In depth study of JVM bottom layer (IV): class file structure

Class file structure

summary

Every line of code we write , To run on the machine , Eventually, it needs to be compiled into binary machine code CPU To recognize . But because of the virtual machine , Shield the operating system from CPU Differences in instruction sets , Be similar to Java This kind of programming language based on virtual machine usually Compiled into an intermediate format file for storage , For example, what we're going to talk about today Bytecode （ByteCode） file .

The cornerstone of irrelevance

Java The designer of virtual machine considered and implemented other languages in the beginning of design Java The possibility of running on a virtual machine . So it's not just Java Language can run in Java Virtual machine , today Such as Kotlin、Groovy、Jython、JRuby Wait for a lot of JVM Language can be in Java Running on a virtual machine . They and Java All languages are compiled into bytecode files by the compiler , And then it's done by the virtual machine . So class files （ Bytecode file ） Have language independence .

Class Class file structure

Class The document is a set of With 8 Bit byte is the base unit of binary stream , The data are arranged in strict order Class In file , There is no separator in between , This makes the whole Class Almost all the contents stored in the file are necessary data for the program to run , There is no gap . When you need to occupy 8 When the data item in the space above the bit byte , It will be divided into several in the way of high position first 8 Bit byte for storage .

Java Virtual machine specification Class The file format is similar to C The pseudo structure of a language structure to store data , There are only two data types in this pseudo structure ： Unsigned numbers and tables ：

• An unsigned number ： It's a basic data type , With u1、u2、u4、u8 To represent respectively 1 Bytes 、2 Bytes 、4 Bytes and 8 An unsigned number of bytes , Unsigned numbers can be used to describe numbers 、 Index reference 、 Quantity value or according to UTF-8 The string value formed by the encoding structure .
• surface ： A composite data type consisting of multiple unsigned numbers or other tables as data items , All watches are used to 「_info」 ending . Tables are used to describe data in a hierarchical composite structure , Whole Class A document is a table , It consists of the data items shown in the table below .

Pictured ,Class The bytes stored in the file are arranged in strict accordance with the order in the table above . Which byte represents what meaning , What is the length , The sequence is strictly limited , No change is allowed .

Magic number and Class Version of a file

1. Every Class Header of file 4 Bytes are called magic numbers （Magic Number）, Its only function is to determine whether the file can be received by the virtual machine Calss file . The reason why magic numbers are used instead of file suffixes is mainly based on security considerations , Because the file suffix can be changed at will .Class The magic value of the file is 「0xCAFEBABE」.

2. Next to the magic count 4 One byte stores Class Version number of the document ： The first 5 And the 6 Two bytes are minor version numbers （Minor Version）, The first 7 And the 8 Byte is the major version number （Major Version）. The high version of the JDK Can be downward compatible with lower versions of Class file , The virtual machine refused to execute more than its version number Class file .

Constant pool

After the major version number is the constant pool entry , Constant pools can be understood as Class The repository of resources in the file , It is Class The most data types associated with other projects in the file structure , It's also occupation Class One of the largest data projects in file space , It's the same thing Class The first table type data item in the file .

There are two main types of constants in the constant pool ： Literal and Symbol reference

• Literal ： Close to Java The concept of constant at the linguistic level , Such as a string 、 Declare as final Constant value of .
• Symbol reference ： The concept of compilation principle , The following three types of constants are included
  • Fully qualified names of classes and interfaces
  • Name and descriptor of the field
  • The name and descriptor of the method

Access signs

The two bytes following the access pool represent （access_flag）, This flag is used to identify some class or interface level access information , Including this Class Class or interface ; Is it defined as public type ; Is it defined as abstract type ; If it's a class , Whether it is declared as final etc. . See the table below for the specific sign position and the meaning of the sign ：

access_flags There's a total of 16 Flags can be used , At present, only one of them is defined 8 individual , All unused flag bits are required to be 0.

Class index 、 Parent index and interface index set

Class index （this_class） And parent index （super_class） It's all one u2 Data of type , And the interface index set （interfaces） It's a group. u2 Type of data set ,Class These three data are used to determine the inheritance relationship of this class in the file .

• The class index is used to determine the fully qualified name of this class
• The parent index is used to determine the fully qualified name of the parent of this class
• The interface index collection is used to describe which interfaces the class implements

Field table set

Field table set （field_info） Used to describe variables declared in an interface or class . Field （field） Including class variables and instance variables , But excluding local variables declared inside methods . Let's look at the structure of the field table ：

The field modifier is placed in access_flags in , It's related to access_flag Very similar , It's all one u2 Data type of ：

Method table set

• Class The description of the method in the document is completely consistent with the description of the field , The structure of the method table is the same as that of the field table .
• because volatile Key words and transient Keywords cannot modify methods , So there is no access flag in the method table ACC_VOLATILE and ACC_TRANSIENT. In contrast ,synchronizes、native、strictfp and abstract Keywords can modify methods , So the access flag of the method table is added ACC_SYNCHRONIZED、ACC_NATIVE、ACC_STRICTFP and ACC_ABSTRACT sign .

Property sheet set

• stay Class file 、 Field table 、 All method tables can carry their own attribute sheets （attribute_info） aggregate , Used to describe specific information about certain scenarios .

• The property sheet collection doesn't look like Class Other data items in the file are so strict , The order of attribute sheets is not mandatory , And as long as it does not duplicate the existing property name , Any compiler implemented by anyone can write their own defined attribute information to the attribute table ,Java The virtual machine will omit the properties it doesn't know at runtime .