JVM: what does the class loading subsystem do? What is it made of? What eight part essay do you need to remember?

jvm： What does the class loading subsystem do ？ What makes up ？ What eight part essay need to be remembered ？

I just worked recently , A little slack , Tiktok , I found that everyone is learning jvm, It happens that Goo Goo has always wanted to learn something in this regard , Just refer to others , To study the b Station Shang Silicon Valley teacher Kang's video , It's really very detailed . I plan to read and record a little .
Recently I finished reading the front 30 Multiple video screens , Mainly about the class loading subsystem .
A little bit jvm, Will know jvm It mainly includes class loading subsystem , Run time data area , And the execution engine .

Three level directory

Here I want to say

Class loading subsystem functions ：

From the file system , load class file , Load only , No matter what .
Someone asked ： Written java How can a file become class The file ？
answer ： adopt javac command
The class loading subsystem includes ：
loading-linking-initializetion The three step

• loading
  You need to know that bootstrap-extension-application Three loaders
  By full class name , Get the binary byte stream of this class =》 The data structure of the method area generates the class in memory class object .

• linking
  Need to know validation , Get ready , Analysis of the three step （ Just feel understood ）

• init
  javac Automatically collect the assignment actions of all class variables in the class Clinnit

1. What are the loaders “？

Class loader （ important ）
bootstrap Boot class loader , use c or c++, For loading ,java,javax,sun The files under the package .
Other loaders （ important ）
extension Extended class loader , and application System class loaders are collectively referred to as other loaders .
Generally, our own classes are written with application Loaded , and jre,lib,ext Some files under the subdirectory are loaded with extension classes .
We can get the system class loader in the following way ,.getParent Get his top .

ClassLoader.getSystemClassLoader()

2. You can also customize the classloader - benefits ：

• Isolation loading class
• Modify the class loading method
• Extended load source
• Prevent source code leakage
  Method ：extends classloader rewrite findclass

3. Parent delegate mechanism （ important ）

• If a classloader receives a load request , It doesn't load itself first , Instead, delegate the request to the parent loader to execute .
• If the delegated parent loader , There is a parent class loader , It will further entrust upward , Total point , Top level boot loader ,
• If the parent class can finish loading, let the parent class load , If it doesn't load , Then the subclass is loaded .
  
  The advantages of parental delegation ：
• Avoid duplicate loading of classes
• Protect program security , Prevention core api Be tampered with at will ,（ For example, custom String class ）

4. Sandbox security mechanism

• Protect the code loaded by your bootstrap class , Do not break custom code
• The class name and the loader used are the same , Can be defined as a class （ Is the class name of a class , Just like the loader that loads it , It's a class ）
• jvm It will protect a boot message of the user class loader , Method to area , For subsequent references

Active and inactive use of class ： Do not initialize .

So you can still see the bold places , It's always good to have a little reflection on the eight part essay .