Talk about all aspects of Java class loading

Linghu Yizhuo 2021-01-22 14:02:41
talk aspects java class loading

What is a class loader

Class loader is a module that loads the description of a class into the memory of a virtual machine ; A typical description of a class is java Source compiled class file , It can also be any other form of data , For example, network byte stream and so on ;

What are the class loaders

java Three types of loaders are defined by default :

  • Start class loader (Bootstrap Class-Loade) Main load jre/lib Below jar package
  • Extend the classloader (Extension or Ext Class-Loader) Main load jre/lib/ext/ In the catalog jar package ;(jre/lib/ext/ The directory can be accessed through the specified java.ext.dirs Cover )
  • Apply the class loader (Application or App Class-Loader), load classpath The content of (classPath It's a set of directories ).

Application class loaders can be “ Cover ” Of , adopt -Djava.system.class.loader=com.yourcorp.YourClassLoader The whole parameter to cover the original application class loader ; The cover here is double quoted , In fact, the custom class loader will be called the father of the default application class loader ;

This is usually done when you need to change the parental delegation model ; About the parental delegation model , See below ;

The parent delegation model of classloader

What is the parent delegate model

When a class loader (Class-Loader) When trying to load a type , Unless the parent loader cannot find the corresponding type , Otherwise, try to delegate this task to the parent loader of the current loader . The purpose of using the delegation model is to avoid repeated loading Java type

Why use the parental delegation model

There are two main reasons for adopting the parental delegation model :

  1. Guarantee the uniqueness of a class , A class is only loaded once , Avoid repeated loading ;( Because in JVM In the specification , The same class The file is loaded by two identical classes , Will be treated as two classes )
  2. Security , Guarantee java The core API No replacement ; That is, the class loaded by the bootloader will not be covered by other class loaders ;

What broke the parental delegation model , Why break

  • JNDI service
  • Tomcat The server
  • OSGI Achieve hot update

The reason why the above scenario breaks the parental delegation model is that the parent class does not have the ability to load or complete the required logic , In order to make the overall architecture design more elegant 、 flexible , It's loaded by the subclass loader ;

The process of class loading

Class loading is mainly divided into three processes , load 、 link 、 initialization ;

load :Java Read bytecode data from different data sources to JVM in , And map it to JVM Approved data structures (Class object )

link : This is the core step , In short, the original class definition information is smoothly transformed into JVM In the process of operation . It is subdivided into verification 、 Get ready 、 Analyze the three processes ;

  • verification : verification Class Whether the object conforms to the virtual machine specification
  • Get ready : Create static variables and prepare memory space
  • analysis : Change the symbolic reference in the constant pool to a direct reference

initialization : The initialization logic of the execution class , Including static field assignments , Static block initialization, etc ; The initialization of the parent type takes precedence over the initialization of the current type .

What are the application scenarios of the custom class loader

  • Modify the bytecode logic of the class , such as : Weave universal logic for external classes ;``
  • According to the needs of users , Create classes dynamically ; such as :JDBC Driver class , Different databases need different driver classes
  • Package name + When class names conflict , A class loader cannot load at the same time , It can be loaded into memory with different class loaders ;
  • Thermal renewal characteristics , Through custom class loader , Realize dynamic loading in the running process 、 Unload class

Implementation of custom class loader

The main core is to get the bytecode phase of the class , For mapping bytecode to Class object , This part generally does not involve ;

public class CustomClassLoader extends ClassLoader {
public Class findClass(String name) throws ClassNotFoundException {
byte[] b = loadClassFromFile(name); // Here's the core logic
return defineClass(name, b, 0, b.length);
private byte[] loadClassFromFile(String fileName) {
InputStream inputStream = getClass().getClassLoader().getResourceAsStream(
fileName.replace('.', File.separatorChar) + ".class");
byte[] buffer;
ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
int nextValue = 0;
try {
while ( (nextValue = != -1 ) {
} catch (IOException e) {
buffer = byteStream.toByteArray();
return buffer;


Today I share with you java The following knowledge about class loaders :

  1. What is a class loader
  2. What are the specific class loaders
  3. Parent delegation model in class loading process
  4. The specific process of class loading
  5. Application scenarios of custom class loader
  6. Implementation of class loader

Zero base learning Java Programming , You can join me for ten years Java Learning garden , Technical communication , Resource sharing , Answer questions and solve doubts , Summary of development experience .

本文为[Linghu Yizhuo]所创,转载请带上原文链接,感谢

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