Accessing the ClassLoader of a Class

JVM Tutorials - Herong's Tutorial Examples

∟Accessing the ClassLoader of a Class

This section provides a tutorial example on how to access the ClassLoader of a given class using the Class.getClassLoader() method.

If you want to access the ClassLoader object of a given class object, you can use the following logic:

   Class cls = obj.getClass();
   ClassLoader loader = cls.getClassLoader();

If you want to access the ClassLoader object of a given class name, you can use the following logic:

   Class cls = Class.forName(className);
   ClassLoader loader = cls.getClassLoader();

Here is tutorial example program on how to example JVM built-in ClassLoaders and access the ClassLoader of a given class:

/* ClassLoaderTest.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class ClassLoaderTest {
   public static void main(String[] a) {
      java.io.PrintStream out = System.out;
      Object o = null;
      Class c = null;
      ClassLoader l = null;

      l = ClassLoader.getSystemClassLoader();
      out.println("");
      out.println("Built-in ClassLoaders");
      out.println("System ClassLoader: "+l.getClass().getName());
      l = l.getParent();
      out.println("Extensions ClassLoader: "+l.getClass().getName());
      l = l.getParent();
      out.println("Bootstrap ClassLoader: "+l);

      o = new java.lang.String();
      c = o.getClass();
      l = c.getClassLoader();
      out.println("");
      out.println("ClassLoader of java.lang.String: "+l);

      try {
         c = Class.forName("sun.security.pkcs11.P11Util");
         l = c.getClassLoader();
         out.println("");
         out.println("ClassLoader of sun.security.pkcs11.P11Util: "
            +l.getClass().getName());
      } catch (Exception e) {
      }

      o = new ClassLoaderTest();
      c = o.getClass();
      l = c.getClassLoader();
      out.println("");
      out.println("ClassLoader of ClassLoaderTest: "
         +l.getClass().getName());
   }
}

When executed with the JVM in JDK, I got this result:

herong$ java ClassLoaderTest.java

Built-in ClassLoaders
System ClassLoader: jdk.internal.loader.ClassLoaders$AppClassLoader
Extensions ClassLoader: jdk.internal.loader.ClassLoaders$PlatformClassLoader
Bootstrap ClassLoader: null

ClassLoader of java.lang.String: null

ClassLoader of sun.security.pkcs11.P11Util:
   jdk.internal.loader.ClassLoaders$PlatformClassLoader

ClassLoader of ClassLoaderTest:
   com.sun.tools.javac.launcher.Main$MemoryClassLoader

The test result confirms that:

The System ClassLoader is implemented by the jdk.internal.loader.ClassLoaders$AppClassLoader class.
The Extensions ClassLoader is implemented by the jdk.internal.loader.ClassLoaders$PlatformClassLoader class.
The Bootstrap ClassLoader is represented as null.
java.lang.String ClassLoader is represented as null too.
sun.security.pkcs11.P11Util is an extension class and loaded by the Extensions ClassLoader.
ClassLoaderTest is my own class, stored in memory (I did not create the .class file), and loaded by the MemoryClassLoader.

Comparing with JDK older versions, HotSpot JVM has changed some class loaders. Here is the result of the same program from JDK 10

herong> java ClassLoaderTest

Built-in ClassLoaders
System ClassLoader: jdk.internal.loader.ClassLoaders$AppClassLoader
Extensions ClassLoader:
   jdk.internal.loader.ClassLoaders$PlatformClassLoader
Bootstrap ClassLoader: null

ClassLoader of java.lang.String: null

ClassLoader of sun.security.pkcs11.P11Util:
   jdk.internal.loader.ClassLoaders$PlatformClassLoader

ClassLoader of ClassLoaderTest:
   jdk.internal.loader.ClassLoaders$AppClassLoader

Here is the result of the same program from JDK 7

herong> java ClassLoaderTest

Built-in ClassLoaders
System ClassLoader: sun.misc.Launcher$AppClassLoader
Extensions ClassLoader: sun.misc.Launcher$ExtClassLoader
Bootstrap ClassLoader: null

ClassLoader of java.lang.String: null

ClassLoader of sun.security.pkcs11.P11Util:
   sun.misc.Launcher$ExtClassLoader

ClassLoader of ClassLoaderTest: sun.misc.Launcher$AppClassLoader