JVM memory model: class loading process

  The class loading subsystem is responsible for loading Class file ,class The file has a specific file ID at the beginning .

 ClassLoader Only responsible for class Loading of files , As for whether it can run , from Execution Engine decision .

The loaded class information is stored in the method area .

ClassLoader Role

Equivalent to a courier role , hold class File loading to JVM Method area .

Class loading process ：

  load :

1、 Get the binary byte stream that defines the class through the fully qualified name of the class

2、 Convert the static storage structure represented by this byte stream into the method area runtime data structure

3、 Generate a representation of this class in memory Class object , As the access entry of various data in the method area

  link ：

verification ：

The purpose is to ensure Class The byte stream of the file contains information that meets the requirements of the current virtual machine , Ensure the correctness of the loaded class .

It mainly includes four kinds of verification ： File format validation , Metadata validation , Bytecode verification , Symbol reference validation .

Get ready ：

* Class variables (static) Allocate memory and set the default initial value of the variable , That's zero .

* The preparation phase does not include final Embellished static, because final It will be allocated at compile time , The preparation phase explicitly initializes .

* Initialization is not assigned to instance variables in the preparation phase , Class variables are assigned in the method area , Instance variables are assigned to... Along with the object java In the pile .

analysis ：

* The process of converting a symbolic reference in a constant pool to a direct reference .

* Parsing is often accompanied by JVM Execute after the initialization of the game .

* A symbolic reference is a set of symbols that describe the target of the reference . A direct reference is a pointer directly to the target 、 Relative offset or an indirect handle to the target .

* Parsing actions are mainly for classes or interfaces , Field , Class method , Interface method , Method type, etc .

* Corresponds to... In the constant pool constant_Class_info,constant_Fieldref_info,constant_Methodref_info etc. .

initialization ：

* The initialization phase is the execution of class constructor methods <clinit>() The process of . This method does not need to define , yes javac The compiler automatically collects the assignment actions of all class variables in the class and the statements in the static code block .

* The instructions in the constructor method are executed in the order in which the statements appear in the source file .

*<clinit>() Constructors different from classes .

* If the class has a parent class ,JVM Will guarantee the subclass <clinit>（） Before execution , Of the parent class <clinit>() Execution completed .

* The virtual machine must guarantee a class of <clinit>() Method is locked synchronously under multithreading .