Java Class File Structrue

A class file consists of a single ClassFile structure: 

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];

}

The interpretation of the field types is as shown in Table 4.2.

BaseType Character	Type	Interpretation
B	byte	signed byte
C	char	Unicode character
D	double	double-precision floating-point value
F	float	single-precision floating-point value
I	int	integer
J	long	long integer
L ;	reference	an instance of class
S	short	signed short
Z	boolean	true or false
[	reference	one array dimension

All constant_pool table entries have the following general format: 

cp_info {

u1 tag;

u1 info[];

}

Each item in the constant_pool table must begin with a 1-byte tag indicating the kind of cp_info entry. The contents of the info array vary with the value of tag. The valid tags and their values are listed in Table 4.3. Each tag byte must be followed by two or more bytes giving information about the specific constant. The format of the additional information varies with the tag value.

Constant Type	Value
CONSTANT_Class	7
CONSTANT_Fieldref	9
CONSTANT_Methodref	10
CONSTANT_InterfaceMethodref	11
CONSTANT_String	8
CONSTANT_Integer	3
CONSTANT_Float	4
CONSTANT_Long	5
CONSTANT_Double	6
CONSTANT_NameAndType	12
CONSTANT_Utf8	1

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;

}

The CONSTANT_String_info structure is used to represent constant objects of the type String: CONSTANT_String_info {

u1 tag;

u2 string_index;

}

The CONSTANT_Integer_info and CONSTANT_Float_info structures represent 4-byte numeric (int and float) constants:

CONSTANT_Integer_info {

u1 tag;

u4 bytes;

}

CONSTANT_Float_info {

u1 tag;

u4 bytes;

}

The CONSTANT_Long_info and CONSTANT_Double_info represent 8-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;

}

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;

}

Each field is described by a field_info structure. No two fields in one class file may have the same name and descriptor (§4.3.2). The format of this structure is

field_info {

u2 access_flags;

u2 name_index;

u2 descriptor_index;

u2 attributes_count;

attribute_info attributes[attributes_count];

}

access_flags

The value of the access_flags item is a mask of flags used to denote access permission to and properties of this field. The interpretation of each flag, when set, is as shown in 

Flag Name	Value	Interpretation
ACC_PUBLIC	0x0001	Declared public; may be accessed from outside its package.
ACC_PRIVATE	0x0002	Declared private; usable only within the defining class.
ACC_PROTECTED	0x0004	Declared protected; may be accessed within subclasses.
ACC_STATIC	0x0008	Declared static.
ACC_FINAL	0x0010	Declared final; no further assignment after initialization.
ACC_VOLATILE	0x0040	Declared volatile; cannot be cached.
ACC_TRANSIENT	0x0080	Declared transient; not written or read by a persistent object manager.

Each method, including each instance initialization method and the class or interface initialization method , is described by a method_info structure. No two methods in one class file may have the same name and descriptor .

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

The value of the access_flags item is a mask of flags used to denote access permission to and properties of this method. The interpretation of each flag, when set, is as shown in Table 4.5.

  
  

   
   

   
   
Flag Name 
Value 
Interpretation 
ACC_PUBLIC 
0x0001 
Declared public; may be accessed from outside its package. 
ACC_PRIVATE 
0x0002 
Declared private; accessible only within the defining class. 
ACC_PROTECTED 
0x0004 
Declared protected; may be accessed within subclasses. 
ACC_STATIC 
0x0008 
Declared static. 
ACC_FINAL 
0x0010 
Declared final; may not be overridden. 
ACC_SYNCHRONIZED 
0x0020 
Declared synchronized; invocation is wrapped in a monitor lock. 
ACC_NATIVE 
0x0100 
Declared native; implemented in a language other than Java. 
ACC_ABSTRACT 
0x0400 
Declared abstract; no implementation is provided. 
ACC_STRICT 
0x0800 
Declared strictfp; floating-point mode is FP-strict 
   
