JVM.Classloading.What is the difference between the Bootstrap ClassLoader, Platform ClassLoader (formerly Extension ClassLoader), and Application ClassLoader?

In Java, class loading follows a delegation model, where each class loader delegates the class loading request to its parent before attempting to load it itself. The three main class loaders in modern Java (JDK 9 and later) are:

1. Bootstrap ClassLoader

• Role:
  Loads the core Java classes that are essential for running Java applications, such as classes from java.lang, java.util, etc.
• Source:
  Loads from the JDK’s runtime image (since JDK 9, the rt.jar file was removed).
• Implemented in:
  Native code (C/C++), as part of the JVM.
• Parent:
  It has no parent, as it is the root class loader.

2. Platform ClassLoader (JDK 9+, formerly Extension ClassLoader)

• Role:
  Loads classes from the Java platform modules that are not part of the core JDK, such as:
  • java.sql
  • java.xml
  • java.logging
• Source:
  Instead of jre/lib/ext/ (which existed in JDK 8 and earlier), it now loads platform-specific modules.
• Implemented in:
  Java (part of jdk.internal.loader.ClassLoaders$PlatformClassLoader).
• Parent:
  Bootstrap ClassLoader.

3. Application ClassLoader (System ClassLoader)

• Role:
  Loads user-defined classes (e.g., application code and external dependencies).
• Source:
  Loads classes from the classpath (-cp or CLASSPATH environment variable) or module path (--module-path in JDK 9+).
• Implemented in:
  Java (part of jdk.internal.loader.ClassLoaders$AppClassLoader).
• Parent:
  Platform ClassLoader.