Structure of class files

1. The class file is a set of binary streams with 8-bit bytes as the basic unit. Each data item is strictly arranged in the class file in order, without any Separator in the middle, this makes almost all the content stored in the entire class file necessary data for running the program, and there is no gap. When a data item needs to occupy more than 8 bytes of space, it is divided into several 8 bytes for storage according to the method of the top position.

2. According to Java Virtual Machine regulations, the class file format is stored by a pseudo mechanism similar to the C language structure. In this pseudo structure, there are only two data types: unsigned number and table.

3. the unsigned number is a basic data type, u1, U2, U4, and u8 represent the unsigned number of 1 byte, 2 byte, 4 byte, and 8 byte, respectively, the unsigned number can be used to describe a number, index reference, number value, or to encode a string value by UTF-8.

4. A table is a composite data type consisting of multiple unsigned numbers or other tables as data items. All Tables habitually end with "_ INFO. A table is used to describe data in a hierarchical composite structure. The entire class file is essentially a table consisting of the following data items.

Class File Format

Type	Name	Quantity
U4	Magic	1
U2	Minor_version	1
U2	Major_version	1
U2	Constant_pool_count	1
Cp_info	Constant_pool	Constant_pool_count-1
U2	Access_flags	1
U2	This_class	1
U2	Super_class	1
U2	Interface_count	1
U2	Interfaces	Interface_count
U2	Fields_count	1
Field_info	Fields	Fields_count
U2	Methods_count	1
Method_info	Methods	Methods_count
U2	Attributes_count	1
Attribute_info	Attributes	Attributes_count

5. cp_info: the constant pool mainly stores two types of constants: literal and symbolic references ). The literal volume is similar to the constant concept at the Java language level. The human text string is declared

The constant value of final. Symbol reference is a concept of compilation principles, including the following three constants:

• Fully Qualified name)
• Field name and Descriptor (descriptor)
• Method Name and Descriptor

Every constant in the constant pool is a table. There are 11 types of table structure data with different structures. These 11 types of tables share a common feature, the first constant at the beginning of the table is a URL-type identifier (TAG, ranging from 1 to 12, without the Data Type of flag 2), which indicates the constant type of the current constant, the meanings of the 11 constant types are as follows:

Project strength of the constant pool

Type	Flag	Description
Constant_utf8_info	1	A UTF-8-encoded string
Constant_integer_info	3	Integer literal
Constant_float_info	4	Float literal
Constant_long_info	5	Long Integer literal
Constant_double_info	6	Double-precision floating-point literal
Constant_class_info	7	Symbol reference of a class or interface
Constant_string_info	8	String Literal
Constant_fieldref_info	9	Symbol reference of a field
Constant_methodref_info	10	Symbol reference of methods in the class
Constant_interfacemethodref_info	11	Symbol reference of methods in the interface
Constant_nameandtype_info	12	Partial symbol reference of a field or Method

The constant pool is the most tedious data because these 11 constant types have their own structures. The structural definitions of the 11 constant pools are summarized as follows:

Structural tables of 11 Data Types in the constant pool

Constant	Project	Type	Description
Constant_utf8_info	Tag	U1	The value is 1.
	Length	U2	The number of bytes occupied by the UTF-8-encoded string
	Bytes	U1	UTF-8-encoded string of Length
Constant_integer_info	Tag	U1	The value is 3.
	Bytes	U4	The Int value stored in front of a high position
Constant_float_info	Tag	U1	The value is 4.
	Bytes	U4	Based on the float value stored in the front
Constant_long_info	Tag	U1	The value is 5.
	Bytes	U8	According to the long value stored in front of the high position
Constant_double_info	Tag	U1	The value is 6.
	Bytes	U8	The double value stored in front of a high position
Constant_class_info	Tag	U1	The value is 7.
	Index	U2	Specifies the index of a fully qualified name constant.
Constant_string_info	Tag	U1	The value is 8.
	Index	U2	Index of the specified string literal
Constant_fieldref_info	Tag	U1	The value is 9.
	Index	U2	Index item of the class or interface descriptor constant_class_info of the declared Field
	Index	U2	Index pointing to the field descriptor constant_nameandtype
Constant_methodref_info	Tag	U1	The value is 10.
	Index	U2	Specifies the index of the constant_class_info class descriptor of the method to be declared.
	Index	U2	Specify the index item of the name and type descriptor constant_nameandtype
Constant_interfacemethodref_info	Tag	U1	The value is 11.
	Index	U2	Index Entry pointing to the constant_class_info interface descriptor of the declared Method
	Index	U2	Specify the index item of the name and type descriptor constant_nameandtype
Constant_nameandtype_info	Tag	U1	The value is 12.
	Index	U2	Index pointing to the constant item of this field or method name
	Index	U2	Index pointing to this field or constant item of the method Descriptor

You can use the-verbose parameter of the javap tool to output the bytecode content of the testclass. Class file.

6. The field table (field_info) is used to describe the variables declared in the interface or class. Fields include class-level variables or instance-level variables, but are not included in variables declared within the method. The field table structure is as follows:

Field table structure

Type	Name	Quantity
U2	Access_flags	1
U2	Name_index	1
U2	Descriptor_index	1
U2	Attributes_count	1
Attribute_info	Attributes	Attributes_count

7. The structure of the method table is the same as that of the field table. It is different only in the selection of the access flag and Attribute Table set.

Method table structure

Type	Name	Quantity
U2	Access_flags	1
U2	Name_index	1
U2	Descriptor_index	1
U2	Attributes_count	1
Attribute_info	Attributes	Attributes_count

8. Attribute Table (attribute_info) can carry its own attribute table set in the class file, field table, and method table to describe specific information in some scenarios.

Unlike other data items in the class file, which require strict sequence, length, and content, the restrictions on Attribute Table sets are slightly looser, and each attribute table is no longer required to have strict order, in addition, as long as it is not the same as an existing attribute name, any compiler implemented by anyone can write its own defined attribute information to the Attribute Table. attributes that are not recognized by the Java Virtual Machine during runtime will be ignored. In order to correctly parse the class file, Java virtual machine specification (version 2) pre-defines the attributes that the virtual machine in Section 9 should recognize, as shown below:

Predefined attributes of Virtual Machine specifications

Attribute name	Usage location	Description
Code	Method table	Bytecode commands compiled by Java code
Constantvalue	Field table	Constant Value defined by the final keyword
Deprecated	Class, method table, and field table	Methods and fields declared as deprecated
Exceptions	Method table	Method throw an exception
Innerclasses	Class File	Internal class list
Linenumbertable	Code attributes	Correspondence between the line numbers of Java source code and bytecode commands
Localvariabletalbe	Code attributes	Local variable description of the method
Sourcefile	Class File	Source File description
Synthetic	Class, method table, and field table	The identifier method or field is automatically generated by the compiler.

Code attribute. After the code in the Java program method body is processed by the javac compiler, the bytecode directive is stored in the Code attribute. The Code attribute appears in the attribute set of the method table, but not all methods must have this attribute. For example, the interface or abstract method does not have the code attribute. If the method table has a code attribute, then its structure will be shown in the table:

Structure of the Code Attribute Table

Type	Name	Quantity
U2	Attribute_name_index	1
U4	Attribute_length	1
U2	Max_stack	1
U2	Max_locals	1
U4	Code_length	1
U1	Code	Code_length
U2	Prediction_table_length	1
Prediction_info	Prediction_table	Prediction_table_length
U2	Attributes_count	1
Attribute_info	Attributes	Attributes_count