   Attributes are used in the ClassFile, field_info, method_info, and Code_attribute  structures of the class file format. All attributes have the following general format: 

  
  
  
  

   
   
attribute_info {
  
  
  
  

   
   
u2 attribute_name_index;
  
  
  
  

   
   
u4 attribute_length;
  
  
  
  

   
   
u1 info[attribute_length];
  
  
  
  

   
   
}
  
  
  
  

   
   
The ConstantValue attribute is a fixed-length attribute used in the attributes table of the field_info structures
  
  
  
  

   
   
The ConstantValue attribute has the following format: 
  
  
  
  

   
   
ConstantValue_attribute {
  
  
  
  

   
   
u2 attribute_name_index;
  
  
  
  

   
   
u4 attribute_length;
  
  
  
  

   
   
u2 constantvalue_index;
  
  
  
  

   
   
}
  
  
  
  

   
   
The Code attribute is a variable-length attribute used in the attributes table of method_info structures
  
  
  
  

   
   
The Code attribute has the following format: 
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 Exceptions attribute is a variable-length attribute used in the attributes table of a method_info 
  
  
  
  

   
   
The Exceptions attribute has the following format: 
Exceptions_attribute {	
  
  
  
  

   
   
u2 attribute_name_index;	
  
  
  
  

   
   
u4 attribute_length;	
  
  
  
  

   
   
u2 number_of_exceptions;	
  
  
  
  

   
   
u2 exception_index_table[number_of_exceptions];
  
  
  
  

   
   
}
  
  
  
  

   
   
The InnerClasses attribute5 is a variable-length attribute in the attributes table of the ClassFile 
  
  
  
  

   
   
The InnerClasses attribute has the following format: 
InnerClasses_attribute {	
  
  
  
  

   
   
u2 attribute_name_index;	
  
  
  
  

   
   
u4 attribute_length;	
  
  
  
  

   
   
u2 number_of_classes;	
  
  
  
  

   
   
{  
  
  
  
  

   
   
u2 inner_class_info_index;
  
  
  
  

   
   
u2 outer_class_info_index;
  
  
  
  

   
   
u2 inner_name_index;
  
  
  
  

   
   
u2 inner_class_access_flags;
  
  
  
  

   
   
} 
  
  
  
  

   
   
classes[number_of_classes];
  
  
  
  

   
   
}
  
  
  
  

   
   
The Synthetic attribute6 is a fixed-length attribute in the attributes table of ClassFile, field_info, and method_info  structures.
  
  
  
  

   
   
The Synthetic attribute has the following format: 
Synthetic_attribute {
  
  
  
  

   
   
u2 attribute_name_index;
  
  
  
  

   
   
u4 attribute_length;
  
  
  
  

   
   
}
  
  
  
  

   
   
The SourceFile attribute is an optional fixed-length attribute in the attributes table of the ClassFile  structure
  
  
  
  

   
   
The SourceFile attribute has the following format: 
SourceFile_attribute {
  
  
  
  

   
   
u2 attribute_name_index;
  
  
  
  

   
   
u4 attribute_length;
  
  
  
  

   
   
u2 sourcefile_index;
  
  
  
  

   
   
}
  
  
  
  

   
   
The LineNumberTable attribute is an optional variable-length attribute in the attributes table of a Code attribute.
  
  
  
  

   
   
The LineNumberTable attribute has the following format: 
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];}The LocalVariableTable attribute is an optional variable-length attribute of a Code  attribute. 
  
  
  
  

   
   
The LocalVariableTable attribute has the following format: 
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];
  
  
  
  

   
   
}
  
  
  
  

   
   
The Deprecated attribute7 is an optional fixed-length attribute in the attributes table of ClassFile (§4.1), field_info (§4.5), and method_info (§4.6)
structures.
  
  
  
  

   
   
The Deprecated attribute has the following format: 
Deprecated_attribute {
  
  
  
  

   
   
u2 attribute_name_index;
  
  
  
  

   
   
u4 attribute_length;
  
  
  
  

   
   
}