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When will class loading be performed ？

The process of class loading

️ load

️ Connect

🪞 verification

🧭 Get ready

️ analysis

️ initialization

* Parent delegation model

️ What is the parent delegate model ？

The advantages of the parental delegation model

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When will class loading be performed ？

java Class name ,Java The entry class of the program , You need to load the class first and then execute main Method
When the program is running , perform Static method calls , Static variables and other operations
new object When
Create a class object through reflection , Then you can generate instance objects through reflection , Or call static methods

Class loading is performed only once , When class loading is completed , The method area will store class information , The heap will hold class objects

In multithreaded environment ,JVM Perform class loading When , Will use synchronized Lock Come on Ensure thread safety , Because class loading can only be performed once

The process of class loading

🧬 Learn about the learning class before loading it The life cycle of a class ：

The first five steps are the process of class loading in order , The middle three steps belong to connection , So for class loading, the steps are as follows ：

load
Connect ： verification , Get ready , analysis
initialization

️ load

Be careful ： The loading here should be distinguished from class loading , Loading is only one stage of the class loading process

During the loading phase ,JVM Virtual opportunity does the following three things ：

Get the binary byte stream that defines this class by using the fully qualified name of a class
Convert the static storage structure represented by the byte stream into the runtime data structure of the method area
Generate a representation of this class in memory java.lang.Class Object is used as the access entry for various data of this class in the method area

In short , Just load class Bytecode （ binary data ） Method to area , A... Is generated in the heap Class object

️ Connect

🪞 verification

Verification is the first step in the connection , This stage is Guarantee Class file The information contained in the byte stream accord with java Virtual machine specification , Ensure that this information , Will not endanger the security of the virtual machine itself

🧭 Get ready

The preparation phase is formal In class Define sub variable （ Static variables ） Allocate memory and set Variable Initial value

For example, a piece of code like this ：

public static int val = 10;

it Initialize to val The value of is 0, No 10, because 10 It was only after initialization that it was assigned 10 This value

The initial value of the object is null, The initial value of the basic data type is the basic type as the default value of the member variable , such as int The initial value is 0,boolean The initial value is false

️ analysis

The parsing phase is Java Virtual machines will Replace symbolic references in the runtime constant pool with direct references The process of , That is to say Initializing constants The process of

🧯 Symbol reference ： Compilation of class In file , There are references to worthy correspondence , In the memory that has not been loaded at this time , Just use “ Symbol reference ” To express this relationship
🧯 Direct reference ： Perform class loading , hold class After the bytecode is loaded into the memory , The relationship between variable and value embodied in memory is direct reference

️ initialization

Initialization phase ,Java The virtual machine really starts to execute... In the class Java Code , This stage is the process of executing the class constructor method

Static variables are only initialized and assigned at this stage

* Parent delegation model

Two points to know when learning the parental delegation mechanism ：

Parent delegation model jdk The loading mechanism of the default class
Perform class loading , yes Java Virtual machines load classes through class loaders

️ What is the parent delegate model ？

If a class loader receives a class load request , it I won't try to load this class first , But put this Request delegate to parent loader To execute , The same is true for the parent loader that receives this request , Finally, the request will be delegated to the top-level startup class loader , Only When the parent loader feedback fails to load the request （ It did not find the required class in its search scope ） when , The child loader will try to load itself

 

BootStrap Start class loader （BootStrap ClassLoader）
Ext Extend the classloader （Extension ClassLoader）
App application / system class loader (Application ClassLoader)
Custom class loaders

From top to bottom corresponds to the hierarchical relationship from parent to child

Two exceptions that class loading must understand ：

🪟ClassNotFoundException： Class cannot be found when loading class file
🪟NoClassDefFoundError： Class found class file , But class loading error , There is no class information in the method area , There are no class objects in the heap

The advantages of the parental delegation model

️ Avoid repeatedly loading classes ： When a class is loaded by the parent class loader , It will not be sent to the subclass loader to perform loading

To ensure safety ： Such as Object,String Classes and so on use jdk Provide classes instead of using self-defined java.lang.Object, If a hacker uploads one from the network java.lang.Object Binary data of , If there is no parental delegation mechanism, security is guaranteed , There will be security risks

Using the parental delegation mechanism , The priority of ensuring loading is jdk Class provided
Do not use the parental delegation mechanism , Load custom classes ,jdk There will also be secure authentication