Java class loaders are divided into four categories. Bootstrap ClassLoader is implemented by C/C and is responsible for loading the JVM core class library such as rt.jar; Extension ClassLoader loads the extended class library, with the default path being java.ext.dirs; Application ClassLoader is responsible for loading classes under the user classpath, with the default path being controlled by java.class.path; Custom ClassLoader inherits the ClassLoader class and is used to implement specific loading logic, such as hot deployment, encrypted class loading, etc., and usually follows the parent delegation model to ensure security.

Java's class loading mechanism is a very important part of the JVM, and the class loader (ClassLoader) is the core of this mechanism. Different types of ClassLoaders are responsible for loading class files from different sources into memory. They have hierarchical relationships and their respective characteristics and uses.

The following describes several common Java class loaders, as well as their responsibilities and usage scenarios.

1. Bootstrap ClassLoader

This is the most basic class loader, implemented by C/C and does not belong to Java itself. Its main function is to load the core class libraries required for JVM startup, such as rt.jar , resources.jar , etc., which include classes in the Java standard library (such as java.lang.* , java.util.* , etc.).

• It does not have a parent class loader.
• The loading path is usually the path specified by sun.boot.class.path .
• Generally, you don't deal directly with developers, but if you extend the core class library or replace certain system classes, it may be related to it.

2. Extension ClassLoader

Extension ClassLoader is the first class loader written in Java and belongs to sun.misc.Launcher$ExtClassLoader . Its task is to load Java's extended class library.

• The default loading path is the JAR package in the directory specified by java.ext.dirs .
• These extension classes can be accessed by all applications and are suitable for some common third-party libraries.
• If you want a library to be available to all applications but don't want to modify the startup parameters, you can put it in the extension directory.

However, it should be noted that after Java 9, the extension class loader still exists, but the module system (JPMS) has changed the way of class loading. Many contents originally placed in the extension directory are now managed by the module system.

3. Application ClassLoader (also called System ClassLoader)

Application ClassLoader is the class loader we deal with most often, which is responsible for loading class files on the user classpath.

• It is an instance of sun.misc.Launcher$AppClassLoader .
• The default loading path is controlled by java.class.path , which is usually what you specify through -cp or -classpath .
• Its parent class loader is the Extension ClassLoader.

When you run a Java application, all classes are loaded by default through this loader. If you write a class loader yourself and don't specify the parent class loader specifically, it will also inherit the Application ClassLoader by default.

4. Custom ClassLoader

In addition to the above three standard class loaders, Java allows developers to customize class loaders, inherit from java.lang.ClassLoader , and rewrite the methods (such as findClass() and defineClass() ), thereby implementing specific class loading logic.

Common uses include:

• Loading classes from the network
• Encrypted and protected class files, decrypted and loaded at runtime
• Realize hot deployment, plug-in and other functions
• Dynamically generated classes (such as AOP proxy)

When using a custom class loader, it is recommended to follow the "Parent Delegation Model", that is, first delegate to the parent class loader to try to load the class, and only handle it yourself if the parent class loader cannot load. This can avoid repeated loading of system classes and ensure the security and consistency of class loading.

In general, Java provides a hierarchical class loading system, and each ClassLoader has its own scope of responsibility. Bootstrap is responsible for core classes, Extension handles extension classes, Application loads user classes, and custom ClassLoader is used for more flexible requirements.

Basically that's it.

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