,可以为以下类型 

Field TypeEntry TypelongCONSTANT_LongfloatCONSTANT_FloatdoubleCONSTANT_Doubleint, short, char, byte, booleanCONSTANT_Integerjava.lang.StringCONSTANT_String

 

 

 

 

3) Code Attribute

Code_attribute { u2 attribute_name_index; u4 attribute_length; u2 max_stack; u2 max_locals; u4 code_length; u1 code[code_length]; u2 exception_table_length; { u2 start_pc; u2 end_pc; u2 handler_pc; u2 catch_type; }exception_table[exception_table_length]; u2 attributes_count; attribute_info attributes[attributes_count]; }

The items of the Code_attribute structure are as follows:

 

attribute_name_index 指向 CONSTANT_Utf8_info "Code"

attribute_length 属性长度,不包括初始的6字节

max_stack 方法的最大栈大小

max_locals 最大局部变量数

code_length 代码长度

code[] 代码数据

exception_table_length 异常表长度

exception_table[] 异常表

 

start_pc, end_pc 异常捕获的指令范围

 

handler_pc 捕获后的跳转位置

catch_type 指向CONSTANT_Class_info, 捕获类型

 

attributes_count 属性计数

attributes[] 所有属性

 

 

 

4) Exceptions Attribute(方法throws的异常)

  
Exceptions_attribute { u2 attribute_name_index; u4 attribute_length; u2 number_of_exceptions; u2 exception_index_table[number_of_exceptions]; }  

 attribute_name_index 指向 CONSTANT_Utf8_info "Exceptions"

 attribute_length 不包括初始的6字节

 number_of_exceptions 异常数

 exception_index_table[] 异常表 指向CONSTANT_Class_info

 

 

5) LineNumberTable Attribute(行号表)

 
LineNumberTable_attribute { u2 attribute_name_index; u4 attribute_length; u2 line_number_table_length; { u2 start_pc; u2 line_number; } line_number_table[line_number_table_length]; }

attribute_name_index 指向CONSTANT_Utf8_info "LineNumberTable"

attribute_length 不包括初始的6字节

line_number_table_length 行号表长度

line_number_table[] 行号表

 

start_pc 开始的指令位置

line_number 行号,对应于源文件

 

 

 

6) LocalVariableTable Attribute(局部变量表)

LocalVariableTable_attribute { u2 attribute_name_index; u4 attribute_length; u2 local_variable_table_length; { u2 start_pc; u2 length; u2 name_index; u2 descriptor_index; u2 index; } local_variable_table[ local_variable_table_length]; }

attribute_name_index 指向 CONSTANT_Utf8_info "LocalVariableTable"

attribute_length 不包括初始的6字节

local_variable_table_length 局部变量表长度

local_variable_table[] 局部变量表

 

start_pc, length 指令开始位置,长度

name_index, descriptor_index 名字索引,描述, 指向CONSTANT_Utf8_info

 

index 局部变量索引,也就是变量占用的slot

 

 

 

 

 

 

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参考:http://java.sun.com/docs/books/jvms/first_edition/html/ClassFile.doc.html